Cannabis

Cannabis (a term used to refer to plants in the cannabis genus, primarily the species of sativa, indica, and ruderalis) is an herb from Traditional Chinese Medicine that also has a history of usage for various non-nutritional and non-medical purposes, such as fiber and textile manufacturing. Its history extends beyond mainland Asia, as it has been detected alongside Egyptian mummies (as hashish).^[1]

The plant is called many names with the most common being marijuana (referring to the plant itself), bhang (referring to a drink produced in some Asian countries from the leaves and flowers^[2]) and hashish (referring to a resinous solution). All parts of the plant tend to be used, usually the leaves and flower buds.^[3]

Cannabis sativa (of the family Cannabaceae) is a somewhat broad plant species, and previous species in the Cannabis genus (Cannabis indica, which is usually the source of hemp, as well as Cannabis ruderalis) are now considered varieties of Cannabis sativa.^[4] Beyond this there are distinct chemotypes of Cannabis sativa;a drug phenotype (defined by a Δ⁹THC exceeding 20%) a fiber type used to create hemp products (0.3% Δ⁹THC or less) and an intermediate type (0.3-1% Δ⁹THC).^[4][5] The fiber type is used to create commercial hemp protein or hemp oil supplements to circumvent their potential for drug abuse.

Cannabis is currently the most widely used illicit substance in the world according to the UN^[6] yet can legally be given as a medical treatment for various forms of cancer, AIDS/HIV, and neurological impairments either as therapy or as adjuvant (to increase appetite, food intake, and thus weight).^[7][8]

Cannabis is a traditional Chinese medicine that was initially spread in part due to its medicinal properties but also for utilizing material from the hemp plant for manufacturing. Nowadays it is most well known and used for its psychoactive properties although it still has some medicinal importance.

In general, the Cannabis sativa plant contains a wide variety of bioactives but those of interest are the cannabinoids. Cannabinoids, in the context of Cannabis sativa, refer to molecules with a C²¹ terpenophenolic skeleton^[9] of which over 86 unique molecules have currently been isolated.^[4][9][10] Known constituents include:

Cannabinoids (aka. phytocannabinoids) including:

• Tetrahydrocannabinol Δ⁹ (Δ⁹THC) type: this includes the major psychoactive (-)-trans-(6aR,10aR) Δ⁹-tetrahydrocannabinol (commonly shortened to Δ⁹THC, but specifically referred to as Δ⁹THC-C^5[9]) but also includes; shortened sidechain variants such as tetrahydrocannabinol C4 (Δ⁹THC-C^4[9]); acid variants tetrahydrocannabolic acid A (Δ⁹THCA-C⁵ A^[9]), tetrahydrocannabinolic acid B (Δ⁹THCA-C⁵ B^[9]), tetrahydrocannabivarinic acid A (Δ⁹THCVA-C³ A^[9]), and tetrahydrocannabiorcolic acid (Δ⁹THCOA-C¹ A/B^[9]); and other variants tetrahydrocannabivarin (Δ⁹THCV-C^3[9]) and tetrahydrocannabiorcol (Δ⁹THCO-C^1[9])
• Tetrahydrocannabinol Δ⁸ (Δ⁸THC) type: Differs from Δ⁹THC via position of the double bond, and only two variants of Δ⁸(6aR,10aR) tetrahydrocannabinolic acid A (Δ⁸-THCA-C⁵ A^[9]) and Δ⁸(6aR,10aR) tetrahydrocannabinol (Δ⁸-THC-C^5[9]) are known to exist currently
• Cannabinol (CBN) type: created from full aromatization of THC type cannabinoids. This includes cannabinolic acid A (CBNA-C⁵ A^[9]), cannabinol (CBN-C^5[9]) and its methyl ether (CBNM-C^5[9]), cannabinol-C4 (CBN-C^4[9]), cannabinol-C2 (CBN-C^2[9]), cannabiorcol-C1 (CBN-C^1[9]), and cannabivarin (CBN-C^3[9])
• Cannabidiol (CBD) type: includes cannabidolic acid (CBDA-C^5[9]), (-)-cannabidiol (CBD-C^5[9]) and its monomethyl ether, cannabidiol C4 (CBD-C^4[9]), cannabidivarinic acid (CBDVA-C^3[9]), (-)-cannabidivarin (CBDVA-C^3[9]), and cannabidiorcol (CBD-C^1[9])
• Cannabitriol (CBT) type: these include cannabitriol in (+)-cis, (+)-trans, and (-)-trans configurations (CBT-C^5[9]) as well as (+)-trans-cannabitriol-C3 (CBT-C^3[9]). Both 8,9-dihydroxy-Δ^6a(10a)tetrahydrocannabinol (8,9-Di-OH-CBT-C^5[9]) and 10-ethoxy-9-hydroxy variants^[9][11][12] belong to this group
• Cannabi_no_diol (CBND) type: formed during full aromatization of CBD type, only cannabinodiol (CBND-C^5[9]) and cannabinodivarin (CBVD-C₃^[9]) are known to exist currently
• Cannabigerol (CBG) type: not psychoactive in the classical sense (effects attributed to cannabis^[13]) and include cannabigerolic acid A (E-CBGA-C⁵ A^[9]) with its monomethyl ether, cannabigerol (E-CBG-C^5[9]) and its monomethyl ether, cannabigerovarinic acid A (E-CBGVA-C³ A^[9]), cannabigerovarin (E-CBGV-C^3[9]), and cannabinerolic acid A (Z-CBGA-C^5[9][14])
• Cannabichromene (CBC) type: these mostly racemic cannabinoids include cannabichromenic acid (CBCA-C⁵ A^[9]), cannabichromene (CDC-C^5[9]), cannabichromevarinic acid (CBCVA-C³ A^[9]), cannabichromenevarin (CBCV-C^3[9]), cannabivarichromene (CBCV-ⁱC^3[9]), and 2-methyl-2(4-methyl-2-pentenyl)-7-propyl-2H-1-benzopyran-5-ol^[9]
• Cannabicyclol (CBL) type: Three known cannabinoids in the (+)-(1aS,3aR,8bR,8cR) configuration including cannabicyclolic acid (CBLA-C⁵ A^[9]), cannabicyclol, (CBL-C^5[9]) and cannabicyclovarin (CBLV-C^3[9])
• Cannabielsoin (CBE) type: These appear to be infrequently identified in natural sources and may be produced by photooxidation from the CBD type, these cannabinoids are all in the (5aS,6S,9R,9aR) configuration and include cannabielsoic acid A (CBEA-C⁵ A^[9]), cannabielsoic acid B (both CBEA-C⁵ B and CBEA-C³ B^[9]), and cannabielsoin (both CBE-C³ and CBE-C^5[9])
• (Other) Cannabinoids found in cannabis sativa not belonging to one of the above groups are known to include dehydrocannabifuran (DCBF-C^5[9]), cannabifuran (CBF-C^5[9]), cannabichromanone (CBCN-C^5[9]), cannabichromanone-C3 (CBCN-C^3[9]), cannabicoumaronone (CBCON-C^5[9]), cannabicitran (CBT-C^5[9]), 10-oxo-Δ^6a(10a)tetrahydrocannabinol (OTHC^[9]), a cis configuration of Δ9THC (_cis_Δ9THC-C^5[9]), cannabiglendol (OH-iso-HHCV-C^3[9]), and isotetrahydrocannabivarin C³ and C^5[9]

Note: In the above acronyms for each molecule the bolded component (ie. CBT) refers to the structural subclass the cannabinoid belongs to while A refers to an acid (carboxylation) and M refers to a monoethyl ether addition. C^x refers to the length of a carbon chain on the backbone.

The exact Δ⁹THC content of cannabis can vary widely. Many studies using Δ⁹THC containing cigarettes tend to use products with a 4.8% Δ⁹THC content, while modern common street cannabis contains 7-9% Δ⁹THC.^[15][16] This content is significantly higher than in the past, since as early as 1980 cannabis has possessed a 1.5% Δ⁹THC content, which has increased steadily as time progressed.^[16][17] There have been strains of home-grown cannabis reporting 20% Δ⁹THC by weight.^[15] As with cannabis, the trend of increased average Δ9THC content over time also extends to hashish.^[15]

There are well over 70 cannabinoids found within Cannabis sativa, which may all contribute to the effects of administering the whole plant extract. Of these, the major cannabinoid is the first highly psychoactive one to be available in a pure form for research, which is (-)-trans-(6aR,10aR) Δ⁹-tetrahydrocannabinol (henceforth simply Δ⁹THC).

Other (noncannabinoid) possible bioactives including:

• Volatile oils (airborne constituents usually implicated in aromatherapy) usually containing a high concentration of myrcene (29.4–65.8% total essential oil^[18][19]) followed by Limonene (up to 16.3-17.7%^[19] although sometimes trace) and various lesser components including linalool, trans-ocimene, α-pinene, β-pinene, and β-caryophyllene (which itself possesses cannabinoid activity^[20])
• Various phenanthrenes including 4,5-dihydroxy-2,3,6-trimethoxy-9,10-dihydrophenanthrene, 4-hydroxy-2,3,6,7-tetramethoxy-9,10-dihydrophenanthrene, and 4,7-dimethoxy-1,2,5-trihydroxyphenanthrene^[21]
• Cannabispiranols such as α-cannabispiranol^[21] and β-acetyl cannabispiranol^[21]
• Cannaflavin A^[21] and C^[21]
• Chrysoeriol (bioflavonoid)^[21]
• 6-prenylapigenin (prenylated apigenin)^[21]

The noncannabinoid constituents will vary depending on growing condition and the strain of cannabis used, but unlike Δ⁹THC they are not commonly quantified so their contribution to the biological effects of cannabis are uncertain. Δ⁹THC and other cannabinoids (primarily cannabidiol) remain the most active components.

The double bond isomer (-)-trans-6a,10a-Δ⁸THC is said to be a biologically active but weaker variant of (-)-trans Δ⁹-THC, differing only by the position of the double bond between carbons 9 and 10 (Δ⁹THC) or 8 and 9 (Δ⁸THC).^[22]

Tetrahydrocannabinol is a terpenoid compound that possesses a double bond that can change location depending on the particular isomer, which is labelled via a delta (Δ) designation.

Passive inhalation refers to second-hand inhalation of smoke from cannabis, and is of concern since many athletes who are prohibited from using cannabis may associate with cannabis users, making exposure to 'side-stream' smoke a concern for positive urine tests.^[23] Whether sidestream smoke exposure is capable of causing a positive result for Δ⁹THC in the urine is also relevant because this is the main excuse employed by those faced with positive test.^[24]

A positive result for the Δ9THC metabolite (11-Nor-9-Carboxy-Δ9-Tetrahydrocannabinol or THCCOOH) (greater than 15ng/mL in urine) generally does not occur with passive inhalation^[25][26] and is not thought to be relevant to athletes who may have been exposed to sidestream cannabis smoke.^[23] Even in a nonventilated room, exposure to the smoke from four joints (2.8% Δ9THC) daily over six days has not been shown to cause a reliable increase in urine THCCOOH.^[26] This has also been tested elsewhere, where four joints over an hour in a similar environment caused a single positive result (3.9ng/mL) out of 80 urinary samples^[27] and three hours of exposure to recreational cannabis usage (joint passing) failed to cause any positive test results.^[28]

A positive result can be forced, since urinary THCCOOH correlates with the amount of Δ⁹THC in the air.^[26] In a small (2.1 x 2.5 x 2.4 meters) nonventilated room, exposure to 16 joints of side-stream over an hour can cause a positive urine test,^[26][29][30] although this is a quantity of cannabis smoke where goggles are actually required for vision due to the high density of smoke.^[26] Only in this latter example are the psychoactive effects of cannabis felt by the nonuser.^[26]

While it is possible to have a positive urine test from secondary (passive) cannabis inhalation by being near cannabis users, this requires the subject to essentially be sitting in a hotbox for an hour with smoke thick enough to require goggles to see properly. Any other condition of cannabis usage including all realistic situations do not seem to be enough exposure for a passive inhaler to test positive.

Tetrahydrocannabinol (THC) appears to be absorbed through the skin when in an appropriate medium (as it is fat soluble), with some studies noting success in transdermal THC absorption in vitro with solutions containing ethanol^[31][32][33] or propylene glycol.^[33][34] This increase in permeability applies to both THC and cannabidiol.^[33] However, the latter appears to be more permeable than THC due to lower lipophilicity.^[34] While mouse skin is more permeable than human skin,^[31] guinea pig skin is very similar in permeability to humans.^[35]

Despite species-dependent differences in absorption, THC has been noted to be topically absorbed in vivo in the mouse^[36] and guinea pig,^[35] with the latter showing a 84 minute lag time to reach a plasma concentration of 4.4+/-0.9ng/mL (with a patch containing 8mg THC in a solution with ethanol and propylene glycol), which was maintained for 70 hours.^[35]

THC can be absorbed through the skin, although its absorption is limited. Absorption can be increased with other agents, and at least in the guinea pig patches have been noted to expose the body to low but prolonged THC levels. Due to the low rates of absorption and lack of any sizeable peak in serum, any appreciable psychoactive effects are unlikely through transdermal absorption.

When cannabis is smoked (toking), a maximum plasma value of cannabinoids is achieved and the onset of psychotropic effects occurs within a few minutes. Psychotropic effects are maximal 15-30 minutes after initial ingestion and taper off 2-3 hours after exposure.^[37] Systemic bioavailability ranges from 10+/-7%^[38] to 27+/-10%^[39], with habitual users absorbing more of the active THC. The reason for the low bioavailability is due to a hypothesized maximum 30% loss from pyrolysis, poor lung absorption, and losses to sidestream smoke not being ingested.^[37] Toking in the form of a pipe seems to eliminate sidestream losses, and absorption rates of up to 45% have been recorded.^[40]

After oral ingestion (via brownies) peak serum levels are achieved at a variable 60-120 minutes after ingestion, due to varying digestive potencies inter-person.^[37][41] Some studies have noted delays of peak values up to 4-6 hours post ingestion,^[42] with some showing multiple plasma peaks.^[43] With a fatty acid vehicle, intestinal uptake of radiolabelled THC (which includes both the active Delta-9 form and its acid hydrolysis-product Delta-8^[44]) exceeds 90% in most cases^[41], although after extensive hepatic first-pass metabolism the amount available to systemic circulation varies from 2-14%, with high interindividual differences.^[37][45]

Ophthalmic (eye) administration has only been researched in rabbits, in which a light mineral solution resulted in 6-40% systemic bioavailability and a peak serum level 1 hour after application, which remained high for several hours.^[46]

Tissue distribution of THC is assumed to be due to the molecule's physicochemical properties, as no THC-specific transporters or barriers that affect tissue concentration are known to exist.^[37][47] Approximately 10% of assimilated THC is bound to red blood cells^[48] while the other 90% is bound to plasma proteins such as lipoproteins and, to a lesser extent, albumin.^[49][50] Due to THC's lipophilicity (fat-solubility), it can readily diffuse through cell membranes.

THC rapidly enters highly vascularized (good blood supply) tissues and organs such as muscle, spleen, heart, lungs, liver, and kidneys.^[51] Due to its lipophilicity, it eventually settles into adipose tissue (body fat) where it can remain for long periods of time.^[52][53]

THC can easily cross the placental barrier, and can appear in a child's blood if a mother ingests cannabis. This is seen across all species to varying degrees.^[54][55][56] Human breastmilk can contain levels of THC up to 8.4 times that found in plasma, and thus a mother smoking 1-2 joints a day can expose their child to values ranging from 0.01-0.1mg active THC through breastmilk.^[57]

THC may also accumulate in the testicles, where it may influence reproductive function.^[51]

Although metabolism exists in the lungs and heart tissue, tetrahydrocannabinoids are metabolized primarily in the liver through the cytochrome P450 (CYP) enzyme system, via hydroxylation and oxidation reactions.^[58][59] A member of the CYP2C subfamily seems to be the most active in humans.^[60]

Although over 100 separate metabolites of THC have been identified,^[61] the major one is produced by hydroxylation of THC at C-11 to form 11-OH-THC, which is further oxidized to THC-COOH.^[62] Becuase breakdown of THC metabolits is mediated by liver P450 enzymes, the rate limiting step seems to be hepatic blood flow.^[37][63]

THC metabolites are commonly excreted in the urine through the acid metabolite 11-nor-9-carboxy-THC glucuronide, a glucuronidated form of THC-COOH.^[64] A proposed mechanism of long-term storage is when 11-OH-THC conjugates with fatty acids in the adipocyte.^[65]

Excretion of THC compounds in the urine and feces begins after a pseudoequilibrium is met between tissues and plasma. The time of equilibrium changes based on dosage, with low dosage (16mg THC) tokes taking 3-12 hours, and high dosage (34mg) tokes taking 6-27 hours.^[66] The carboxylated metabolite (THC-COOH) can be detected in the plasma for up to 7 days after both dosages.^[66] This long duration for metabolism is partially explained by the slow release of THC conjugates from adipose and other body tissue into the bloodstream,^[47] and partially due to the half-life of various THC conjugates, which although not accurately known, ranges from 12-36 hours for 11-OH-THC and 25-55 hours for THC-COOH. Half lives in the 20-30 hour range are typically reported for the THC molecule itself.^[39][67] THC metabolites typically have longer half-lives than the parent THC molecule. Complications arise in measuring the half-life of Delta-9-THC due to interpersonal and interspecies differences, with some complications in distinguishing THC from its metabolites in vivo.

Excretion of THC occurs primarily as acid metabolites rather than the parent molecule, with 20-35% being excreted in the urine and 65-80% excreted in the feces.^[41][63] THC feces-eexcretion can be attributed to to the molecule's fat-solubility, extensive enterohepatic recirculation, and resorption from the renal tubules (which minimizes urine excretion).^[41][68] Roughly 65% of THC and THC metabolites are excreted after 72 hours from both routes.^[37][41] Full elimination of THC from the body may take up to two weeks to occur.^[42] There also seems to be differences between chronic and first-time users. Chronic users tend to take much longer to fully metabolize all THC from the body; in some cases under urine analysis metabolites can be traced in the urine up to 46-77 days after administration.^[69] Average elimination times have been reported as 12.9 days for light users and 31.5 days for chronic users.

Despite the lipophilicity of cannabinoids, which usually suggests fecal elimination via the liver, cannabinoids also tend to be eliminated in the urine leading to urinalysis of Δ⁹THC metabolites as a reliable way to detect cannabis abstinence.^[70] Blood testing^[70] as well as hair testing^[70][71] have both also been investigated.

Δ⁹THC is known to accumulate in body fat (adipose), with one study noting that in heavy users who were given two joints daily for two days (an estimated intake of 56mg labelled Δ⁹THC) inhaled Δ⁹THC resulted in between 0.4-8.0ng/g Δ⁹THC in fat deposites that remained for over four weeks.^[52]

Δ⁹THC can be retained at a low concentration in body fat for a month or more.

Smoking (cannabis or tobacco) can lead to increases in CYP1A2 activity, although cessation is known to return levels to normal. At times, it has been noted that the reduced liver enzyme activity from stopping cannabis smoking resulted in an overdose of some antipsychotics (clozapine and olanzapine) due to reduced metabolism.^[72][73]

When tested in vitro, all three enzymes (CYP1A1/2 and CYP1B1) are competitively inhibited by both cannabidiol (CBD) and Δ⁹THC with the most potent inhibition on CYP1A1 noted with cannabidiol (IC₅₀ of 537nM) Both CYP1A2 and CYP1B1 were inhibited by potently by cannabinol, (CBN) at 188nM and 278nM respectively.^[74] The actions of CBD on CYP1A1, however, have been shown to be related to enzymatic inactivation in an NADPH-dependent manner.^[74]

Δ⁹THC was relatively weaker at inhibiting CYP1 isoforms, having a K_i for competitive inhibition in the 2.47μM (CYP1B1) to 7.54μM (CYP1A2) range.^[74]

Cannabis smoking increases expression of the CYP1 enzymes. In vitro studies have noted that constituents of cannabis quite potently inhibit these enzymes (with CYP1A1 being inactivated from cannabidiol), suggesting that induction of CYP1 enzyme expression may be compensatory mechanism for suppressed enzyme activity.

Cannabidiol (CBD) appears to be a mixed inhibitor of CYP2C19, with an IC50 of 2.51-8.70µM (Ki value of only 793nM),^[75] The phenolic hydroxyl groups on the resorcinol moiety seem crucial for this inhibition, and CBD is thought to be more potent since the group is rotatable on this molecule (not so much on Δ9THC).^[75] Cannabidivarin (a minor constituent of Cannabis sativa) also has inhibitory activity.^[75]

CYP2C19 appears to be inhibited by relatively low concentrations of both Δ⁹THC and cannabidiol when tested in vitro, suggesting possible drug-drug interactions.

Cannabinoids appear to inhibit CYP2C9 potently when tested in vitro, with Δ⁹THC possessing a K_i of 937-1,500nM and IC₅₀ in inhibiting warfarin hydroxylation of 2.29μM.^[76] Moreover, cannabidiol (CBD) has a K_i of 882-1,290nM and IC₅₀ of 4.8μM while ^[76] cannabinol (CBN) and polyaromatic hydrocarbons are comparatively weaker. Notably no cannabinoids have been noted to have metabolism-dependent inhibition.^[76]

CYP2C9 is inhibited by all cannabinoids when tested _in vitro. Relative to the IC₅₀ values (a proxy measurement of potency) of all the P450 enzymes CYP29c9 as well as CYP1 isoforms appear to be most likely inhibited by cannabis.

CYP2D6 is inhibited by both Δ⁹THC (IC₅₀ of 17.1-21.2μM in inhibiting various substrate metabolism^[77]) and CBD (IC₅₀ of 4.01-6.52μM^[77]), with CBD being a more potent competitive inhibitor that is dependent on the resorcinol moiety.^[77] This inhibition was significantly greater than tested polyaromatic hydrocarbons (PAHs; produced from the smoke in cannabis inhalation) which had IC₅₀ values exceeding 100μM.^[77]

In regards to the isoforms CYP2A6 and CYP2B6, cannabinoids appear to be noncompetitive inhibitors with comparatively weaker effects in inhibiting CYP2A6 in vitro (K_i ranging from 28.9-55μM in inhibiting the function of this enzyme) with appreciable inhibition of CYP2B6 by Δ⁹THC (2.81μM), cannabidiol (695nM), and cannabinol (2.55μM).^[78]

Repeated exposure of a cell to cannabidiol and THC has been noted to result in an induction of CYP2B and CYP2C mRNA but without apparent changes in catalytic activity of these enzymes in vitro.^[79]

CYP2D6 and CYP2A6 are both inhibited by cannabinoids, with competitive inhibition by cannabidiol on the former and noncompetitive inhibition by all three cannabinoids on the latter (the latter being relatively weak compared to the others). CYP2B6, however, is potently noncompetitively inhibited by all three cannabinoids in vitro.

The resorcinol moiety also plays a role with CYP3A4 (another enzyme involved in cannabidiol metabolism^[80]) and CYP3A5 as cannabidiol has inhibited both with IC₅₀ values of 11.7μM and 1.65μM respectively. This is significnatly more potent than Δ⁹THC and cannabinol, which required over 35μM to reach their IC₅₀ values).^[81] CYP3A7 was inhibited in a mixed manner by all three cannibinoids to comparable degrees with IC₅₀ values in the range of 23-31μM.^[81] Cannabidiol has also been reported to be an inhibit CYP3A4 among other P450 enzymes when tested in vitro.^[82][83]

In contrast to the above information assessing acute enzyme interactions, induction of CYP3A isoforms has been noted with repeated exposure of cells to inactivating cannabinoids (usually cannabidiol^[79]) although the increase in CYP3A4 mRNA and protein content was met with no significant change in catalytic activity in vitro.^[79]

One study in cancer patients given an oral Cannabis sativa supplement (1g prepared as tea containing 18% THC and 0.8% CBD) once daily in the evening for 15 days prior to testing of two chemotherapeutics metabolized by CYP3A (irinotecan^[84] and docetaxel^[85]) failed to find any significant alterations in clearance or AUC for either drug.^[86]

Other human evidence is limited to a suspected acute inhibition of CYP3A4 due to a case study where the combination of Viagra and cannabis resulted in a myocardial infarction^[87] and a study in people with HIV on antiretroviral therapy where two weeks supplementation of dronabinol (2.5mg) or inhalation of cannabis thrice daily resulted in nonsignificant reductions in drug area under the curve (AUC).^[88]

All tested cannabinoids are acute inhibitors of the CYP3A isoforms of P450 when tested in vitro, with the most potent one being cannabidiol, which also potently inhibits CYP3A7 and appears to inactivate CYP3A4. Subchronic treatment of cells with these cannabinoids appears to cause a refractory induction of CYP3A activity. When tested in humans limited evidence suggests that two weeks of consistent cannabis use does not significantly affect drug kinetics.

Vaporization, as the name implies, is the creation of vapor using heat but not combustion. A potential advantage of this is less cannabinoid loss from burning.^[89] Another potential advantage is reducing combustion-formed toxicants such as benzene, toluene, naphthalene, and pyrene.^[90][91]

In a randomized, double-blind, placebo-controlled crossover trial, 17 healthy participants smoked or vaporized different doses of cannabis on separate occasions.^[92] The study found that a vaporizer led to higher maximum whole blood levels of THC, especially 11-OH-THC and THCCOOH, while Tmax was longer for vaporizers. For oral fluid levels, however, the difference was considerably smaller. There weren’t notable differences for acute adverse effects.

In another placebo-controlled trial, smoking cannabis led to a higher THC Cmax in oral fluid than vaporization, and both saw a higher Cmax than oral consumption (via brownies).^[93] Tmax was faster for smoking and vaporization than for oral consumption in regular smokers, but occasional smokers saw a longer Tmax for vaporization, which was comparable to oral consumption. Smoking led to higher 11-OH-THC Cmax than the other two, which were comparable, and Tmax was comparable between the three. Cmax was comparable for THCCOOH between the 3, though Tmax was much longer for oral intake. CBD Cmax was much higher for smoking and vaporization than for oral intake, and Tmax was higher for oral intake and comparable for smoking and vaporization.

A highly prevalent clinical form of cannabinoid administration is via an oromucosal spray called Nabiximols (Sativex®). It contains 2.7 mg of THC and 2.5 mg of CBD per 0.1 ml spray. It is typically aimed at the sublingual area and buccal mucosa. One study found that, compared with a synthetic THC oral capsule, THC levels were increased comparably, without statistically significant differences.^[94] There was somewhat less conversion to 11-OH-THC, which would make sense due to the sublingual absorption and bypassing of digestion and the portal vein to the liver. Another study found that the spray produced dramatically lower Cmax and higher Tmax than smoking cannabis, suggesting an overall much slower delivery rate.^[95] This is consistent with how it is generally used clinically: as regularly spaced or as-needed doses that may provide consistent pain relief without being as intoxicating as smoked cannabis.

PTL401 is an oral cannabis formulation with a 1:1 ratio of THC:CBD. It utilizes a self-nanoemulsifying technology, which spontaneously forms drug-encapsulating nano-lipopheres when it reaches the aqueous phase of the GI tract. In one study, it had notably greater absorption of THC, CBD, and 11-OH-THC than Nabiximols oromucosal spray.^[96]

Namisol is a patented extract of pure emulsified THC in tablet form; it uses a small amount of surfactant, less than 10% of concentration. In a randomized, double-blind, double-dummy crossover study, 14 participants took 5 mg of THC orally or sublingually, and 9 of them remaining took 6.5 mg, 8 mg, or matching placebos.^[97] Sublingual dosing led to a higher Tmax and lower Cmax for THC in oral dosing, and naturally the same for 11-OH-THC. The AUC was greater for sublingual dosing, however. 8 mg of oral Namisol led to marginally higher Cmax and Tmax than 6.5, while 6.5 led to notably higher levels of THC than 5 mg orally, though the effects on AUC were upwardly linear among the 3. Nonetheless, as dose increased, so did subjective feelings. This may be a faster increase in concentration than nabilone and Sativex, though head-to-head trials haven’t been performed.^{[94][98][99][100]}

Blunts are cannabis within cigar or cigarillo paper, which is thicker than usual joint paper. While some blunt papers are similar in size to standard joint papers, larger papers and scooping out the tobacco from a large cigar and replacing it with cannabis are common practices. In a secondary analysis of a cannabis cessation trial, blunt consumption was associated with more intense withdrawal symptoms^[101], likely due to being associated with and conditionally causal of higher cannabis use and greater dependence.^[102].

Dabbing is the practice of heating concentrated cannabis-rich wax (usually butane hash oil) on a hot surface and then inhaling the vapor (there is a lack of combustion) with the assistance of a special type of water pipe device. Butane hash oil has a highly variable THC profile, and confiscated products from 1993-2008 contained on average 16.8% THC, with a standard deviation of 16.3 and a range (of averages on different years) of 6.4 top 31.3%.^[103]

One preliminary experiment found that dabbing led to a drastically higher percentage of THC from the plant being retained in vapor as compared with cannabis smoking, to the tune of 75.5% compared with 26.7.^[104]

Various researchers are concerned that the large doses of THC could put some people at risk for psychosis, and there have been case reports.^{[105][106][107]}. However, it is difficult to discern how much higher, if at all, this is than for more conventional uses of cannabis. One study found that butane hash oil use was associated with a higher rate of cannabis dependency and related behavioral issues, though causality is difficult to determine.^[108] Another study found a higher rate of mental illness as compared with herbal cannabis, but the causality issue remains the same.^[109]

Due to the lack of combustion from dabbing, it is sometimes viewed as higher than smoking, but this is poorly studied, and a number of researchers are concerned with lung and heart damage from dabbing.^{[110][111][112][113][114][115]}

The main site of activity for the cannabinoid constituents of cannabis are the cannabinoid receptors, named cannabinoid receptor 1 (CB1) and cannabinoid receptor 2 (CB2), with the former being the found mostly in the brain where it modulates psychoactive effects whereas the latter is found mostly immune tissue where it modulates inflammation and immune response.^[116]

CB1 receptors are prominent in neural tissue but are also found in pituitary and peripheral tissue such as the thyroid, adrenals, gastrointestinal tract, and reproductive organs.^[117] The affinity (assessed by K_i for diplacement of known high-affinity ligands) of Δ⁹THC for the CB1 receptor is approximately 35-80nM, which is more potent than both the endogenous cannabinoids (or 'endocannabinoids') anandamide (60-540nM) and 2-AG (60-470nM).^[116]

CB1 exerts its effects through coupling with G proteins.^[116] Specifically, CB1 is coupled to:

• G_i/o (also known as G_i, which stands for G protein Inhibitory), known to have inhibitory actions on adenylyl cyclase (reducing cAMP).^[118] Through this coupling, CB1 activation is known to reduce cAMP-activated Protein Kinase A (PKA) activity, as well as activating MAPK and mixed effects on nitric oxide signalling, and inhibition of voltage-gated calcium channels and activation of inwardly-rectifying potassium channels.^[119] Na⁺K⁺ATPase activity can also be enhanced by CB1 activation.^[120] One subtype, Gα_i3, which is preferrentially coupled with CB1 is also involved in opiod signalling^[121]
• G_s (G protein Stimulatory), which has a stimulatory effect on adenylyl cyclase, although CB1 couples less selctively with this compared to G_i/o^[122]
• G_q/11 which is coupled to the phospholipase C pathway and leading to increased intracellular calcium, although Δ⁹THC only induces this particular pathway weakly as compared to other CB1 ligands^[123][124]

Activation of the CB1 receptor by any ligand, which usually refers to Δ⁹THC when looking at cannabis, results in psychoactive effects of cannabis. This receptor acts by coupling with G proteins.

CB2 receptors are found mainly in immune cells of which include some brain cells (glial cells) and, like CB1 receptors, are coupled with pertussis toxin-sensitive G_i/o proteins, which inhibit adenylyl cyclase formation and and affect the MAPK pathway.^[116] CB2 has also been found in keratinocytes as well, where they facilitate the release of endorphins.^[125] The affinity (as K_i) for Δ⁹THC towards CB2 is in the range of 4-75nM, which is more potent than both endocannabinoids anandamide (280-1,900nM) and 2-AG (150-1,400nM).^[116]

CB2 receptors are also G-protein coupled receptors, and do not mediate psychoactive effects of cannabis but instead act mostly in regards to regulating the immune system, including immune cells in the brain such as glial cells.

The CB1 receptor has been noted to heterodimerize (pair with another, distinct, receptor) with multiple other receptors including the dopamine D2 receptor, adenosine A2A receptor, β2 adrenergic receptor, and the μ, κ, and δ opioid receptors, which leads to different signaling pathways being activated.^[126]

There are some actions of cannabinoids which still occur when CB1 or CB2 or blocked or deleted entirely in knockout mouse strains, yet are sensitive to pertussis toxin that inhibits any G-protein coupled receptor's actions, suggesting that cannabinoids act directly on receptors that are not one of the two aforementioned cannabinoid receptors. Δ⁹THC seems to have little effect on these receptors, however, and these effects are mainly seen from other endogenous and synthetic cannabinoids.^[127][128] The former orphan receptor (receptors without known ligands) GPR55 is one such cannabinoid receptor, as it responds to some synthetic cannabinoids as well as most endocannabinoids such as 2-AG (EC₅₀ of 3nM) and oleoylethanolamide (440nM), as well as Δ⁹THC (8nM) while cannabidiol acts only as an antagonist.^[129] GPR55 is expressed in the adreanal glands, parts of the gastrointenstinal tract, and central nervous system (although at lower levels than CB1) in mice, and it is thought to be coupled to the G-protein known as Gα₁₃.^[129] This receptor's function is not entirely clear, but preliminary evidence suggests that it may play a role in vascular tone and have some anti-inflammatory effects. ^[129]

Some G-protein receptors, which were initially 'orphan' receptors due to no known ligands such as GPR55 have affinity for molecules which the cannabinoid receptors also respond to. Hence they may be involed in cannabinoid signalling.

Cannabinoids (not specifically Δ⁹THC) are known to influence a variety of other receptor functions including inhibitory actions on some serotonin receptors, cholinergic, glutaminergic, and mixed inhibitory or potentiation of GABAergic and glycinergic receptors.^[130] Each are elaborated more on in their respective neurotransmission subsections (in the Neurology section), and it seems that these receptors are common to one another as they are the members of the Cys-loop ligand-gated ion channel (LGIC) superfamily.^[131]

The Cys-loop ligand-gated superfamily appears to respond to cannabinoids, with most members of the superfamily being allosterically inhibited by various cannabinoids (with exception for the glycine and GABA receptors, which are sometimes potentiated).

The effects of CB1 receptor activation on ion channels, and ultimately the electrical conduction and action potential in neurons are mediated primarily through CB1's effects on PKA and G proteins. ^[116] Some evidence in guinea pigs suggests that the effect of CB1 on some ion channels may may be subject to sex-related differences.^[132]

Specifically, CB1 receptors can activate A-type potassium channels (K_V1.4^[133] and K_V4.2^[116]), which is secondary to a decrease in cAMP via inhibiting adenylyl cyclase^[134] and it appears that activation of CB1 is also capable of inhibiting L-type calcium channels (arterial cells^[135]) and N-type calcium channels (neuronal cells) secondary to the G_i protein inhibition of adenylyl cyclase.^[136][137] T-type calcium channels also appear to be inhibited by CB1 activation.^[138]

Activation of CB1 receptors influences neurons by modifying ion flux, which modifies the frequency of neuronal 'firing' (action potentials) and thus their signalling. CB1 activation results in activation of potassium channels and inhibition of calcium channels, ultimately causing a 'suppressive' or 'downer' effect (similar to GABA or adenosine signalling).

A family of ion channels called the transient receptor potential (TRP) channels depolarize the cell membrane upon activation, and play a wide role in sensory perception, including pain.^[139] Cannabinoids have been noted to interact with TRP channels including activating both TRPA1^[140] and TRPV1^[141] depending on the cannabinoid investigated. Δ⁹THC specifically appears to be an agonist of TRPA1, although differing in potency based on cellular localization. THC activation of extracellular TRPA1 is comparatively weak (K_i greater than 20µM) relative to intracellular TRPA1, with a K_i of 700nM.^[142] The latter affinity of 700nM is close to the levels seen acutely after inhalation.^[143] As TRPV1 is known to be involved in neuropathic pain^[144] it is also a plausible target for cannabis's pain killing effects. In vitro evidence suggests that several cannabinoids from cannabis, including cannabidiol and cannabinol (a degradation product of THC) are able to desensitize TRPV1 and TRPA1, which could possibly lead to analgesia.

Cannabinoids interact with a kind of ion channel called TRP channels, some of which are involved in pain signaling. Some cannabinoids have been shown to desensitize these channels in cell models, which could in part explain cannabis's analgesic benefits beyond their cannabinoid receptors activating-effects.

The 15-lipoxygenase (15-LOX) enzyme, which is known to play a role in oxidizing low density lipoprotein (LDL) and a key step in the development of atherosclerosis, appears to be inhibited by Δ⁹-THC with an IC₅₀ of 2.42μM in vitro.^[145] It is uncertain if this is biologically relevant, however, since THC has been noted to accumulate in tissues up to a concentration of around 1uM with standard usage^[52] while the active metabolite tends to be detected in plasma up to 200nM only.^[145]

Cannabidiol also has the ability to directly inhibit 15-LOX,^[146] with its metabolite cannabidiol-2',6'-dimethyl ether (CBDD) being significantly more potent (complete inhibition at 2μM^[145] and an IC₅₀ of 280nM)^[146] The other cannabidiol metabolite, cannabielsoin, is wholly inactive at these concentrations.^[145]

None of the above tested cannabinoids appear to inhibit 5-LOX, the molecular target of boswellia serrata, in the range where they were effective on 15-LOX^[145] and the selectivity of CBDD towards 15-LOX (relative to 5-LOX) was greater than 700.^[146]

While numerous cannabinoids show the ability to inhibit 15-LOX, an enzyme that plays a role in inflammation and atherosclerosis, this mechanism is of uncertain status in regards to cannabis use.

The CB1 cannabinoid receptor is G protein coupled receptor that signals through G proteins of the Gα_i/o subfamily to inhibit adenylyl cyclase.^[116] In this way CB1^[147] and adenosine A1^[148] receptors have much in common; both are located presynaptically and in similar brain regions, where they exert collective suppressive effects on glutamate release.^[149][150] Furthermore, CB1 and A1, along with GABA_B, signal through a common pool of G proteins. Activation of both CB1 and A1 result in less-than-additive G protein activation,^[151] while stimulation of the A1 pathway with Caffeine reduces the activation of G proteins through CB1.^[152]

While both cannabinoid CB1 receptors and adenosine A1 receptors clearly share common second messengers in their respective signaling cascades, the exact nature of their crosstalk is complex, and not completely understood. During tolerance to cannabinoids, A1 mediated G protein activation is not affected,^[153] while CB1-dependent activation is suppressed.^[154] Moreover, there is mixed evidence as to whether adenylyl cyclase inhibition by the A1 receptor is affected upon tolerance to cannabinoids, with one study finding no effect,^[155] and another finding a reduction in inhibition upon stimulation of A1 and GABA_B of around 18%.^[151] One study assessing motor coordination also noted cross tolerance with A1 and CB1 receptors.^[156]

Conversely, tolerance to Caffeine (which increases A1 receptor density in neurons^[157]) decreases CB1 receptor density. ^[152] The simultaneous increase in A1 receptors and decrease in CB1 receptors led to a condition where A1 activation suppressed activity of CB1 presynaptic receptors in an in vitro assay, where A1-dependent activation of the common Gα_i/o pool limited signaling through the CB1 receptor.^[152]

The CB1 receptor (the psychoactive target of Δ9THC) and the adenosine A1 receptor (activated by caffeine) are located in similar regions of the brain and signal through common secondary messengers (inhibitory G-proteins) to inhibit adenylyl cyclase. This may confer an element of negative cross-talk between these pathways, as some studies suggest.

One study has found that acute oral ingestion of 30-70mg (a high dose) of Δ⁹THC may have an influence on urinary catecholamines, with noradrenaline not being affected but with adrenaline being increased by 57% at the two hour mark (returned to baseline after four hours).^[158]

Agmatine is a neurotransmitter derived from L-Arginine^[159] that appears to interact with cannabinoid signalling, in particular the CB1 receptor^[160][161] and imidazoline receptors^[162] (which agmatine can act upon) and may act as a co-transmitter released alongside glutamate. ^[163]

The analgesic properties of agmatine appear to partially depend on imidazoline receptors, as the ability of mice to tolerate pain caused by heat decreases when given both imidazoline and CB1 receptor blockers.^[164] Since agmatine also synergistically enhanced the analgesia of test cannabinoids (WIN 55212-2 and CP55,940), it seems that agmatine can augment the analgesic properties of cannabinoids.^[164] Agmatine has also been noted to augment cannabinoid-induced hypothermia.^[165][166]

Evidence suggests that agmatine is is associated with cannabinoid signalling in the brain, where it appears to synergistically augment pain and hypothermic responses in animal testing, although this remains to be tested in humans.

The cannabinoid receptors CB1 and CB2 are named after the plant Cannabis sativa, since its main psychoactive componetn, Δ⁹THC, was the first potent ligand discovered for this class of receptors; since then, endogenous cannabinoids (endocannabinoids) such as the arachidonic acid derivative anandamide have been discovered.^[167] These receptors are G protein-coupled, with CB1 coupled to G_i/o^[120][168] and CB2 coupled only to G_i.^[168] Internalization (drawing a receptor into a cellular membrane) and externalization (shuttling the receptor to the cell surface) of the receptors help to regulate activity.^[169][170]

Δ⁹THC is a partial agonist of CB1, and does not maximally activate either G protein coupled to it.^[168]

Cannabidiol is known to be an inverse agonist of both CB1^[171][172] and CB2^[172][173] receptors in vitro, and is active as an inverse agonist on CB2 at 1µM with a potency comparable to 1µM rimonabant.^[172] It appears to be able to block the activation of these receptors by other agonists at a lower concentration though, blocking the nonselective agonist CP55940 with a K_B of 79nM and the CB1 agonist WIN55212 with a K_B of 138nM. The ability of cannabidiol to block agonists seems to occur at around a 64.5-fold lower concentration than its inverse agonist ability.^[172] Cannabidiol is also able to block the effects of Δ⁹THC.^[174]

Cannabidiol (CBD) appears to block the effects of ligands binding to the cannabinoid receptors, and as such negatively interacts with the actions of Δ⁹THC on these receptors.

Subchronic exposure to cannabis is known to downregulate the CB1 receptor in humans^[175] and a downregulation has been noted as acutely as after three days of exposure in rats.^[176] In the case of the CB2 receptor, at least in rats, exposure to cannabis results in an increase in cerebral mRNA for the CB2 receptor after cessation.^[177]

Δ⁹THC does not appear to influence α7-nicotinic acetylcholine receptors in vitro at concentrations up to 1μM, despite the endocannabinoid 2-AG being a noncompetitive inhibitor with an IC₅₀ of 168nM.^[178] This inhibition is also seen with anandamide,^[179] which is additive with alcohol in vitro,^[180] and anandamide also inhibits α4β2 nicotinic receptors which is not seen with Δ⁹THC at concentrations up to 1μM.^[181] When this inhibition occurs, it seems to be independent of the CB1 receptor^[179][181] or CB2 receptor.^[179]

At the level of the muscarinic acetylcholine receptors, anandamide has shown weak (and possibly not biologically relevant) inhibition of M1 and M4 receptors with an IC₅₀ value ranging between 10-50μM.^{[182][183][184]}

While endocannabinoids seem to have an inhibitory role on nicotine receptors, this inhibitory property does not appear to be replicated with Δ⁹THC as it doesn't seem to interact with nicotinic acetylcholine receptors.

Dopamine D2 receptors in the prefrontal cortex has the potential to couple with 5-HT_2A receptors, forming heteromers and thereby enhancing the actions of 5-HT_2A^[185][186]. This heteromer formation is enhanced after cannabinoid treatment in rats in vivo following a week of treatment; increased membrane localization of individual 5-HT_2A, D₂S, and D₂L receptors also occurs, and together these effects may be involved in the mechanism by which cannabis induces mood and cognitive dysfunction in susceptible individuals.^[187]

Activation of cannabinoid signalling in neurons for a subchronic length of time (week) has been noted to increase D2 dopamine receptors alongside the serotonin receptor 5-HT_2A, and has been noted to encourage their ability to bond together which enhances 5-HT_2A signalling.

The increase in 5-HT_2A expression upon subchronic cannabinoid exposure appears to be due to CB2 activation^[187] which has been noted elsewhere^[188] while the suppression of D2 mRNA is due to CB1 activation;^[187] this suppression of D2 mRNA has been found in utero with mothers who use cannabis.^[189]

Low D2 receptor content and the resulting lower dopamine release from drugs is implicated in various drug dependency situations including alcohol,^[190] amphetamines,^[191] and cocaine.^[192] Chronic cannabis usage, however, despite the above in vitro study noting reduced D2 mRNA, does not appear to be associated with any abnormalities in dopamine D2 or D3 receptor availability or dopamine release in any tested brain region relative to controls.^{[193][194][195][196]}

Based on limited evidence, activation of the CB1 receptor for a subchronic period (week) can reduce the transcription of the D2 receptor. With a combined CB1/CB2 agonist (such as Δ⁹THC) this may occur alongside an increase in receptor content on the cell membrane and an increase in dopamine signalling. A reduced expression of the D2 receptor seen with chronic usage of cannabis has failed to be demonstrated thus far.

The CB1 receptor is expressed on GABAergic interneurons in the hippocampus and cerebellum, usually to a larger relative degree than glutaminergic neurons.^[197][198] These interneurons (around 10% of total neurons in this brain region^[199]) appear to be the target of some CB1 actions in the hippocampus.^[200] In this brain region, Δ⁹THC can act as a full agonist of CB1 receptors^[201] ultimately influencing GABA signalling through stimulation of CB1.^[202] In addition to stimulation of CB1, some endocannabinoids are known to act on GABA_A receptors as positive allosteric modulators (directly binding to GABA_A and, while not stimulating the receptor alone, enhancing GABA's effects upon binding); Δ⁹THC was also found to interact with GABA_A directly, but only weakly.^[203]

When looking at GABA transmission itself, there appears to be an inhibition of hippocampal GABA-mediated transmission associated with cannabinoids.^[204] Specifically, CB1 modulates the synchronicity by which GABA is released from different types of interneurons.^[202] Prolonged CB1 activation leads to interanalization of CB1 receptors and tolerance, which attenuates this inhibition and has been noted to make the cells relatively more susceptable to excitotoxicity.^[205] Cannabinoid tolerance may also reduce the possible therapeutic effects of cannabinoids for epilepsy, since the benefits are mediated in part through GABA.^[206]

Δ⁹THC appears to influence GABA in an inhibitory manner, secondary to acting on CB1 receptors; there is also a weak direct interaction between Δ⁹THC and GABA_A receptors. Chronic administration of cannabinoids appears to attenuate this response.

During tolerance to Δ⁹THC in rats, GABA_B-mediated G protein activation does not appear to be altered.^[154]

The effects of cannabis usage on glutamate, a major excitatory neurotransmitter, appear to partially underlie its impairment of working memory.^[207]

The impact on working memory seems to rely mostly on the effects of Δ⁹THC on astrocytes, as abolishing the CB1 receptor on these cells, but not on glutamatergic or GABAergic neurons, abolishes its negative effects on spatial memory and long term depression (LTD) of synaptic strength between hippocampal synapses.^[208] CB1 activation on astrocytes increases their release of ambient glutamate, which activates a particular subset of glutamate receptor on neurons known as the NMDA receptor (NR2B subunit);^[208] This activation of NMDA receptors lead to internalization of another type of glutamate receptor on neurons, the AMPA receptor, which leads to synaptic LTD and working memory impairment.^[208]

Subchronically (one week of THC infusion at 5-10mg/kg in rodents), it appears that Δ⁹THC causes a downregulation in NMDA receptor expression (specifically the GluR1, NR2A and NR2B subunits^[209][210]). The downregulation is dose- and time-dependent, and is mediated through the upregulation of COX2.^[211] The glutaminergic target CREB, whose activation downstream of both NMDA and AMPA receptor activation is pivotal for memory formation^[212], has its activation reduced relative to baseline, which may in turn lead to decreased NMDA receptor expression.^[209]

This results in a reduction in hippocampal plasticity, which is noted in vitro and seen in vivo with subchronic Δ⁹THC administration, and is thought to underlie its impairment of memory.^{[208][209][213]}

Δ⁹THC is implicated in impairments to working memory related to glutamate signalling, and while this extends to both acute (one time use) and subchronic (week long) usage, they are mediated by different means. Acute usage is met with an increase in glutamate overall leading to reduced AMPA receptor availability, and lower synaptic plasticity, whereas for subchronic usage the NMDA receptor gets internalized and therefore is not available to the same degree for signalling.

Besides effects on astrocytes, activation of CB1 can also affect neurons directly; in both the ventral tegmental area (VTA)^[214] and in hippocampal slices^[215] activation of CB1 receptors presynaptically appears to attenuate glutamate release from neurons. Since mixed cultures (containing both glial cells such as astrocytes alongside neurons) incubated with Δ⁹THC results in an overall increase in synaptic glutamate,^[208] it seems the suppressed release of presynaptic glutamate may not be directly practically relevant.

Although activating presynaptic CB1 receptors suppresses glutamate release, the increase in glutamate release from astrocytes appears to be larger and overrides this possible protective effect.

CB1 can also reduce the levels of NMDA receptors at the cell surface by binding to them via a protein known as HINT1^[216] and being co-internalized,^[217][218] and subchronic Δ⁹THC exposure will internalize CB1 bringing NMDA into the cytosol alongside CB1.^[209][219] This results in less glutaminergic activity via the NMDA receptor.^[217][218] This is thought to be relevant to the actions of Δ⁹THC since noncompetitive NMDA antagonists can block CB1-mediated analgesia^[218][220] and reduce zinc mobilization (involved in epilepsy),^[218] both functions known to be relevant to inhalation of cannabis, and in mice lacking the binding protein HINT1 this effect is not observed.^[218]

The coupling of CB1 with NMDA ultimately seems to reduce signalling via NMDA.^[216][218]

This reduction in NMDA signalling due to an association with CB1 is also thought to underlie some of the neuroprotective aspects of cannabis, since mice lacking HINT1 do not experience protection by CB1 agonists from glutamate-induced neurotoxicity,^[216] which is known to be mediated by uncontrolled NMDA signalling.

CB1 and NMDA can physically bind to each other, which can lead to less NMDA signalling. This mechanism may underlie some of the possible benefits of cannabis, such as a reduction in epilepsy and neurodegeneration, as these seem to be linked to a long-term suppression of reduced glutamate signalling via the NMDA receptor.

Glycinergic neurotransmission (referring to the signalling properties of glycine and D-serine) appears to interact with cannabinoids,^[221][222] with Δ⁹THC having a direct potentiating role.^[223] The ionic currents activating by glycine in vitro appear to be enhanced by Δ⁹THC with an EC₅₀ value in the range of 73-115nM depending on the structure of the receptor,^[224] are replicated albeit less potently with anandamide,^[224] and is unlike both alcohol and volatile anaesthetics (which also potentiate glycinergic neurotransmission) as it is thought to directly allosterically modify the receptor^[224] via hydrogen bonding from the hydroxyl groups on the cannabinoids to the α1 subunit of glycine receptors.^[225][226]

This potentiation has been noted to contribute in part to analgesic properties of cannabis with or without psychoactive effects, since the effects of cannabis are lessened in mice lacking a glycine receptor (α3GlyR).^[225]

Glycinergic neurotransmission appears to be potentiated with cannabinoids, and THC has been noted to exert some pain relieving effects in part due to this pathway in mice.

The CB1 receptor is known to participate in crosstalk with opioid signalling, where animal evidence initially noted that administration of an μ-opioid agonist augmented the effects of Δ⁹THC in rodents^[227] and primates^[228] where blocking μ-opioid signalling decreases cannabis self-administration in rodents^[229][230] and primates.^[231]

Mechanistically, CB1 activation and µ-opioid receptors are synergistic in a manner dependent on the A2a receptor as blocking this adenosine receptor prevents synergism between the two receptors with respect to drug-seeking behaviour.^[232]

Blocking μ-opioid signalling has been noted to precipitate withdrawal from Δ⁹THC or other CB1 agonists in chronically treated rats,^[229][233] but similar designs failed to find this effect in primates^[234] or heavy cannabis-smoking humans^[235] suggesting species differences.

At least when looking at animal evidence, the opioid signalling system and the cannabinoid signalling system seem to positively influence each other in regards to drug self-administration and reward.

In heavy smokers, administration of naltrexone (opioid antagonist) alongside cannabis inhalation appeared to increase the perceived high and was also noted to cause impairment to psychomotor function (in a study which cannabis alone was insufficient to do so).^[236]

Δ⁹THC appears to be able to noncompetitively inhibit the effects of serotonin on 5-HT_3A receptors (a serotonin receptor subset involved in pain,^[237] drug abuse,^[238] and anxiety^[239]) with an IC₅₀ of 1.2μM, with stronger inhibitory actions in vitro in oocytes when there are less overall 5-HT_3A receptors (and accordingly, less inhibitory effects when receptors are abundant).^[240] The endocannabinoid anandamide has similar inhibitory actions on this receptor.^[241][242] Furthermore, 5-HT₃ and CB1 receptors are colocalized on interneurons of the hippocampus, suggesting that Δ⁹THC may affect these cells in multiple ways to the effect of reducing overall GABA neurotransmission.^[243]

It has been noted that, despite glycine and serotonin receptors both being allosterically modified by Δ⁹THC, that the hydrogen bonding necessary for glycine receptor interactions does not extend to 5-HT_3A receptors.^[226]

Noncompetitive inhibition of one serotonin receptor subset (5-HT_3A) has been noted with endocannabinoids as well as a low concentrations of Δ⁹THC.

In regard to other receptor subsets, it has been noted that activation of cannabinoid signalling causes an upregulation of 5-HT_2A receptors,^[244] including in the the paraventricular nucleus (PVN) of the hypothalamus^[188] which is likely mediated by the CB2 receptor activating ERK1/2 signalling, since this upregulation is stimulated by CB2-specific agonists^[244] and by blocking CB2 or ERK1/2 signalling selectively.^[245]

Subchronic, but not acute, exposure to CB2 agonists has been noted to cause anxiety in rodents.^[246] This was noted to occur alongside a downregulation of GABA receptors (and prevented by blocking the CB2 receptor^[246]) and was replicated elsewhere where an increase in cannabinoid-induced anxiety was also associated with increased 5-HT_2A receptor density.^[188] The actions of 5-HT_2A are also thought to be increased in vivo, since production of prolactin and corticosterone (increased in part by 5-HT_2A activation^[247]) are increased in response to weeklong cannabinoid treatment.^[188]

Activation of the CB2 receptor subchronically (at least a week) has been noted to increase the receptor density of 5-HT_2A in the hypothalamic PVN and subsequent signalling through this receptor.

Subchronic (12 day) administration of the synthetic cannabinoid HU-120 has been noted to suppress the activity of the 5-HT_1A receptor subset while upregulating the aforementioned 5-HT_2A receptor and inceasing the actions of its agonists.^[248] The release of corticosterone from a 5-HT_1A agonist appeared to be blunted with pretreatment of cannabinoid.^[248]

It is possible that, alongside an increase in 5-HT_2A receptor activity there is a suppression of 5-HT_1A receptor activity induced by cannabinoids in a subchronic manner.

In vivo, administration of Δ⁹THC to rats for seven days has been noted to increase the mRNA and protein content of brain-derived neurotrophic factor (BDNF), which plays a role in neurogenesis,^[249] in several brain regions in particular the nuclear accumbens (NAc; 5.5-fold increase in protein content^[250]), the ventral tegmental area (VTA; 4-fold^[250]) and the paraventricular nuclei (PVN; 1.7-fold^[250]) with no influence in the hippocampus.^[250] Another rat study, however, found that higher doses did in fact raise the BDNF levels in the hippocampus.^[251]

It appears that THC can increase the transcription of brain-derived neurotrophic factor (BDNF), which helps promote neurogenesis.

Endocannabinoids, via acting on CB1 (which Δ⁹THC also acts upon, although nonselectively), seem to additively promote GABAergic interneuron migration alongside BDNF in a manner dependent on the TrkB receptor and Src which lay downstream of it.^[252]

The BDNF-induced morphogenesis of interneurons, however, appears to be suppressed in the presence of CB1 activation in a manner also dependent on TrkB-Src signalling.^[252]

Activation of CB1 receptors appears to have some effects on interneuron migration and morphogenesis; the practical relevance is unknown.

Injections of Δ⁹THC into participants has been noted to have differential effects on serum BDNF based on cannabis tolerance, with cannabis users (defined as using at least 10 times in the last month) having lower baseline BDNF in serum.^[253] The nonusers, aside from having a higher baseline BDNF, experienced an acute increase in serum BDNF 20 minutes after injections, which was normalized after an hour; the cannabis users, however, experienced no such increase.^[253] The authors of this study suggest that serum levels of BNDF have a reasonable chance of reflecting levels in the brain.^[253]

The influence of Δ⁹THC on BDNF is subject to cannabis tolerance due to past use. Cannabis users had a reduced baseline BDNF compared to controls, and their blood level of BDNF did not change significantly upon acute IV adminstration of Δ⁹THC, whereas healthy control's BDNF levels did. While brain BNDF was not measured, serum levels may be a reasonable proxy for them, and may reflect what occured in the brain.

Glial cells are macrophage-like brain cells that support neuronal function and highly involved in neurodegenerative disorders when overactivated.^[254][255] Δ⁹THC can suppress inflammation in these cells secondary to acting on CB2 receptors,^[256] and despite cannabidiol (CBD) not activating CB2 receptors it appears to suppress the inflammatory genomic response to lipopolysaccharide^[257] (LPS, an inflammatory stimuli) and has been noted to reduce the expression of IL-1β by 81% at 10µM.^[257] The mechanism of cannabidiol may be secondary to PPARγ, as it has been noted to act on this nuclear receptor to exert antiinflammatory effects on glial cells not related to cannabinoid receptors.^[258][259]

Cannabinoids appear to acutely suppress inflammation in the brain, and both Δ⁹THC and CBD appear to be active, although by different mechanisms.

Orthostatic hypotension/dizziness upon reaching a standing position from a supine position has been noted with intravenous administration of isolated Δ⁹THC in otherwise healthy people^{[260][261][262]} (around 28% of users^[263]) thought to be related to decreased cerebral blood velocity as assessed by transcranial Doppler measures seen with cannabis inhalation.^[264] This orthostatic hypotension appears to be subject to tolerance with chronic Δ⁹THC administration, which was noted alongside edema and thought to be related to a counterregulatory increase in blood volume.^[260]

Naive users to cannabis may experience orthostatic hypotension upon standing up, and this orthostatic hypotension is reduced in those tolerant to cannabis.

The decrease in middle cerebral blood velocity is seen with mild and major instances of orthostatic hypotension associated with cannabis after 10 minutes^[263][264] and not related to plasma THC content.^[263]

When looking at the hemispheres of the brain there appears to be an increase in cerebral blood flow relative to placebo inhalation between 30-120 minutes (no longer present at the two hour mark) in both hemispheres and globally, which is dose-dependent.^[265][266] This increase in cerebral blood flow is mediated by CB1 receptors^[267][268] (despite the CB1 receptors not being highly involved in blood flow regulation under baseline conditions^[267]). In humans subject to positron emission tomography with intravenous Δ⁹THC,^[265][269] the increase in blood flow specifically favors the frontal lobes, right hemisphere, and especially the anterior cingulate cortex (which is highly involved in cardiovascular functions such as heart rate).^[269]

Orthostatic hypotension shortly after inhaling cannabis corresponds with reductions in blood flow velocity which is thought to be the cause of said orthostatic hypotension, while after 30 minutes or longer, there appears to be an increase in overall blood flow to the brain.

Preclinical studies have noted that the CB1 receptor is expressed in brain regions important for sensing and responding to pain (nociception),^{[270][271][272]} suggesting that the cannabinoids may play an important role in nociceptive transmission.^[273] Cannabis inhalation does appear to mitigate pain, however this is more of a dissociative effect, reducing the reported unpleasantness of pain rather than perceived intensity.^[274]

The effect of CB1 activation on pain dissociation can occur in those who have never used cannabis before^[274] and has a rapid onset, occurring within 45 minutes of inhalation.^[273] Efficacy may also correlate with perceived severity of pain, as CB1 agonists have been found to be especially effective in hyperalgesic states.^[273][275] Interestingly, cannabis has been shown to have a biphasic effect on pain, with medium doses causing the most robust pain relief and higher doses actually increasing pain.^[273] Consistent with this idea, at least one study using an active dose of 20mg Δ⁹THC failed to find any pain relieving effects 2.5 hours after oral ingestion of a capsule, instead finding evidence of increased sensitivity to pain.^[276]

There appears to be an acute analgesic effect of Δ⁹THC when administered either as capsules or as inhaled. This has been most tested in the capsaicin model for neuropathic pain. Notably, one study has indicated that cannabis may have a biphasic effect on pain, with medium doses causing pain relief and higher doses actually increasing the perception of pain.

The dissociation of pain seen in otherwise healthy volunteers (naive to Δ⁹THC and given 15mg) where pain was induced by topical capsaicin appears to be associated with right amygdala activity;^[274] a brain region that preferentially processes highly salient stimuli^[277] and is known to be involved in pain perception.^[278][279] This is thought to partially explain the analgesic effects, since some peripheral mechanisms may also contribute to the observed analgesic effects from CB1 activation.^[280]

The effects of cannabis on pain relief appear to be mediated by CB1 activation in the amygdala region of the brain.

Cannabis has been tested in the treatment of neuropathic pain, and found to be effective in reducing pain when smoked compared to placebo (0% THC content).^[281][282] Vaporized cannabis was also found to be effective in reducing pain relative to placebo, including in patients with neuropathic pain resistant to traditional treatments.^[283] Smoking cannabis was also found to be effective specifically in HIV-related neuropathic pain in placebo-controlled trials, providing benefit similar to other therapies for neuropathic pain^[284] and providing additional relief when added on top of other therapies to manage pain.^[285]

Vaporized and smoked cannabis appears effective in reducing neuropathic pain due to various causes, including physical trauma and HIV.

Cannabis is well known to increase hunger, a major cause of the 'munchies' in recreational users. The appetite-increasing effects of cannabis have also found medicinal use however, to bolster appetite in patients with HIV, cancer, or other diseases associated with muscle-wasting (cachexia).^[286] The appetite-stimulating effects of cannabis are driven by Δ⁹THC activation of the CB1 receptor and intimately intertwined with ghrelin, a peptide hormone that is secreted from the stomach and intestines to increase hunger.^[287]

Ghrelin ultimately increases food intake via signaling in the hypothalamus, in part via increased hypothalamic AMPK activity, a central control point for nutrient sensing.^[288][289] This action is dependent on both the CB1 receptor^[290] and the ghrelin (GHS-R1a) receptor,^[291] while the same holds true for cannabinoids such as Δ⁹THC.^[291] Ghrelin can increase synthesis of the endocannabinoid 2-arachidonoylglycerol,^[290] suggesting that there is much crosstalk between AMPK, ghrelin, and the endocannabinoid system for the control of appetite and energy homeostasis.

Interestingly, the requirement of CB1 activation for ghrelin appetite-inducing effects may occur in the periphery rather than the central nervous system, as CB1 receptor antagonists that cannot reach the brain are still highly effective in blocking the appetite-increasing effects of centrally administered ghrelin.^[292] For this reason, CB1 antagonists that act on the periphery are currently being tested as antiobesity agents^[293] due to the lack of anxiety-inducing effects that tend to be associated with centrally acting CB1 antagonists.^[294]

The appetite increasing properties of cannabis and other cannabinoids occur via CB1 receptor activation in addition to increased release of the 'gut-brain' hunger hormone ghrelin.

In addition to increasing ghrelin levels,^[295] inhalation of cannabis in HIV patients (blinded) has been noted to reduce levels of the appetite suppressing peptide PYY.^[295]

An increase in serum ghrelin and decrease of the appetite suppressing peptide PYY has been noted in humans who smoke cannabis.

In heavy cannabis users (average six sessions weekly with 5.4+/-1 joints each) who are given cigarettes containing 800mg cannabis (5.5% or 6.3% Δ⁹THC) and then given an attention test involving tracking an item on a computer screen for 10 minutes, cannabis did not appear to worsen attention relative to placebo inhalation.^[296]

Chronic cannabis usage has been associated with a reduction in motivation (ie. apathy), which has been associated in humans (PET imaging) with a reduction in dopamine synthesis seen with chronic cannabis usage.^[297]

A reduction in motivation seen in chronic cannabis use may be associated with a reduction in dopamine synthesis.

A randomized trial in heavy cannabis users (consumption of at least 4 times per week for at least 2 years) found that acute cannabis use with medium-dose THC (5.5 mg Δ⁹THC delivered) had no effect on measures of creativity (specifically, convergent and divergent thinking measures). However, contrary to the experimenters expectations, acute cannabis with higher doses of Δ⁹THC (22 mg) actually impaired divergent thinking without affecting convergent thinking.^[298]

One study in heavy cannabis users found that acute use of cannabis with a medium dose of Δ⁹THC did not affect two measures of creativity, whereas high doses worsened one measure of creativity (divergent thinking).

A systematic review of the effects of cannabinoids (including cannabis use) was recently completed, and overall found mixed results and general study quality to be poor, with most studies done in the short-term, with a high risk of bias, and done mainly on small and limited populations in North America only.^[299] The authors' main conclusions were that marjuana tends to decrease slow wave sleep (deep sleep which plays a role in memory consolidation^[300]), with inconsistent effects otherwise, and little effect on total sleep time.^[299] There may also be a beneficial effect on sleep quality only in cases where it is being used medically, where its beneficial effects on sleep may be due to its ability to reduce some sleep-disturbing symptoms of some disease states.^[299] A second review of the literature agrees in some aspects, finding that most studies show decreased slow wave sleep, but also notes that acute increases in slow wave sleep have been seen and that decreases occur with more chronic use,^[301] while also claiming that most studies show a decrease in REM sleep,^[301] which is in direct contrast with the systematic review,^[299] which found highly mixed results.

In one of the higher-quality, more recent studies, healthy volunteers were administered a nasal spray containing either 5mg or 15mg each of Δ⁹THC and cannabidiol (CBD), 15mg Δ⁹THC alone, or placebo in a crossover trial.^[302] A decrease in stage 3 sleep (one of the slow wave stages) was seen after use of the combination spray at night, along with an increase in wakefulness with the higher combination dose.^[302] While the 15mg dose of Δ⁹THC alone had no sleeptime effect, on the day following spray administration, sleep latency in the morning was decreased compared to placebo.^[302] This, combined with an increased duration of wakefulness during the night in the 15mg Δ⁹THC/CBD group, was interpreted to suggest that Δ⁹THC may be somewhat sedating while CBD may increase alertfulness in terms of sleep.^[302]

While several studies have been done on the effects of cannabis and its components, the studies tend to be small and uncontrolled, and yield mixed results. One of the more consistent results is an acute decrease in a type of deep sleep important in long-term memory consolidation.

Endocannabinoids are thought to have a protective effect in epilepsy, since they are released from neurons when a seizure occurs^[303] and control seizure frequency and duration via activating CB1 receptors.^[304][305] Consistent with this idea, blocking the CB1 receptor can increase the severity of seizures in in vitro models^[303][306] and downregulation of CB1 receptors seen with prolonged cannabinoid exposure in vitro also results in less tonic inhibition^[205] and reduced anticonvulsant activity.^[206]

The anti-epileptic activity of CB1 receptor activation^[305] is thought to occur via formation of a heterodimer between CB1 and the NMDA receptor, which functions to suppress glutamate signaling.^[218]

Excessive glutamate signaling promotes seizures by activating NMDA, which then increases calcium influx and the production of intracellular Nitric Oxide (via increased nNOS activity) which becomes oxidized to form peroxynitrate. Subsequent peroxynitrate-induced Zinc release in neurons then increases excitatory signaling from glutamate leading to dysregulated neuronal firing.^[307][308] CB1 activation in neurons suppresses glutamate signaling by blocking the initial activation of the NMDA receptor.^[218][309] Although control of glutamate signaling is quite complex, CB1 is thought to be the endogenous regulator that best hinders excessive NMDA signaling.^[210][217]

The antiepileptic actions of THC occur via activation of the CB1 receptor, which suppresses excitatory glutamate signaling to reduce seizures. Although this mechanism appears to be subject to tolerance in vitro, it is not currently known whether this phenomenon may also occurs in vivo and at least for this reason Δ⁹THC is not seen as a practical epileptic medication

Cannabidiol (CBD) is a nonpsychoactive bioactive in cannabis which is primarily looked into for epilepsy. Rather than acting upon the CB1 receptor it is thought to act on the TRPV1 calcium channel, seen in vitro to activate and rapidly desensitize this channel^[310] ultimately resulting in less potential for hyperexcitation.^[310] This property has been noted to apply to its propyl analogue, cannabidiverin (CBDV), as well^[310] with an EC₅₀ on TRPV1 3.6μM (EC₅₀ of TRPA1 at 420nM and TRPV2 7.3μM)^[311][312] and a similar function seems to exist on TRPA1.^[311]

In rodents it is known that these cannabinoids exert anti-epileptic effects^{[313][314][315][316]} in a manner not dependent on CB1 activation like Δ⁹THC is,^[315][316] and since the ability of plant cannabinoids to block sodium channels may not underlie therapeutic benefits to epilepsy in rodents^[317] it is thought that the actions on TRP channels underlies therapeutic benefits. Furthermore, it is suggested to be a potential therapeutic molecule since tolerance to CBD is not known to be a prominent feature of prolonged usage (again unlike Δ⁹THC which is subject to rapid tolerance).^[318]

In surveys assessing the use of cannabis products in childhood epilepsy, it has been reported that in a small sample (19) that 84% of parents reported benefits to symptoms with cannabidiol-rich products with just under half reporting at least an 80% reduction in symptoms;^[319] this survey noted a wide variety in reported CBD content of products used.

Two nonpsychoactive components of cannabis, cannabidiol (CBD) and cannabidiverin (CBDV), appear to activate yet rapidly desensitize the calcium channel known as TRPV1. This desensitization results in the neuron becoming less likely to become hyperexcitable and is thought to underlie the benefits of chronic cannabis usage in the treatment of epilepsy. While human studies are lacking, one survey suggests potent therapeutic effects

One meta-analysis of numerous cohort studies assessed the association between cannabis use or cannabis use disorders in the general population^[320] and various states of anxiety such as trait/state anxiety,^{[321][322][323]} anxiety disorders (social, general, and other),^{[324][325][326][327][328][329][330][331][332][333][334][335][336][337]} anxiety and mood disorders (AMD),^{[338][339][340][341][342][343][344]} post-traumatic stress disorder (PTSD) and panic disorders,^{[345][346][347]} and other types of anxiety diagnoses that often co-occur with other mental disorders such as anxiety during obsessive compulsive disorder.^{[329][348][349]} While there appeared to be an overall positive relationship with long-term cannabis use, it was relatively small in magnitude.^[320] Associations between anxiety and cannabis use had an Odds Ratio (OR) of 1.24, (95% confidence interval (CI): 1.06-1.45) while anxiety and cannabis use disorders had an OR of 1.68 (95% CI: 1.23-2.31), and cannabis use with combined anxiety and depression had an OR of 1.68 (95% CI: 1.17-2.40).^[320]

Cannabis use in the general population is positively associated with anxiety.

While only five studies were available for establishing a temporal relationship between cannabis use and anxiety (by measuring the same individuals at two time points) it does seem that people who used cannabis early on had an increased risk of developing anxiety later in life compared to users who did not use cannabis (OR of 1.28; 95% CI: 1.06-1.54).^[320] This study also extracted the higher doses and usage times of cannabis but used the more conservative ORs from each study, and controlled for confounds when possible^[320] since some drugs commonly ingested alongside cannabis (such as Nicotine via tobacco products) may also increase long-term risk for anxiety symptoms.^[349]

Cannabis appears to be associated with an increased risk of developing anxiety and panic symptoms (to the degree where a disorder can be diagnosed) over the course of numerous years of continuous usage. Increased risk with long-term use appears to be relatively small, not usually exceeding an odds ratio of 2.00 (meaning double risk).

In heavy adolescent and adult cannabis users (approximately daily usage) there does not appear to be any increased risk for depression in later in life when compared to non-smokers.^[330][350] Although at times a positive association has been noted between cannabis use and depressive symptoms,^[351][352] unlike anxiety this was eliminated after controlling for confounding factors in some studies.^[353][354] Moreover, when intentionally seeking out heavy cannabis users with significant depression, cannabis use appeared to be associated with less depressive symptoms relative to those who did not use cannabis.^[355] However, a meta-analysis of purely prospective studies which controlled for baseline depression upon entrance to the study found that cannabis use is correlated with depressive symptoms, with an odds ratio (OR) of 1.17 (95% CI of 1.05-1.30) compared to non-users, and a stronger relationship of heavy users with an OR 1.62 (95% CI 1.21–2.16) compared to non-users or light users; no relationship between the age of the users was seen, and there was a high heterogeneity among the studies.^[356]

A mildly increased risk for diagnosis of combined depression and anxiety in subjects who chronically use cannabis was noted in another meta-analysis (OR of 1.68 with a 95% confidence interval of 1.17-2.40).^[320] Importantly, combined depression and anxiety is a distinct diagnosis from solely depression.

The evidence of concerning cannabis use and the development of depressive symptoms is mixed. Some studies indicate that long-term cannabis use does not appear to be associated with increased long term risk for developing depressive symptoms when compared to nonusers. However, some positive associations have been noted, but there are often confounding factors present in people who use cannabis (such as social status or other drug usage) that contribute to the development of depression, and meta-analyses of the subject often find high heterogeneity, making generalizations somewhat difficult.

The effects of cannabis memory and learning can be divided up into short, medium, and long-term effects. In the short term (within hours of smoking), cannabis has consistently been shown to impact working and short-term memory.^[357][358] In the medium-term (after days to weeks of abstinence), no lasting effect on working memory has been found,^[357] although some lasting effect on decision-making has been observed that seems to correlate with the self-reported frequency of smoking.^[359][360] In cannabis users who have abstained, at least one twin study has failed to find a long-term impairment to cognition (with a median 20 years of cannabis cessation),^[361] another study found no difference between abstinent long-term cannabis users and users of other substances,^[362] and a meta-analysis assessing shorter term studies suggested that possible impairment is limited to 25 days as it failed to find any impairment for periods longer than that.^[363] When abstaining from cannabis for one month, heavy users (94+/-15 joints a week) do not return to baseline working memory and cognition whereas moderate (42+/-18 joints a week) and light (11+/-4 joints a week) users appear to fully normalize.^[359]

Another meta-analytic study of the non-acute effects of cannabis found small but significant effects on learning and forgretting, but noted a few caveats: firstly, the population used in most studies were young, healthy, and better-educated suggesting that the results may not be generalizable to other populations; secondly, there were few studies of good quality done examining this issue; and finally "residual effects" of smoking cannabis on the order of days may account for the overall effect found, and, based on the work of one month-long study in long-term cannabis users remaining abstinent,^[364] that it is likely that these effects disappear after less than a month has passed.^[365]

While cannabis may affect multiple parameters of cognition, the one that appears to be most frequently affected in an adverse manner is the attention/working memory domain,^[366] with an increase in forgetting and decrease in motor control also being common.^[366] In studies of heavy users given cannabis under acute conditions, inhalation has also been noted to impair immediate word recall as assessed by a digit recall task.^[296]

Cannabis impairs memory, attention, and motor control for hours after smoking, and may have small effects impairment effects in the days following smoking. However, the best evidence to date suggests no long-term impact on memory.

The memory-impairment effects of cannabis have been attributed to its Δ⁹THC content, since this cannabinoid in its pure form has been shown to suppress working memory in research animals.^[367] Δ⁹THC-induced memory impairment occurs via CB1 mediated induction of cyclooxygenase-2 (COX-2),^[211] an enzyme that converts arachidonic acid to prostanoids in the brain. Cox-2 induction downstream of CB1 appears to be necessary for memory impairment by Δ⁹THC,^[211] as inhibiting either the CB1 receptor^{[208][209][368]} or COX-2^[211] can limit the effects of CB1 activation on memory impairment.

Chronic administration of Δ⁹THC may also negatively effect memory by impairing synaptic plasticity. CB1 receptor agonists have been shown to suppress NMDA glutamate receptor-mediated calcium influx,^[210] resulting in less CREB phosphorylation and decreased synaptic plasticity that may ultimately impair memory formation.^[209]

Δ⁹THC appears to impair memory via CB1 receptor activation, which affects multiple downstream signaling pathways important for memory. CB1 activation has been shown to impair memory by activating COX-2, suggesting that COX-2 inhibitors may ameliorate some of the negative effects of cannabis on working memory. CB1 activation has also been shown to limit NMDA glutamate receptor-mediated calcium influx that is important for memory formation.

Inhalation of cannabis smoke is known to increase heart rate at rest.^[369][370] This increase in resting heart rate varies from 20-100% greater than baseline (when measured supine; it is lower when standing), occurs within 10 minutes of inhalation, and persists for two to three hours.^[371] Although cannabis increased heart rate relative to control during submaximal exercise,^[369] no differences were noted at intensities that exceeded 80% maximum, nor were there any differences in maximal exercise-induced heart rate between groups.^[369]

Increases in heart rate upon smoking cannabis can occur even in heavy users^[236] and have been suggested to correlate with the percieved high.^[236] In contrast, one study noted that attenuation in subjective intoxication with oral synthetic Δ⁹THC adminstration over the course of a week was not met with an attenuation of heart rate, suggesting that heart rate may increase independently of psychotropic effects.^[372] More prolonged usage of Δ⁹THC is associated with not only a normalization of heart rate, but a refractory decrease in heart rate relative to baseline.^[260]

There is an increase in heart rate associated with both cannabis smoking and oral ingestion of pure Δ⁹THC. The increase seems to scale with the psychoactive effects of cannabis acutely, but the increase in heart rate does not attenuate with tolerance to the psychoactive effects over longer periods of time. Heart rate may in fact decrease below baseline with longer-term exposure to Δ⁹THC.

Cannabis-induced increases in heart rate can be attenuated with beta-blockers^[261][262] or atropine^[261], the combination of which nearly abolish any changes in heart rate^[261] or other cardiac measures.^[373] This suggests that the heart-rate increasing effects of cannabis are mediated by both cholinergic (atropine) and adrenergic (beta-blockers) mechanisms.

These effects may originate from the central nervous system,^[373] since Δ⁹THC has been noted to increase blood flow to the anterior cingulate cortex in a manner correlating with the depersonalization effects of this drug.^[269] The activity of this brain region, which is enhanced with cannabis inhalation,^[374] is also involved in the regulation of cardiovascular function.^[375][376] The anterior cingulate cortex has also been observed to develop hypoactivity with chronic usage^[377] (possibly resulting in atrophy^[378]), correlating with the time course of the effects of cannabis on heart rate.

Cannabis's actions on the heart seem to occur through central (brain-mediated) means rather than directly, and are correlated with increased activity of the anterior cingulate cortex.

The cannabinoid receptor CB1 has been detected in endothelial cells from the human aorta^[379] where its activation leads to increased free radical production and Mitogen-Activated Protein Kinase (MAPK) activation which leads to cell death.^[379][380] These tissues also express CB2 receptors which appear to have antiinflammatory actions when activated resulting in less immune cell adhesion to these tissues as the adhesion factors (induced by inflammation) are suppressed.^[381][382]

The cannabinoid receptors CB1 and CB2 are expressed throughout the cardiovascular system. Neither the degree to which cannabis consumption may activate them, nor the resulting physiological responses are well-understood.

Atherosclerosis is a disease that is driven by chronic inflammation, where fatty deposits and immune cells enter the walls of blood vessels causing progressive narrowing and restriction of blood flow. It has been noted that cannabinoids have both immunosuppressive and anti-inflammatory properties,^{[383][384][385]} suggesting they may have potential as therapeutics for atherosclerosis.^[386] CB2 has been found to be upregulated in diseased arteries due to the accumulation of immune cells expressing this receptor on the diseased arterial wall.^[386] Notably, signaling through the CB2 receptor is both immunosuppressive and anti-inflammatory, suggesting that cannabinoid signaling through CB2 receptors may be part of a negative feedback mechanism that suppresses the formation of atherosclerotic plaques.

Consistent with this idea, human foam cells, which are damaged macrophages that develop after engulfing oxidized lipids,^[387] have been found to have greater amounts of CB2 than the macrophages from which they are derived.^[388] In vitro evidence also has indicated that activation of the CB2 receptor on macrophages could be protective against atherosclerosis by reducing macrophage recruitment^[389] and suppressing the accumulation of oxidized lipid droplets,^[388] resulting in reduced accumulation of foam cells.^[387] The anti-atherosclerotic effects of the CB2 receptor have also been confirmed in vivo, where Δ⁹THC orally at 1mg/kg (peak effective dose) daily in ApoE-/- mice hindered progression of atherosclerosis via the CB2 receptor.^[386] Notably, the concentration of Δ⁹THC in serum from this study was well- below levels required for psychoactive effects.

In vitro and animal data suggest that activation of the CB2 receptor via Δ⁹THC confers an antiinflammatory effect that reduces the progression of atherosclerosis. This occurs at serum concentrations lower than required for psychoactive effects.

Activation of the CB2 receptor appears to work via reducing the amount of oxidized LDL cholesterol (oLDL) that is accumulated by macrophages, secondary to downregulating the binding protein CD36 (aka. the macrophage binding receptor).^[388] CD36 is central to accumulating oLDL and initiating the production of foam cells, and is known to have its own expression enhanced during the inflammatory process initiated when binding to oLDL;^[390][391] this inflammation is reduced by CB2 activation as evidenced by a subsequent reduction in the inflammatory cytokines TNF-α, IL-10, and IL-12.^[388]

There may be an efflux of cholesterol out of already-formed foam cells secondary to CB2 activation, since it has been noted that CB2 activation has caused an increase in ATP-Binding Cassette sub-family G member 1 (ABCG1) protein levels,^[388] which is responsible for transporting excess cholesterol out of cells.^[392] It is hypothesized that the 15-lipoxygenase inhibition seen with some cannabinoids (most notably the cannabidiol metabolite CBDD) may have a role in reducing LDL oxidation^[146] as this enzyme is able to oxidize LDL when active^[393][394] and cannabinoids are highly lipophilic and carried in plasma by these lipoproteins.^[70]

Uptake of oxidized LDL cholesterol into macrophages produces foam cells, a critical factor for atherosclerotic buildup. Activation of CB2 via Δ⁹THC appears to prevent oxidized LDL from binding to the macrophage, reducing its uptake and ultimately reducing foam cell production.

In contrast to CB2, activation of the cannabinoid signaling system through the CB1 receptor is proatherogenic^[395][396] (although some compounds that inhibit CB1 also have confounding actions which may also be beneficial (direct acyl CoA:cholesterol acyltransferase inhibition^[397][398]). CB1 receptor activation has been shown to promote cholesterol accumulation in macrophages, leading to foam cell formation.^[399] This has been confirmed in vivo, where the CB1 receptor antagonist rimonobant inhibited atherosclerosis in LDL receptor-deficient mice.^[400] Although cannabis activates CB1 and CB2 receptors with similar affinity, cells of the immune system tend to have much higher levels of CB2, which may explain why the mixed agonist Δ⁹THC ultimately has protective effects in mice when orally ingested.^[386] Moreover, the negative effects of CB1 activation may explain why the benefits of Δ⁹THC are not dose dependent.^[386]

Although macrophages are traditionally considered as the source of foam cell formation, resident vascular smooth muscle cells are also capable of developing into foam cells.^[401][402] The Transient Receptor Potential Vanilloid Type 1 (TRPV1) receptor, a protein widely expressed in vascular smooth muscle cells that also reduces lipid accumulation,^[403] is known to be activated by components found in cannabis.^[404][405] This is known to reduce foam cell formation.^[406] This receptor is also found in macrophages and is downregulated in foam cells derived from them, although activation in these cells may actually increase lipid droplet accumulation.^[388]

While cannabis smoking may increase resting systolic and diastolic blood pressure in otherwise healthy subjects who do not identify as heavy users,^[370] many others studies have seen either no effects or a decrease; for instance, heavy users experience either no changes or a potential decrease in diastolic blood pressure when given Δ⁹THC,^[372] a result also seen with smoked cannabis.^[407] Another study with heavy users with isolated Δ⁹THC administered continuously over six days also showed a consistent hypotensive effect.^[372] Glaucoma patients were also seen to have decreased blood pressure when inhaling cannabis acutely, which was frequently associated with postural hypotension.^[408] Changes in blood pressure seem to be more marked with diastolic pressure, and tend to occur alongside alterations in heart rate.^[369][370]

Increased blood pressure may be partly driven by the action of THC on CB1 receptors in the brain, which is known to increase heart rate and produce acute increases in blood pressure.^[409]

While limited evidence suggests that those who do not smoke cannabis heavily may experience and acute rise in blood pressure, more evidence exists that these increases are subject to tolerance and may result in a refractory decrease in resting blood pressure in heavy users.

Abrupt cessation of cannabis usage (usage at least 25 times monthly for minimum of one year) was associated with an increase in systolic (7.8%), diastolic (9.3%) and mean arterial blood pressure (8.7%) for the first three days of cessation in a manner that was not associated with heart rate. These changes were normalized when cannabis was resumed,^[410] and most subjects (69%) in this study did not experience increases large enough to meet hypertension criteria.^[410]

Abruptly stopping chronic cannabis usage can result in a refractory increase in blood pressure.

In vitro evidence suggests that some components of cannabis may inhibit platelet aggregation by blocking ADP-induced platelet aggregation.^[411] The ADP-induced aggregation was inhibited in a dose-dependent manner, with cannabigerol being the most potent, followed in decreasing potency by cannabidiol, olivetol, Δ¹THC and cannabinol.^[412] The inhibition caused by cannabidiol and Δ¹THC is countered by adding higher levels of ADP.^[413]

Serotonin is taken up and stored by platelets; when these platelets are activated, serotonin is released, which functions both as a vasoconstrictor as well as an activator of additional platelets.^[414] Pure Δ⁹THC at 30-70mg acutely does not appear to modify platelet serotonin concentration relative to baseline two hours after administration,^[158] as the noncompetitive inhibition of Δ⁹THC on platelet serotonin uptake occurs at very high concentrations (IC₅₀ of 139 µM).^[415] Serotonin release from the platelets of migraine sufferers was also inhibited at similar levels^[416]. An in vitro study found that the inhibition of serotonin release by cannabinoids was not correlated to their ability to inhibit platelet aggregation.^[412] Despite this, male moderate cannabis users (3-7 joints per week for 2-12 years) appeared to have a slightly increased V_max for serotonin uptake in platelets when compared to nonsmoking controls (this effect was not observed in females).^[415]

Intraperitoneal injection of 10mg/kg body weight cannabidiol into Wistar rats did not affect triglyceride levels, although injection of 5mg/kg THC was seen to significantly raise them.^[417]

Although present in rats, the effects of cannabis on triglycerides do not appear to occur in humans. A large longitudinal study examining cannabis usage found a correlation between cannabis use and elevated triglyceride levels in young users, although this correlation disappeared when concurrent alcohol use was taken into account.^[418] A smaller case-control study found no association between cannabis use and triglyceride levels.^[419] An additional study which examined self-reported past or present use of cannabis in adults found no association with triglyceride levels.^[420]

Based on observational human evidence, there seems to be no correlation between cannabis use and triglyceride levels once confounders are taken into account.

Intraperitoneal injection of 10mg/kg body weight cannabidiol into Wistar rats was observed to reduce HDL levels, while a 5mg/kg injection decreased both HDL and total cholesterol.^[417]

Observational studies have failed to note changes in total cholesterol levels among cannabis users,^[418] and either a decrease,^[419] increase,^[421] or no change^[418][420] in HDL-C. No interventional research concerning the effects of active components of cannabis on cholesterol has been performed to date.

When total cholesterol and LDL-C are investigated, similar mixed results are noted, with one study reporting a possible decrease^[421] and another reporting no significant changes.^[418]

Current research on the effects of cannabis on cholesterol levels is inconclusive.

Current usage of cannabis appears to be associated with a reduction in fasting insulin concentrations (16% after adjusting for multiple possible confounders) relative to those who report having never used cannabis.^[420] This reduction is not thought to be due to pancreatic damage since a different case-control study failed to find any association with cannabis usage and β-cell function.^[419]

Although not currently well understood, the mechanisms responsible for cannabis-induced reduction in fasting insulin may occur via attenuation CB1 receptor signaling that could increase adiponectin levels,^[422] resulting in increased insulin sensitivity^[423] and therefore reduced production of insulin.^[420] Consistent with this idea, CB1 receptor knockout mice are resistant to diet induced obesity,^[424] which is suggestive of an important role for this receptor in glucose homeostasis. Moreover, in addition to containing the cannabinoid receptor agonist THC, cannabis also contains cannabidiol, which is a cannabinoid receptor antagonist. Thus, it is plausible that known or yet-to be identified components of cannabis may reduce fasting insulin concentrations via an adiponectin-mediated mechanism that is driven by decreased signaling through the CB1 receptor.

Insulin sensitivity as calculated by the Homeostasis Model Assessment of Insulin Resistance (HOMA-IR) was observed to be higher in current users of cannabis (used at least once in the past 30 days) when compared to controls who never used cannabis. No dose-dependency between cannabis use and insulin sensitivity was noted, however.^[420] This increase in insulin sensitivity was not observed in another study of long-term heavy cannabis users (average of six joints daily over 9.5 years),^[419] although this study used calculations of insulin sensitivity which differed from the previous study. However, the study of chronic users did reveal an increase in adipocyte insulin resistance despite no change in the other measures of insulin sensitivity.^[419]

Observational studies have yielded mixed results concerning cannabis's effect on insulin sensitivity. One study noted that recent use may increase sensitivity, while another study found no effect in long-term heavy users. Thus, the effects of cannabis on insulin sensitivity in different populations are not well-understood.

Data from the National Health and Nutrition Examination Survey (NHANES III) showed that past and current cannabis users appear to have lower concentrations of fasting glucose (92.3-93.1mg/dL) when compared to those who have never used cannabis (97.8mg/dL).^[421] In contrast, a small study examining the acute effects of cannabis on carbohydrate metabolism saw no evidence of hypoglycemia in the fasted for fed state in chronic cannabis users after smoking,^[425] suggesting that the effects of cannabis on blood glucose may manifest over time, rather than acutely.

There are no clinical studies on the effects of cannabis on glycogen. However, some data from animal models and in vitro experiments do exist.

In animal models, acute administration of cannabis extract to rats decreases glycogen stores in the liver,^[426] while repeated exposure to cannabis extract decreases uterine glycogen stores in rats.^[427]

Indirect in vitro evidence using human tissue suggests that the cannabinoid system may affect glycogen levels. AMP-activated protein kinase (AMPK) functions as a cellular energy sensor, inhibits glycogen synthesis in muscle cells, and is involved in a number among other metabolic processes associated with energy homeostasis.^[428] The endocannabinoid system is present in human skeletal muscle and plays a role in oxidative energy metabolism, with CB1 antagonism affecting the expression of several proteins including AMPK. However cellular and molecular control of energy homeostasis is quite complex, requiring a number of other receptors and signaling pathways that also play a role and complicate the picture.^[429] Some of these additional receptors include Transient Receptor Potential channel-Vanilloid sub-family member 1 (TRPV1) and CB2.^[430] Rat studies indicate that CB1 agonism using Δ⁹THC did not affect AMPK activity, however.^[431]

Data from NHANES III suggests that current and past usage of cannabis appears to be associated with a significantly reduced prevalence of elevated HbA1c in serum (3.2-4.2% of users having an HbA1c exceeding 6.0%) when compared to persons who have never used cannabis (8.7% of the sample exceeding an HbA1c of 6.0%).^[421] However, another study using NHANES III data found that, while unadjusted average HbA1C levels were significantly lower in current and past users, this effect disappeared once the data were adjusted for possible confounders.^[420]

A collection of case studies has suggested that cannabis usage may 'mask' ketoacidosis, with less than expected increases in blood acidity relative to ketone concentration and symptoms. This ketonuria was remedied with intravenous insulin.^[432] It has also been noted that cannabis given to obese rats, was associated with weight reduction and an increase in pancreas weight, suggesting that it may have conferred a protective effect to pancreatic beta cells.^[433] This suggests that cannabis may prevent the onset type I diabetes in certain animal models.

In a cross-sectional analysis of NHANES data, it appears that current or past cannabis users were at a lower risk of having type II diabetes when compared to those who never used cannabis (overall OR 0.42 and a 95% CI of 0.33-0.55). In this study past users, (OR 0.44, 95% CI 0.33-0.59), light current users (OR 0.29, 95% CI 0.13-0.65), and heavy current users (OR 0.47, 95% CI 0.22-0.98) were all found to be at lower risk for type II diabetes.^[421] These protective effects occured alongside reductions in serum C-reactive protein, which is suggestive of a mechanism related to inflammation.^[421] Another study also using NHANES data found reductions in fasting insulin (16%) and improved insulin sensitivity (17%) in cannabis users.^[420]

Self-reported usage of cannabis appears to be associated with a significantly reduced risk of developing type II diabetes when compared to those who do not use cannabis.

Both cannabis^[434] and the endogenous cannabinoid anandamide^[435][436] have orexigenic (appetite increasing) effects. It was noted in one study that the appetite-increasing effects of ghrelin, an orexigenic peptide, were blocked by the cannabinoid receptor antagonist rimonabant,^[437] suggesting the some of the appetite increasing effects of cannabis occur via increased ghrelin levels. Moreover, rimonabant also reduced ghrelin secretion, leading to reduced food intake in food deprived rats.^[438]

The hypothalamus is highly enriched in CB1 receptors,^[437][439] and it has been demonstrated in rodents that rimonabant blocks the appetite-stimulating effects of ghrelin when infused directly into this region of the brain.^[437] Thus, the appetite stimulating effects of cannabis appear to occur via increased ghrelin levels that activate CB1 receptors in regions of the hypothalamus important for appetite control.

Cannabis also has been shown to directly increase ghrelin levels in humans. In a prospective subgroup analysis of a trial investigating the effect of smoked cannabis on neuropathic pain in HIV-positive men,^[285] inhalation of cannabis was noted to increase circulating ghrelin by 42.4% (range of 27-59%).^[295]

Inhalation of cannabis increases ghrelin levels, which is partly responsible for its appetite-increasing effects.

Inhalation of cannabis over a week has also been noted to increase circulating levels of leptin to a large degree in one preliminary trial in an HIV-infected population (67.1% relative to 11.7% in placebo).^[295] Considering that cannabis stimulates appetite via increased ghrelin levels, this result was counterintuitive, as leptin normally acts to suppress appetite by upregulating the production anorexigenic (appetite-reducing) neuropeptides^[440][441] and downregulating the production of orexigenic ones.^[442]

It is not clear whether the counterintuitive cannabis-induced increase in leptin levels may be population-specific, or even of functional significance. Given the effects of cannabis on ghrelin, it is possible that leptin levels may increase as part of a negative feedback mechanism. Because calorie intake was not monitored in the HIV study,^[295] it also cannot be ruled out that the increase in leptin levels may have occurred via increased food intake, a known stimulus for leptin production.^[443][444]

One study noted that cannabis increased leptin levels. While large in magnitude, this has not been replicated in other studies. Thus, the reproducibility and practical significance of this result is not clear.

It has been noted that cannabinoids, similar to the peptide ghrelin,^[445][446] can directly suppress lipolysis in adipocytes via CB1 activation resulting in an increase in lipid accumulation. This causes a shift in energy utilization away from fatty acids and towards glucose.^[447] Localized activation of CB1 receptors may also explain the increase in adipocyte insulin sensitivity observed in cannabis users despite no alterations in whole-body insulin sensitivity.^[419] In vitro studies suggest that this phenomenon may indeed be due to CB1 activation.^[448] This effect appears to be dependent on cannabinoid concentration as well as the presence of insulin; in vitro studies have shown that 5μg/mL Δ9THC increases triglyceride accumulation whereas 2.5-10μg/mL Δ9THC in the presence of insulin suppresses triglyceride accumulation and adipogenesis.^[449]

Glucose uptake into adipocytes also appears to be enhanced subsequent to CB1 activation,^[450] despite decreased AMPK activity.^[431] This appears to occur via increased PI3K signaling, leading to enhanced GLUT4 mobilization and glucose uptake.^[450] Notably, PPARγ does not play a role here.^[451] Both insulin-stimulated as well as basal glucose uptake are enhanced by cannabinoids in vitro.^[450][451]

Other studies assessing the anti-lipolytic effects of cannabinoids have found a suppression of peripheral AMPK (mostly in visceral fat tissue)^[431] as well as carnitine palmitoyltransferase 1 (CPT1)^[452][453] in adipose and hepatic tissue despite both being elevated in neuronal tissues.^[431][454]

Activation of the CB1 receptor in fat cells appears to reduce fat oxidation rates, switching energy utilization away from lipids and towards carbohydrates.

Beyond suppressing lipolysis, CB1 activation can also increase lipogenesis via increasing the expression of proteins such as SREBP-1c,^[455][456] fatty acid synthase (FAS),^[455] and lipoprotein lipase (LPL)^[457] while inhibiting adenylyl cyclase.^[458] This has also been noted to increase adipocyte maturation and triglyceride content^[458] due to increased activity of the nuclear receptor PPARγ.^[450][458] Because adipocyte proliferation is abolished when PPARγ is blocked, CB1 activation also appears to have a proliferative effect in adipocytes.^[459]

The endocannabinoid system appears to have a location-specific influence, and during the state of obesity subcutaneous body fat exhibits a relative decrease in endocannabinoid (anandamide and 2-AG) concentrations^{[460][461][462]} whereas visceral body fat exhibits an increase.^[463][464] While it is thought that this difference may play a role in imbalances in a viceral:subcutaneous body fat ratio by favoring deposition of lipids in the former,^[465] studies investigating weight circumference (a proxy measure of visceral fat content) have found both increases^[466] and decreases^[467] in cannabis users relative to controls.

Few studies have looked at cannabis effect on the weight of healthy subjects, and the results are conflicting. Cross-sectional observational studies are lower-quality studies that collect data at a single point in time. One such study associated cannabis with greater fat mass,^[466] but the others associated it with a smaller waist,^[467] lower BMI,^[468][469] and lower prevalence of obesity.^[470] Even a higher-quality prospective observational study found no association between extensive cannabis use and higher BMI.^[471]

Keep in mind, however, that those studies all relied on self-reported use, and that such reports are always, to some extent, unreliable. Studies that used more reliable methods paint a different picture. Three clinical trials (whose participants were largely confined to a controlled hospital environment where food intake is controlled and accounted for) saw increases in body weight.^{[472][473][474]} However, all three trials were relatively short-term and had modest sample sizes.

Cannabis was associated with little or no weight gain in observational studies, but with weight gain in a few higher-quality studies (clinical trials).

Researchers have used endocannabinoid antagonists (compounds that block CB1) to treat obesity caused by compulsive binging or irresistible cravings for sweets and snacks. Rats given the anti-obesity drug rimonabant, an endocannabinoid antagonist, lost weight and experienced a reduction in their blood levels of insulin.^[475] Human trials of the same drug showed weight losses of 2.6–6.3 kg (5.7–13.7 lb) above placebo.^[476] Most of these trials lasted over a year, including one that saw a plateau at 9 months.^[477]

Yet in spite of these successes, rimonabant failed to earn approval from the U.S. Food and Drug Administration (FDA) and was withdrawn from the European market,^[478] due to side effects that include nausea, dizziness, severe depression, and suicidal thoughts.^[479][480]

Since CB1s are found throughout the body, it is difficult to pinpoint the causes and mechanisms of these side effects. Still, in the future, safer endocannabinoid antagonists may play a role in treating obesity by blocking CB1 in order to increase adiponectin production and reduce appetite.

Drugs that block CB1, such as rimonabant, lead to significant weight loss, but the side effects are severe. Still, future studies on endocannabinoid antagonists (i.e., CB1 blockers) might lead to breakthroughs in the fight against obesity.

While many people wish they could lose weight, people with HIV-associated wasting syndrome, cancer-associated cachexia, or anorexia nervosa, notably, are often underweight. Two synthetic THC-based drugs have been developed to address the issue: nabilone (Cesamet®) and dronabinol (Marinol®, Syndros®).

HIV-associated wasting syndrome

This unintentional, steady weight loss associated with HIV can lead to poor health outcomes. Oral cannabinoids (dronabinol) may help increase weight but the results are not entirely conclusive.^[481][482] The various doses used by the different studies may have been lower than optimal.^[483]

Cancer-associated cachexia

The loss of skeletal muscle associated with cancer is called cancer-associated cachexia, or simply cancer wasting.^[484] In two large RCTs, 2.5 mg of dronabinol (synthetic THC) twice daily increased body weight, but not enough to improve health.^[485][486] This 5 mg daily dose, like the doses used in the HIV-wasting studies, may have been too small to elicit a beneficial effect.

Anorexia nervosa

Two small, short RCTs have investigated THC as a treatment option for anorexia nervosa.^[487][488] People in the intervention groups received 2.5 mg of dronabinol twice daily for 4 weeks. People in the intervention groups gained, on average, 0.70 kg^[488] and 0.73 kg^[487] (1.54 lb and 1.6 lb) more than people in the placebo groups. This increase is small, but it may still benefit this population.

THC, natural or synthetic, may increase body weight in people with HIV-associated wasting syndrome. In people with anorexia nervosa or cancer-associated cachexia, it might have the same effect, but the evidence is weaker.

The inhibitory effect of cannabis on motor control may be attenuated in heavy users (near daily), as a dose of 800mg cannabis (3.7% Δ⁹THC) is insufficient to impair psychomotor performance^[236] while 5.5-6.4% Δ⁹THC of the same amount of cannabis is.^[296]

Acute inhalation of cannabis (1.4g conferring 18.2mg Δ9THC) in self-reported cannabis smokers was unable to influence power output as assessed by a grip strength test.^[370]

Cannabis use is prevalent in athletes, with upwards of 10% of Olympic athletes reporting usage prior to its status as a prohibited substance by the IOC (1998-2003) with it being the second most used prohibited substance in the Olympics behind anabolic agents.^[23] While its use is not seen as physically ergogenic, it is thought to be used primarily due to its psychoactive effects of relieving anxiety and tension.^[23][489] Due to its impairment to learning also applying to adversive learning (conditioned fear^[490][491]) it is thought that cannabis is also used to reduce fear associated with competition.^[23][489] This function, to some,^[492] has been described as ergogenic since the end result is an increase in performance.

In regards to exercise (both aerobic and anaerobic) cannabis does not appear to have inherhent ergogenic effects. Cannabis use in athletes may help to reduce fear and anxiety with competing, however, leading some to consider it 'performance-enhancing'.

Inhalation of cannabis (20-25 tokes of 1.4g cannabis, conferring 18.2mg Δ⁹THC) has been noted to acutely increase heart rate and diastolic blood pressure at rest which resulted in an acute 25% reduction in steady state performance on a bicycle ergometer (maintaining cycling at 170 BPM);^[370] smoking the placebo, relative to the control, did not influence performance.

Interleukin 6 (IL-6) is an inflammatory and immunostimulatory cytokine produced by the immune system which normally increases with age.^[493] Self-reported usage of cannabis in middle-aged African-Americans appears to be associated with lower resting IL-6 concentrations when compared to similarly-aged nonsmoking cohorts (58% of control value).^[494] Although this cohort was on average obese (another possible reason for elevated IL-6^[495]) and their BMI was also correlated with IL-6 in serum, IL-6 levels were still lower in the cannabis users even after adjustment for these and other social and physical factors.^[494]

The cannabinoid system appears to affect natural killer cells (NK cells), as the endocannabinoid 2-arachidonoylglycerol (2-AG) which is involved in immunological signalling^[496] has been noted to induce the migration of NK cells partially via CB2 receptors;^[497] when Δ⁹THC was tested, it not only failed to induce migration of NK cells at 1µM but blocked 2-AG from doing so at the same concentration.^[497] While not specifically demonstrated, these differences may be due to differences in binding strength to CB2, since Δ⁹THC and anandamide, which also failed, are partial CB2 receptor agonists while 2-AG is a full agonist.^[496][498]

In contrast to the inability of Δ⁹THC to induce cell migration, intravenous administration of 2.5mg/kg cannabidiol (CBD) for two weeks appears to increase the total count and percentage of NK cells in rats despite reductions in other lymphocytes (B and T cells).^[499]

While the cannabinoid system is involved in natural killer cell migration, Δ⁹THC does not induce cell migration at moderate concentrations.

Usage of bhang in youth (around 1.5-3g daily) appears to be associated with a reduced amount of natural killer cells in serum (26%) relative to youth who did not report usage of cigarettes or cannabis; this association was noted in those using for 6-24 months yet was not present in those who reported usage for longer periods.^[500]

One study assessing an association between immunological parameters and Cannabis sativa usage (as bhang) noted a reduction in NK cell count associated with bhang.

B Lymphocytes (B cells) express CB2 receptors to a degree greater than NK cells, macrophages, neutrophils, and T cells (descending order of receptor abundance^[501][502]) and the mRNA for this receptor appears to be responsive to various cytokines in vitro with lipopolysaccharide potentially suppressing CB2 mRNA^[502][503] and stimulation of STAT6 via IL-4 increasing CB2 receptor expression.^[504]

Activation of this receptor in B cells increases differentiation,^[505] migration,^[506] and activation^[507] with at least one study also implicating this receptor in antibody class switching since incubation of B cells with cannabinoid agonists can increase immunoglobulin E (IgE) at the cost of IgM secretion in a manner blocked by CB2 antagonists.^[508] An increase in IgE without allergic reaction has been noted in a few cases given cannabis inhalation.^[509]

When tested in vitro, Δ⁹THC caused a dose-dependent increase in B cell proliferation with an EC₅₀ of 2nM when in the presence of costimulatory agents depsite anandamide being ineffective up to 1µM.^[510]

The antiinflammatory cannabinoid receptor (CB2) appears to be expressed to a relatively large degree on the particular kind of white blood cell known as B cells, which are involved in adaptive immunity. When this receptor is activated, B cells appear to generally be activated and secretion of IgE may be preferred over other immunoglobulins.

When investigating chronic cannabis smokers, baseline B cell count appeared to be lower than nonsmoking controls^[509] (lower baseline B cell count has also been noted in chronic bhang users relative to nonusers^[500]) but this was normalized over the course of 64 days with cannabis usage in hospitalized settings.^[509] Trends for the initial reduction^[500][509]and restoration to baseline levels with cannabis usage^[509] have also been noted with T cells.

There are no controlled interventions comparing the effects of cannabis against placebo in regard to B cell mediated immunity, but it seems that chronic users have lower baseline B cell count than nonusers. Reductions in B cell levels may not persist with longer term use, however.

Many case studies have reported that heavy cannabis usage often precedes the development of gynecomastia (breast tissue growth in males) suggesting that cannabis may have intrinsic estrogenic properties that may disrupt normal hormonal balance in males.^[511][512] Some in vitro studies corroborate this; both an ethanolic Cannabis sativa extract and cannabis smoke condensate with a THC content equivalent to 24µM competes with estradiol for binding to the estrogen receptor.^[513] This may not be due to THC, however; neither pure Δ⁹THC nor ten of its metabolites were noted to have any estrogenic activity, and cannabidiol showed estrogenic activity only at very high concentrations.^[513] Moreover, cannabis smoke condensate and several components of cannabis showed neither estrogenic nor anti-estrogenic activity in an in vivo animal uterine tissue model, bringing the biological relevance of in vitro studies into question.^[513] Failure of phytocannabinoids from Cannabis sativa to either stimulate or suppress the estrogen receptor of breast cancer cells in culture has been noted elsewhere, further questioning the purported estrogenic activity of cannabis.^[514]

A more recent study using in vitro as well as in vivo methods found that cannabis smoke condensate has an estrogenic effect that can be traced to phenolic compounds generated upon the combustion of plant materials. Thus, cannabis smoke may have intrinsic estrogenic properties that occur via estrogenic polyphenols, rather than cannabinoids as previously assumed.^[515]

It has also been noted that Apigenin in Cannabis sativa is an estrogen antagonist at 500-5,000nM^[513] while both formononetin^[514] and 4,4,dihydroxy-5-methoxybibenzyl from Cannabis sativa^[516] are agonists.

Human case studies suggest that cannabis possesses estrogenic activity. If an estrogenic effect does exist, this is likely attributed to combustion products from natural polyphenols found in cannabis, rather than cannabinoids.

In a study on rats fed Cannabis sativa as aqueous Bhang solution (at 6mg/mL), 36 days of administration decreased testosterone by almost half at a dose of 3mg/kg bodyweight, with further decreases noted at 6mg/kg bodyweight.^[2] This decrease in testosterone was thought to be due to inhibitory effects on 3β-Hydroxysteroid Dehydrogenase (3βHSD), the final enzyme in testosterone synthesis.^[2] Another study found that THC can inhibit gonadotropin-induced testosterone synthesis even in abundance of gonadotropins, suggesting inhibition at the level of testicular tissue.^[517]

Another study also found that while cannabinoids do not prevent gonadotropins from binding to receptors, they still lower testosterone by inhibiting cholesterol esterase, an enzyme needed for testosterone synthesis.^[518] These effects are also seen with cannabidiol and cannabinol, and are more effective than THC.^[519]

In research animals and animal tissue, several cannabinoid components of Cannabis sativa such as THC can suppress testosterone levels.

In humans, infusion of 10mg THC over 50 minutes (as 0.02% solution) showed a time-dependent decrease in testosterone over a 6-hour interval. Whereas the control group fluctuated at around 5.5+/-0.5ng/mL testosterone, the THC group dropped to around 3.5+/-0.5ng/mL at 4-6 hours despite THC concentrations in the blood disappearing by 1 hour post-test.^[520] A decrease in testeosterone was also noted after subjects smoked a cannabis cigarette, where testosterone levels appeared to reach about 66% of baseline values after 3 hours (time after not recorded).^[520] Other studies suggest nonsignificant reductions in testosterone levels after 1-2 2.8% THC joints,^[521] including a slight (8%) transient decreased in testosterone after 20mg THC as a joint.^[522] Both of these studies controlled for cannabis use prior to the study, whereas aforementioned studies that noted larger testosterone decreases did not.^[520] Interestingly, a study using isolated THC did not find any effect on testosterone level, suggesting that other compounds in the Cannabis sativa plant may be affecting testosterone levels.^[523]

Two studies found that chronic users of cannabis did not display significantly different baseline levels of testosterone (either gender, tested under non-smoking conditions) up to daily smoking sessions, or 7 joints weekly.^[524][525] However, another study noted depressed testosterone levels in men who used cannabis at least 4 days a week for at least 6 months compared to age-matched controls that did not smoke cannabis.^[526]

It has been noted^[527] that all human studies showing decreases are still within the normal biological range, suggesting that cannabis use is unlikely to influence behavior secondary to testosterone.

Human studies the effects of cannabis on testosterone levels have yielded mixed results. Smoking cannabis probably causes an acute decrease in circulating testosterone levels, but the effect of chronic, heavier use on testosterone is much less clear. The clinical relevance of depressed testosterone levels is not clear; although most studies indicate that although circulating hormone levels may drop, they remain within the normal range.

Another possible mechnanism by which testosterone is depressed is through reducing hypothalamic and pituitary output of gonadotropin hormones, as administration of hCG (Human chorionic gonadotropin) in one study that noted cannabis-induced testosterone suppression also noted that the admistrration of hCG reversed it.^[526]

Other possible mechanisms of testosterone suppression include decreased testosterone synthesis in the testes (extrapolated from mouse studies)^[2], increased liver conjugation and metabolism of testosterone,^[528] or direct antagonism at the level of the androgen receptor,^[529][530] with Δ⁹THC possessing the ability to prevent DHT from binding to the androgen receptor. ^[531] The last possible mechanism was noted in in vivo animal studies, as castrated rats still experience anti-androgenic effects from THC and this effect is independent of circulating testosterone levels.^[530] Many of the testosterone-related effects may be due to cannabis's action on the pituitary gland, as endocannabinoids cannot suppress testosterone in rats lacking a CB1 receptor.^[532]

The possible mechanisms by which cannabis can suppress testosterone synthesis are quite numerous, but are mostly due to decreased synthesis of testosterone in the testicles.

Cannabis use may suppress testosterone levels for up to 48 hours, as based on a mathematical simulation.^[520] Most time curves, however, indicate maximal suppression of testosterone 4-6 hours after consumption, which is well after THC has been cleared from circulation.^{[520][520][521]}

Acute smoking of two 2.8% joints is associated with a slight, statistically insignificant increase in circulating growth hormone levels 2 hours after smoking in men, from about 1ng/mL to 2ng/mL when compared to control (smoking placebo).^[521] Abnormally high oral doses (210mg) of Δ⁹THC have been noted to acutely suppress circulating growth hormone concentrations.^[533]

Cannabis smoking causes an acute reduction Luteinizing Hormone (LH) levels in males. ^[521] Chronic use is not associated with depressed LH levels, although moderate use (5-6 tokes weekly) is associated with a nonsignificant increase in baseline LH levels.^[525]

Chronic usage of cannabis, when tested under non-smoking conditions, is not associated with significant changes in follicle-stimulating hormone levels in men or women.^[525]

One study found that inhalation of cannabis smoke (from 1-2 cigarettes containing 2.8% Δ⁹THC) acutely increased cortisol alongside the psychoactive effects in healthy men.^[521] This effect has been confirmed elsewhere to occur with intravenous Δ⁹THC in naive users in a dose-dependent manner, but is subject to tolerance as chronic users do not appear to have increased cortisol at similar doses.^[534]

The magnitude of change noted with doses of cannabis corresponding to recreational use is not thought to be of a clinically relevant magnitude,^[535] and there is no alteration in the diurnal rhythm of cortisol when comparing chronic users of cannabis to nonusers.^[523]

Very high doses (210mg) Δ⁹THC can reduce the cortisol response to low blood glucose in hospitalized patients.^[533] A plausible mechanism for this effect is via suppressed ACTH activity, which has been observed in rats with the CB1 agonist HU-120 at high doses. In contrast, lower doses have been found to have a mild stimulatory effect, suggesting that this mechanism may be subject to tolerance subchronically.^[536]

Inhalation of cannabis seems to promote an acute spike in cortisol, which is attenuated with chronic use. The overall size of the cortisol spike noted with normal use does not appear to be of a clinically relevant magnitude.

The cannabinoid system plays several roles in the regulation and pathophysiology of the intestines, and cannabis and its constituents could theoretically play a role in aiding intestinal disorders by direct suppression of proinflammatory mediators, inhibition of intestinal motility and diarrhea, and attenuation of visceral sensitivity.^[537] Specifically, the CB1 receptor plays a role in decreasing gut motility when activated,^[538] and can also reduce gastrointestinal inflammation in animal models,^[539] while CB2 activation reduces intestinal inflammation and may also play a minor role in gut motility, while both may be involved in disorders involving intestinal pain.^[540]

The effects of cannabis on intestinal disease in humans is limited but suggests that cannabis may be effective in reducing the symptoms of inflammatory bowel disease.^[541]

The cannabinoid system plays several roles in the intestines, and limited human evidence suggests that cannabis may be helpful in inflammatory bowel diseases.

Cannabis usage is thought to be a contributing factor in the development of nonalcoholic fatty liver disease (NAFLD)^[452] as activation of the CB1 receptor in the liver (hepatic tissue) seems to promote lipogenesis in the liver^[455] and CB2 receptors appear to be expressed in the NAFLD-affected liver but not healthy livers.^[542]

Mechanistically, when CB1 is activated in vitro the expression of the lipogenic factor SREBP-1c and the target enzymes ACC1 and fatty acid synthase (FAS) are increased^[455] and injections of CB1 agonists causes lipogenesis in the liver of mice correlating with subsequent weight gain.^[455]

Inhalation of cannabis has been noted to account for some instances of acute pancreatitis, albeit less than 2% of what is seen in emergency settings (as a more common cause is alcohol).^[543] While it is not certain that cannabis was the sole causal agent, it is known that the CB1 receptor exacerbates preexisting pancreatitis and blocking this receptor can prolong survival in rats.^[544]

Cannabis is associated with a few instances of pancreatitis (acute inflammation) causing pain. Causation has not been placed on cannabis for this, but it is possible that cannabis could have a role, as the cannabinoid system has been seen to play a role in animal models of pancreatitis.

In rats exposed to a standard Western diet, injections of cannabis (equivalent to 5mg/kg Δ⁹THC) increased ab libitum food intake in lean rats, yet not in obese rats, and reduced weight in both groups;^[433] this observation was noted alongside protection of pancreatic β-cell function.^[433] This study did use a cannabis extract containing cannabidiol, which independently (at 5mg/kg i.p.) can reduce the incidence of diabetes in rats at the level of the pancreas.^[545]

In diabetic mice (streptozotocin-induced) with pancreatic inflammation, Δ⁹THC given alongside the inflammatory stressor at 150mg/kg appeared to partially prevent inflammation of the pancreas resulting in preserved insulin content of this organ and less of an increase in blood glucose.^[546]

Limited animal evidence assessing the effects of cannabis on the pancreas varies; while there may be a protective effect with cannabis usage chronically, an acute protective effect seen with Δ⁹THC requires a very high dose.

Cannabis has effects on the lungs secondary to both the plant constituents but also due to the act of inhalation per se, since any combustable organic material (regardless of its constituents) inhaled may have a damaging effect on lung tissue.^[547]

The act of chronic cannabis inhalation has been associated with acute and chronic bronchitis as well as cases of cellular dysplasia, which are thought to be related to volatiles produced during combustion similar to tobacco-containing cigarettes.^[547]

The smoke from cannabis has similar properties to tobacco smoke in that they both create harmful byproducts due to organic material combustion, which can damage lung tissue and possibly has a carcinogenic role.

In self-reported heavy smokers (four times a week minimum) while one study failed to note any abnormalities in pulmonary function or lung power when tested^[548] an assessment of cannabis-dependent adults noted that the percentage of subjects with a FEV1/FVC less than 80% was higher in users (36%) than nonusers (20%).^[549]

The state of cannabis tolerance (assessed by surveying dependent adults) is also associated with higher baseline sputum production in the morning, waking during the night with chest pains, wheezing (outside of sickness), and exercise-induced shortness of breath were also increased in frequency relative to nonsmokers;^[549] these effects were also noted in tobacco users, and hypothesized to be related to the inhalation of smoke per se.^[549]

However, a 20-year follow-up as part of Coronary Artery Risk Development in Young Adults (CARDIA) study found that long-term cannabis use at low, common levels had a slight postive effect on pulmonary function measures; heavier users had an FEV1 measure no different from baseline, while FVC remained slightly improved even in heavy users, although data for very heavy users was sparse.^[550]

The effects of cannabis smoking on lung function are mixed; while some observational studies have found an effect on lung function and sputum production, a longer-term study of baseline lung power in cannabis users, relative to nonusers, found little long-term effect.

Cannabis has a bronchodilation effect, and is able to increase FEV₁ and airway conductance when inhaled by otherwise healthy persons acutely^[369][551] with actions present within 20 minutes and persisting for up to an hour;^[551] this appears to be due to the Δ⁹THC content, since oral supplementation is also effective, albeit slower-acting,^[551] and inhalation of placebo or cigarettes acutely impair airway conductance.^[551]

The bronchodilation seen with cannabis does not appear to be additive with exercise when inhalted at 7mg/kg (1.7% Δ⁹THC).^[369]

Acute inhalation of cannabis appears to have a slight bronchodilating effect, and is significantly more effective than oral supplementation of Δ⁹THC due to the direct application to the airway via inhalation. This effect lasts for approximately one hour.

Glutamate-mediated cytotoxicity plays an important role in the pathophysiology of retinal damage,^[552] and both cannabidiol and Δ⁹THC have been shown in vitro to exert neuroprotective effects via this pathway.^{[553][554][555]}

Additionally, components of cannabis affect intraocular pressure; when given to subjects with glaucoma, sublingual (5mg Δ⁹THC),^[556] intravenous Δ⁹THC,^[557] and inhalation^[408] of Δ⁹THC via cannabis has been shown to decrease IOP more than placebo. The decrease in IOP seen with cannabis peaks 60-90 minutes after inhalation^[408] although a decrease may occur before 30 minutes,^[558] with these changes parallelling a decrease in peripheral blood pressure at a concenration which coincides with the psychoactive effects of cannabis.^[408] This effect may be subject to tolerance, since a low dose of Δ⁹THC (12mg) has once shown efficacy in only those naive to cannabis usage.^[558]

The reduction in IOP has been noted in young adults who do not suffer from glaucoma as well.^[558]

Cannabis appears to be able to reduce intraocular pressure (IOP) in subjects with glaucoma associated with the reduction in peripheral blood pressure, and the dose required for this is similar to the one required to have a psychoactive effect. This decrease in IOP may be subject to tolerance.

The testicles are known to express both CB1 and CB2 receptors in mice^[2] and also express the fatty acid hydrolase (FAAH) enzyme of cannabinoid metabolism.^[2]

Bhang (leaf/flower extract of Cannabis sativa with over 25% Δ⁹THC content) orally at 3-6mg/kg daily in mice for 36 days has been noted to reduce testicular weight in a dose-dependent manner by 14-27%. This was associated with a reduction in 3βHSD and FAAH activity alongside reduced serum testosterone, thought to be related to testicular toxicity as assessed by histology.^[2]

The cannabinoid system in cancer cells differs from normal cells; cancer cells often overexpress cannabinoid receptors, and the level of these receptors on cancer cells correlates with tumor agressiveness, which suggests that the cannabinoid system plays some role in cancer development.^[559]

One mechanism by which cannabinoids may inhibit cancer cell growth is via inducing cell death through stimulation of the cannabinoid receptors on these cells, which stimulates the production of ceramide, in turn upregulating the stress-regulated protein known as p8, which ultimately leads to apoptosis.^[560] In addition to inducing apoptosis, Δ⁹THC has been observed to slow tumor growth by cell cycle arrest in both human prostate cancer cells in vitro through binding to both CB1 and CB2^[561] and in human breast cancer cells through a mechanism primarily dependent on CB2 stimulation.^[562] Similar reductions in VEGF, along with decreased tumor vascularization, have been seen in skin cancer models as well.^[563]

An additional effect of cannabinoids seems to be to reduce the blood supply of tumors. Cannabinoid-induced, ceramide-dependent downregulation of vascular endotherlial growth factor (VEGF) and reduction in the stimulation of its receptor has been noted in gliomas.^[564]

A third mechanism by which cannabinoids may inhibit cancer growth is by limiting its spread. This effect has been seen with Δ⁹THC in glioma cells through downregulating cancer cells' production of matrix metalloproteases (MMPs) in a ceramide-dependent fashion.^[565] In addition, Δ⁹THC has been shown to increase the expression of tissue inhibitors of MMPs in cervical cancer cells, leading to decreased invasiveness and migration in vitro.^[566] The reduction of cells' invasiveness has been verified in in vivo models of lung cancer, where Δ⁹THC inhibited metastasis in mice^[567] synthetic cannabinoids produced similar effects in vivo for models of lung^[567] and breast^[568] cancer.

Cannabis may inhibit the growth and spread of cancers through several mechanisms: inducing the cells to die, reducing tumors' blood supply, and slowing their spread.

An in vitro study of glioblastoma cells indicates that Δ⁹THC at physiologically relevant concentrations may accelerate cancer cell proliferation by transactivation of the epidermal growth factor receptor (EGFR) leading to activation of MAPK/Erk and Akt/PKB pathways.^[569]

A pilot study which administered Δ⁹THC directly into the tumor of 9 patients with glioblastoma multiforme, a severe form of brain tumor, who failed standard therapy found that administration was safe and no overt psychoactive effects were seen.^[570] Since there was no control group, and since the trial was primarily designed to assess safety of Δ⁹THC adminstration via this route, it cannot be said for certain whether the administration of Δ⁹THC increased survival time; the median survival time was found to be 24 weeks (95% CI: 15–33 weeks), which the authors of this study note is similar to the current benchmark treatment for malignant gliomas, temozolomide.^[570] In vitro studies from biopsies of these tumors showed that Δ⁹THC inhibited tumor cell proliferation.^[570]

An uncontrolled pilot study in humans found that Δ⁹THC directly injected into a severe kind of brain tumor was safe, with no overt psychoactive effects. The study was uncontrolled, so the effectiveness of this treatment cannot be assessed from this study, but the median survival time of the patients was 24 weeks. No studies on the effects on brain cancer of oral Δ⁹THC or cannabis have been performed to date.

In vivo studies of a mouse model of HER2-positive breast cancer suggests that Δ⁹THC at a dose of 0.5mg/day may an effective anti-tumor agent in this subtype, as the treatment reduced both tumor growth and tumor number, as well as decreased the number of blood vessels in the tumors compared to control; this effect seemed to be mediated by the CB2 receptors, as the CB2-specific agonist known as JWH-133 induced similar changes.^[571] In addition, cannabidiol, when administered via intratumor injection at a dose of 5-6.5mg/kg, reduced tumor growth a highly-metastatic human triple-negative (does not express HER2, estrogen receptor, or progesterone receptor) breast cancer injected into immune-deficient mice.^[572]

However, a study using a mouse mammary carcinoma cell line that naturally does not express cannabinoid receptors showed that near-physiological levels of Δ⁹THC increased the rate of tumor growth and metastasis in vivo by suppressing the immune Th1 response against the tumors via increased IL-4 and IL-10 production.^[573] Since some human cancers also do not express cannabinoid receptors, the researchers speculate that Δ⁹THC may promote tumor growth in such cancers by also suppressing the host's natural immune response against these tumors while having no direct suppression of the tumors due to lack of cannabinoid receptors.^[573]

Mouse data is mixed. Certain types of breast cancer responds well to treatment with Δ⁹THC and cannabis. However, other studies suggest that Δ⁹THC can suppress the host's anti-tumor immune response, which can lead to the exacerbation of some breast cancers that do not express cannabinoid receptors.

An early study found that many components of cannabis, including Δ⁹THC and cannabinol (but not cannabidiol) were found to inhibit the growth of lung adenocarcinoma both in vitro and in vivo in mice via oral adminstration.^[574] A later in vitro study of a lung carcinoma cell line found that Δ⁹THC at physiologically relevant (nanomolar) concentrations may accelerate cancer cell proliferation, which differs from other in vitro studies which examined the effects of Δ⁹THC at less physiologically relevant micromolar concentrations.^[569]

In vitro studies of non-small cell lung cancer (NSCLC) indicates that Δ⁹THC inhibits EGF-induced migration and growth of these cells in a manner dependent upon the EGF-induced phosphorylation of AKT and MAP kinases (specifically, ERK1/2 and JNK1/2).^[567] These studies were confirmed in an in vivo mouse model using severe combined immunodeficient (SCID) mice which were injected with cancerous cells; in this mouse model, Δ⁹THC inhibited tumor cell growth as well as metastasis.^[567]

In vitro and mouse model studies indicate that Δ⁹THC may inhibit non-small cell lung cancer growth and metastasis, although no data exists as to whether this effect extends to humans or extends to the whole cannabis plant.

When assessing rates of cannabis usage and lung cancer incidence (survey research) and investigating research that exludes tobacco usage, there was no increased risk for habitual (frequent) users relative to either nonhabitual (infrequent) or nonusers in regards to developing lung cancer.^[575] This has been noted elsewhere, where no association was noted after tobacco was controlled for,^[576][577] although increased biomarkers that would suggest lung damage (tar exposure, alveolar macrophage dysfunction, etc.) were noted^[577] likely due to the inhalation of smoke per se.

A lack of association between cannabis usage and development of adenocarcinoma has been noted with cannabis usage despite a positive (nonsignificant) trend^[575] although there may be some premalignant changes in the respiratory tract.^[577]

Cannabis usage, either light or heavy, does not appear to be associated with an increased incidence of lung cancer when compared to nonusers after tobacco has been controlled for. There may still be some procarcinogenic effects due to inhaled smoke which require further study as it applies to cannabis, as smoke is known to be carcinogenic.

A case-control study among head and neck cancer patients found no statistically significant association between cannabis smoking and head or neck cancer, even when restricting the analysis to those who never smoked tobacco and those who both never smoked tobacco and never drank alcohol (both being additional risk factors for some head and neck cancers).^[578]

The healthy prostate normally expresses both CB1 and CB2 receptors, as well as some TRPV/TRPA channels which cannabis constituents are also known to target.^[579][580] The expression of the cannabinoid receptors in cancerous prostate cells such as LNCaP, PC3, and DU145 appears to be higher than in normal prostate cells (PZ-HPV-7 and PrEC,^[581] and activating these receptors in cancerous cells appears to cause a dose- and time-dependent reduction in cell viability and increases apoptosis in a manner blocked by receptor antagonists.^[581] Higher CB1 immunoreactivity seems to be associated with worse prognosis in prostate cancer,^[582] although this is thought to be a consequence of rather than a cause of the worsening state.^[582][583]

Δ⁹THC may be an androgen receptor antagonist, antagonizing DHT binding with a dissociation constant of 210nM,^[531] and is active in prostate tissue of the rat where a week-long administration of 10mg/kg hindered the effects of testosterone injections on DNA synthesis in this tissue,^[530] which has been noted elsewhere.^[529] Due to the therapeutic role inhibiting DHT signalling through the androgen receptor plays in prostate cancer,^[584] it is thought that this property of cannabis could be beneficial.^[583]

It should also be noted that, in cancerous prostate cells, activation of cannabinoid receptors decreases the protein expression of the androgen receptor and decreases the production and secretion of PSA^[581] suggesting dual effects on the androgen receptor.

Cannabinoid receptors appear to be expressed on prostate cells, and their expression increases during cancer and seems to be correlated with the severity of the cancer prognosis. Activating the cannabinoid receptor can cause apoptosis and decreases androgen receptor content, and, due to Δ⁹THC also blocking DHT binding to the androgen receptor, it could have a therapeutic role in prostate cancer, although this has not been explored experimentally as of yet.

Other components of cannabis beyond Δ⁹THC may have anticancer effects at the level of the prostate,^[585] and in one study assessing 12 cannabinoids found in cannabis it was found that cannabidiol (CBD) was the most potent inhibitor of prostate cancer cell growth, and cannabis extracts (varying between 24.1-67.5% pure compound) tended to be similar potency in vitro; depleting the extract of cannabinoids eliminates efficacy.^[579] These cannabinoids were additive with the chemotherapeutics docetaxel and bicalutamide in vitro.^[579]

1-100mg/kg injections of a CBD-rich cannabis extract in vivo in mice appeared to inhibit the growth of LNCaP (androgen-dependent) cells in a manner equipotent to 5mg/kg docetaxel, and while inherently ineffective in DU-145 (androgen independent) cells, it augmented docetaxel's growth-inhibitory effects.^[579] These inhibitory effects are associated with increased p53 expression and increased ROS production leading to apoptosis^[579] and the estrogen receptor known as G-protein coupled estrogen receptor 1 (GPER) significantly hindered these effects.^[579]

Other cannabinoids in cannabis appear to have a role in promoting apoptosis of prostate cancer cells in vitro.

A case-control study of cases of testicular germ cell tumor patients age 18-35 at time of diagnosis matched with controls by age, race, and neighborhood found almost a doubling of risk in those who self-reported ever used cannabis compared to those who never did (OR=1.94, 95% CI 1.02–3.68), although further analysis revealed no simple dose-response relationship; further analysis found that nonseminoma and mixed histology tumors were the main contributors to the increased risk.^[586]

In a large cohort study of older men (aged 45-69), cannabis use was correlated with a lower risk of bladder cancer (hazard ratio of 0.55, 95% CI 0.31-1.00 compared to those who never used it and never used tobacco) after adjustment for age, race and BMI.^[587] A clear dose-response relationship was not found, however, although the study may have been underpowered to detect it; when cannabis users were stratified into number of times of use over their lifetime, only those who used it 3-10 times had a statistically significant lower hazard ratio.^[587] Among those who used used tobacco, cannabis users also had a lower risk of bladder cancer compared to those who never used it, but the difference between the two groups was not statistically significant.^[587]

A standardized high-cannabidiol (65.9% w/w of the extract) cannabis extract has been shown to reduce colorectal carcinoma profiferation in vitro at concentrations of 3-5μM in a manner dependent on CB1 and CB2 without affecting normal cell growth; a xenograft mouse model of colon cancer also showed a reduction in tumor growth with administration of 5mg/kg of the extract via intraperitoneal injection over the course of 7 days.^[588]

Several mouse tumor types of immune origin are susceptible to Δ⁹THC in vitro at concentrations of 1-20μM, which reduced cancer cell viability and increased apoptosis in these cells.^[589] Δ⁹THC was also effective in vivo in mice with lymphoma given intraperitoneal injections at a concentration of 5mg/kg, reducing tumor load as well as increasing survival rates.^[589]

Cannabis has been investigated for its usage in treating pain associated with chemotherapy,^[590][591] although currently trials tend to be of relatively low quality (based on a GRADE approach) and are mostly conducted in chronic pain in general.^[592] In an assessment of 18 trials (four scoring a 4 or above on the Jadad scale) totaling 809 patients, it was noted that usage of cannabis or oral Δ⁹THC was associated with a significant reduction in chronic pain intensity with a standardized mean difference (SMD) of -0.61 (95% CI of -0.84 to -0.37) alongside psychoactive effects.^[591]

While cannabis is at times used for reducing pain associated with chemotherapy, there does not appear to be much evidence specific for this usage. There is evidence for the benefits of cannabis in chronic pain in general, which may extend to pain during chemotherapy.

Cannabis is also reported to be used as adjuvant in chemotherapy for its appetite-stimulating effects, since weight loss from reduced food intake may worsen the prognosis of some cancers, and any attempt to circumvent weight loss is seen as protective,^[590][593] although the clinical evidence supporting this specific use is relatively sparse.^[590]

The appetite-stimulating properties of cannabis may also be of benefit to cancer therapy as an adjuvant, since preventing weight loss during chemotherapy is a therapeutic goal. While some evidence exists to support cannabis's adjuvant use, more is needed.

The endocannabinoid system is involved in food and appetite regulation,^[594] and has been shown to be involved in increasing lifespan under dietary restriction in a nematode model by reducing endogenous N-acylethanolamines (NAEs) (signalling molecules that activate the endocannabinoid system).^[595] Artificially increasing levels of the enzyme Fatty Acid Amide Hydrolase (FAAH), an enzyme which breaks down NAEs, mimics the life extension effects of dietary restriction, while supplementing a particular NAE (eicosapentaenoyl ethanolamide) abolishes these effects, lending further support to the idea that NAEs play a role in longevity in nematodes.^[595]

In transgenic mice possessing a mutated superoxide dismutase gene to model amyotrophic lateral sclerosis (ALS), it has been noted that abolishing the FAAH enzyme fails to modify lifespan despite an increase in anandamide, however knocking out the CB1 receptor promotes increased lifespan.^[596]

Dietary restriction has been shown in a worm model to increase lifespan, and the endocannabinoid system seems to play a role here. The endocannabinoid system also plays a role for longevity in a mouse model of ALS. No studies to date have examined the relevance of these animal studies in humans.

Oral ingestion of 3-6mg/kg bodyweight liquid Bhang (aqueous Cannabis Sativa) is able to dose-dependently decrease testicle weight in rats over 36 days, by 17% at 3mg/kg and 33% at 6mg/kg.^[2]

Histologically, an absence of spermatozoa has been seen after oral administration of 3-6mg/kg Bhang ingestion in up to 40% of tubules observed.^[2] Spermatozoa apoptosis (sperm death) appeared to increase as observed via staining.^[2]

Alzheimer's disease is a neurodegenerative disease which, on a biochemical level, is characterized by an accumulation of plaques composed mainly of β-amyloid and 'tangles' usually comprised of a protein known as tau.^[597] Neuroinflammation around these plaques and tangles is associated with neuronal damage,^[598] with a critical intermediate in the process being inflammatory activation of neuroglial cells.^[599][600] The cannabinoid system, particularly CB2 (the receptor that mediates antiinflammation but not psychoactive effects), appears to be involved in this pathology since a deficiency of this receptor aggravates amyloid pathology^[601] and activation of the system reverses pathology.^[602]

The CB2 receptor is expressed in various immune cells^[603] including microglia^[604] The gene which expresses the CB2 receptor, CNR2, it may have its transcription upregulated during Alzheimer's disease relative to control,^[605] which is correlated with cognitive impairment^[606]) resulting in more CB2 receptors (demonstrated in vivo in humans^[607]) alongside a reduction in endogenous cannabinoid activity, thought to be due to upregulation of Fatty Acid Amide Hydrolase (FAAH), which is an enzyme that breaks down endogenous cannabinoids such as anandaminde.^[608]

There appears to be dysregulation of the endocannabinoid system in Alzheimer's disease. One facet of this is a reduction in endocannabinoid activity, at least in part due to the upregulation of FAAH, an enzyme that breaks them down. Additionally, CB2 receptors are overexpressed in brain tissue afflicted with Alzheimer's disease.

It is known that Aβ_1-40 (amyloid beta) fibrils activate microglial immune cells localized near neurons^[609][610] by acting upon surface receptors such as scavenger receptor A, CD36, α6β1 integrin, CD47, and TLRs.^[611][612] This overall process is blocked when CB2 is activated in vitro.^[613]

In rats injected with Aβ_1-40 fibrils to mimic Alzheimer's pathology, activation of CB2 (via the synthetic cannabinoid MDA7^[614]) is able to near-fully preserve memory relative to control, which is thought to be due to reduced microglial activation in the hippocampal CA area.^[602] It was also observed that CB2 activation promoted Aβ_1-40 clearance in the hippocampal CA1 area as well. ^[602] Treatment with MDA7 also prevented abnormalities in this region such as CB2 receptor upregulation or impaired glutaminergic signalling.^[602] The toxic effects of injected Aβ have been ablated by various other cannabinoids in rodents.^[613]

Activation of CB2 by various cannabinoids has been seen to reduce many of the toxic effects of induced Alzheimer's disease in rodent models.

Cannabis has been investigated for the treatment of multiple sclerosis (MS) in part due to self-reports that it improves several physical and pain symptoms associated with MS.^[615] In places where medical cannabis is prescribed, such as The Netherlands, MS is one of the more common medicinal uses for cannabis.^[616] Some evidence for its possible usefulness in MS also comes from animal data; for instance, cannabinoids have been shown in mice to modulate the immune response in a manner that could be beneficial to MS.^[617]

In terms of the overall efficacy of cannabis in MS, a systematic review of the evidence to date by the American Academy of Neurology found that oral cannabis extract is effective in reducing spasticity and central pain or painful spasms in MS; smoked cannabis was of uncertain efficacy in both of the previous symptoms and cannabis overall was possibly or probably ineffective for other symptoms of MS.^[618]

According to a systematic review of the evidence, oral cannabis extract is probably effective in reducing pain and spasms in MS, although smoked cannabis has less good evidence to support its efficacy for any symptoms.

Clinical trials have been performed to examine the effects of oral cannabis extract on MS. Oral ingestion of a cannabis extract or pure Δ⁹THC (up to 25mg based on weight) in people with MS over 15 weeks failed to improve physical mobility as reported by a physician/physiotherapy using the Ashworth scale,^[619] although patient-reported spasticity and pain was significantly reduced relative to placebo, and secondary improvements in sleep were also noted (with no change in fatigue or mood).^[619] However, this study (the CAMS study) has been followed up after a year and in those who chose to maintain therapy, a small therapeutic effect as assessed by the Ashworth scale and self-reported benefits to spasticity and pain were noted with no major safety concerns.^[620] Another large trial (MUSEC) in persons with MS given oral Cannabis sativa (two weeks used to titrate up to 25mg Δ⁹THC daily) for ten weeks noted that the percent of patients reporting significant reductions in muscle stiffness reached 29.4% with cannabis despite only reaching 15.7% in placebo, giving an odds ratio (OR) of beneficial effects of 2.26.^[621]

Clinical trials of oral cannabis extract or oral Δ⁹THC have found that daily usage can reduce self-reported levels of pain, spasticity, and stiffness associated with MS.

The cognitive effects of cannabis on patients with MS have also been examined. In people with MS who report (mostly daily) cannabis smoking, following at least 12 hours cessation the smoking group appears to perform worse on tests of acute recall and working memory relative to those with MS who do not use cannabis;^[374] fatigue and depression did not differ between groups.^[374] Similar negative associations have been noted in one study of 10 subjects who inhaled or ingested street cannabis who performed slower on an Symbol Digit Modalities Test (SDMT) for working memory and sustained attention (with no differences on a Neuropsychological Battery for MS, or NPBMS)^[622] and in a cross-sectional study where 25 cannabis-using people with MS performed worse than disease controls on information processing, working memory, and executive function.^[623]

When looking at placebo-controlled trials (the aforementioned studies being correlations), those with MS who took Cannabis sativa (personally titrated to the optimal dose to reduce muscle spasticity) for eight weeks at different times did not experience memory impairment with the former relative to placebo.^[624]

Usage of cannabis in those with multiple sclerosis, relative to those with MS but with no reported usage of cannabis, appears to be associated with impaired working memory and acute recall with at least one study noting reduced executive function. One double-blind controlled study involving cannabis, while small in size, failed to replicate these observations.

While less commonly reported, cannabis usage appeared to be associated with self-reported improvements in incontinence in people with MS^[615] and in a substudy of a major study (CAMS) it was noted that the 18% of subjects in placebo who reported significant improvements in incontinence (specifically urgency incontinence) were outperformed by both Cannabis sativa capsules and pure Δ⁹THC (33-38%) both dosed at 25mg Δ⁹THC equivalents for fifteen weeks.^[625] it should be noted that the primary CAMS trial failed to find any influence on bladder symptoms unlike the substudy,^[619] but is still considered an up-and-coming treatment option requiring further evidence.^[626]

Based on limited evidence, it appears that the side-effect of incontinence seen in multiple sclerosis may be beneficially influenced by usage of cannabis capsules or Δ⁹THC, particularly in the urgency thereof.

Amyotrophic Lateral Sclerosis (ALS) is a progressive neurological disorder in which glials cells may play a role, with human post-mortem evidence suggesting that spinal cord damage in this disorder is associated with CB2-positive microglial activity, suggesting that cannabinoids could in theory affect the progression of the disease.^[627] This possibility has been supported in a mouse model of Δ⁹THC improved symptoms when administered either before or during the onset of symptoms.^[628]

Cannabis has been noted to have effects which may be beneficial in the treatment and palliation of ALS.^[629] A survey of people with ALS revealed self-reports of improvements of appetite loss, depression, pain, spasticity, and drooling, although other symptoms of ALS were not affected, and the sample size was very small.^[630]

Animal data suggests that the cannabinoid system may be involved in the disease progression of ALS, and Δ⁹THC in these models can delay progression. ALS patients have also reported relief from some symptoms of ALS with cannabis use, although there have been no clinical trials to date to assess the effect of cannabis or its components to support these claims.

While cannabis was historically given to patients with tremor associated Parkinson's disease, a small pilot study treating 5 Parkinson's patients with a cigarette in the morning containing 1g of cannabis with a THC content of 2-9% found no benefit in subjective relief or tremor severity.^[631] A small double-blind crossover trial using oral cannabis extract to alleviate levodopa-induced dyskinesia in Parkinson's patients trial also found no improvements in dyskinesia or in the symptoms of Parkinson's disease.^[632] However, an open-label study of smoked cannabis in Parkinson's disease patients found a signficant decrease in the Unified Parkinson Disease Rating Scale (UPDRS) in patients after use, including a reduction in tremors.^[633]

Despite this, a larger survey of Parkinson's patients from Prague revealed that almost half of patients who used cannabis reported a general improvement of their symptoms with about 30% reporting an improvement in tremor.^[634]

The evidence of cannabis's efficacy on Parkinson's disease is mixed at this time; while sometimes patients report subjective symptom relief, some studies have found litte to no effect.

A small double-blind trial using 75 or 300mg/d of CBD for six weeks in patients with Parkinson's disease and on a stable dose of anti-Parkinson's medications found no change in symptoms as scored by the UPDRS, but some improvement in quality of life as reported by the Parkinson's Disease Questionnaire (PDQ-39) in the 300mg dose group.^[635]

Usage of cannabis recreationally is consistently correlated with various forms of mental illness, particularly psychosis,^[636] but it is not entirely clear whether or not there exists a causal relationship; some reviews claim that more evidence exists for cannabis exacerbating existing schizophrenia rather than causing a "cannabis psychosis,"^[637] while others suggest causation.^[638] The evidence to date seems to rule out the hypothesis that increased cannabis use can be attributed to attempts at self-medication of previously-existing psychotic symptoms.^[639][640] It also appears there may be a common genetic component which can predispose people to both cannabis use and schizophrenia, although the genetic contribution is small.^[641]

Because case-control studies may be prone to recall bias,^[642] and since randomized controlled trials of medicinal cannabis use would not be applicable to populations which use the drug recreationally,^[636] the best evidence for a causal link between cannabis use and symptoms of psychosis would be from longitudinal cohort studies. Several such studies have been performed and seem to demonstrate a link between cannabis use and the development of psychotic symptoms. A follow-up to the first such study examining a large all-male cohort of Swedish conscripts found a dose-dependent increase in risk of developing schizophrenia in those who used cannabis without using other illicit substances (OR 1.9, 95% CI 1.1-3.1) even after adjusting for multiple confounders.^[643] Such a dose-dependent relationship for the development of any psychotic symptoms in cannabis users was also found in a Dutch sample from the general population over 3 years when adjusted for multiple confounders (adjusted overall OR 2.76, 95% CI 1.18-6.74).^[644]. A New Zealand sample also showed an positive association with dose-dependence between cannabis use and psychotic symptom development even after statistically taking into account symptoms previous to cannabis use, which ruled out pre-existing psychotic symptoms as the cause for cannabis use and other confounders.^[645] A separate New Zealand study using a different sample confirmed an increased risk of schizophrenic symptoms in those who used cannabis, with early age of first use leading to worsening risk.^[646] Finally, a German cohort study of cannabis use in adolescents over 10 years also confirmed a link between cannabis use and psychotic symptoms even after taking into account other variables including other psychiatric diagnoses; this study also noted that the self-medication hypothesis was unlikely as psychotic symptoms previous to cannabis use did not predict future cannabis use.^[647]

In addition, early exposure to cannabis during adolescence seems to be associated to development of psychosis later in life.^[648] One propspective study found that cannabis use before the age of 14 predicted the development of symptoms of schizotypal personality disorder (SPD) in adulthood, even after adjustment for multiple confounders; the use of cannabis in those with SPD symptoms in adolescence did not seem to effect symptoms in adulthood, however.^[649] In addition, a 14 year prospective study in The Netherlands starting with a group of randomly-selected 4-16 year-olds found that psychotic symptoms were associated with cannabis use regardless of the initial presence of psychological symptoms as assessed by the Child Behavior Checklist; the authors of this study note that although cannabis use was assessed by self-report, these reports are more likely to be accurate in The Netherlands due to the legal status and relative acceptablility of cannabis use in that culture.^[650]

While no long-term controlled trials of cannabis use have been done due to practical and ethical concerns, there have been placebo-controlled trials of intravenously-infused Δ⁹THC and its effects on certain psychotic symptoms. In one study, an 2 mg/mL IV infusion administered over 20 minutes in both healthy and frequent users of cannabis to mimic Δ⁹THC blood levels obtained during recreational cannabis use found that increases in both the Positive and Negative Syndrome Scale (PANSS), a measure of psychosis, over placebo, but no change in "panic" or "anxious" visual rating scales; the subjective experience of the infusion was reported to be similar to that of recreational cannabis use.^[651] A similar experiment in stabilized schizophrenics being treated with antipsychotics a significant transient increase in positive and negative psychotic symptoms.^[652] Another placebo-controlled study using IV-infused Δ⁹THC in individuals who had at least one paranoid thought in the past month and assessing paranoia via a realistic virtual reality simulation and focusing on which congnitive components contributed to paranoia found that paranoia was increased over placebo, and found that an increase in negative affect and anomalous experiences such as thought echoing, hallucinations, and unusual changes in sensory experiences.^[653]

Several observational studies have suggested that cannabis use, especially at a young age, may be associated with an increase in the risk of developing psychotic symptoms later in life, and short-term placebo-controlled studies have verified that Δ⁹THC can briefly increase psychotic symptoms in both healthy and schizophrenic individuals. It is important to note, however, that association does not show causation, and more research is needed to establish a definitive link. Although the body of evidence leans toward causation, it is not currently clear whether this may occur in certain populations predisposed to psychosis.

While the mechanisms of psychosis are not completely clear, it is biologically plausibile that cannabis may have a causative role in the development of psychosis and schizophrenic symptoms.^[654][655] The general mode of hypothetical causation involves the effects of THC on CB1 receptors, which is distributed in brain regions implicated in psychosis such as the frontal regions of the cerebral cortex, basal ganglia, hippocampus, cerebellum, and anterior cingulate cortex.^[654] The primary general role of the CB1 receptor is to presynaptically modulate the release of various neurotransmitters upon being stimulated, which suggests the general mechanism by which THC could induce its psychological effects.^[654] In particular, THC has been seen to enhance doapamenergic activity in the striatal and mesocorticolimbic areas of the brain, which may explain how cannabis use could induce some of the positive symptoms of psychosis, which are thought to be related to increased dopaminergic activity.^[655]

A second mechanism by which THC may induce psychotic symptoms may be through reducing glutamate activity. Starting in 1980, hypoglutamatergic (reduced glutamate) neurotransmission has been implicated in schizophrenia,^[656] specifically due to reduced activity of the NMDA receptor.^[657] The Δ⁹THC content found in cannabis can also decrease NMDA receptor activity by activating the CB1 receptor which can then bind to the NMDA glutamate receptor and acutely reduce signalling via NMDA.^[217] Cannabinoids can do this through the internalization of CB1 receptors, which can co-internalize NMDA receptors into the cytosol.^[218] In addition, subchronic administration of Δ⁹THC has been seen to reduce expression of NMDA receptors subunits (NR1 and NR2 subunits in the hippocampus in animal models.^[209]

Finally, there seems to exist a genetic mediator which predisposes adolescents who use cannabis to mediate the emergence of psychosis in some young users; specifically, a longitudinal study found that carriers of a variant of the catechol-O-methyltransferase (COMT) gene (valine-158 allele) who used cannabis when young were more likely to develop schizophreniform disorder.^[658] Carriers of this variant catabolize dopamine more quickly,^[659] leading to diminished dopamine activity in the prefrontal cortex, which can lead to increased dysregulation and indirect increase in striatal dopaminergic activity and increased positive psychotic symptoms.^[660] This, when compounded with similar increased striatal activity induced by cannabis,^[661] may combine and ultimately lead to increased risk of psychotic symptoms, an effect which has been confirmed in the short-term experimentally when adminstering Δ⁹THC to those with the valine-158 allele.^[662]

It is biologically plausible that cannabis could cause psychotic symptoms. The mechanisms by which it may do so may involve the dopamine and glutamate systems.

It should be noted that while the symptoms of psychosis discussed above are suspected to be attributable to the action of Δ⁹THC, the actions of CBD seem to act in the opposite direction, possessing some anti-psychotic properties.^[663] These effects may carry over to cannabis use in real life; one group found that use of cannabis with a higher CBD content was associated with a lower incidence of psychotic symptoms.^[664] Another study found that subjects with Δ⁹THC found in their hair samples had a higher incidence of psychotic symptoms when compared to those who had both Δ⁹THC and CBD found in their hair or those with neither found.^[665] Furthermore, a case-control study in those who exhibited an first episode of psychosis found that the cases tended to use high-potency cannabis, which has a high Δ⁹THC content and a low CBD content, more than the controls, who tended to prefer hash (resin), which has a higher CBD to Δ⁹THC ratio.^[666]

Cannabis has varying ratios of CBD to Δ⁹THC ratios; that sold in Dutch coffee shops typically have a CBD to Δ⁹THC of around 0.01 for herbal cannabis and 0.44 for the resin.^[667] That found in the UK has a CBD/Δ⁹THC ratio of 0.05 for the herb and 1.2 for the resin.^[668]

CBD found in cannabis may possess anti-psychotic effects. There is some evidence to suggest that the ratio of CBD to ⁹THC in cannabis when used is correlated with the chance of developing symptoms of psychosis.

A systematic review and meta-analysis of the literature of prospective cohort studies examined whether chronic cannabis use was associated with manic symptoms in those without a psychotic disorder; 6 studies were found which met the inclusion criteria, and 2 studies were combined for meta-analysis.^[669] Three of the studies found involved participants who had pre-existing bipolar disorder,^{[670][671][672]} and the authors of the systematic review concluded from these that cannabis use may worsen the course of bipolar disorder by exercerbating the manic phase.^[669] The authors also concluded from three prospective studies of those without pre-existing disorders that cannabis use was associated with the development of either bipolar disorder^[673] or manic symptoms^[674][675], although in the latter case, the manic symptoms sometimes remained below the clinical threshold, and so the clinical relevance of this finding is not completely clear.^[669] The meta-analysis of two of the studies done in those who entered the studies without a clinical disorder^[674][675] found an odds ratio for developing manic symptoms in those who used cannabis to be 2.97 (95% CI 1.80-4.90), with low heterogeneity between the studies.^[669] There were several limitations to the systematic review, including the low number of prospective studies found by the authors, the variability in assessing cannabis use between the studies, the potential for bias in many of the included studies, and the observational nature of the included studies which precludes proving causality.^[669]

Cannabis use is associated with increased manic symptoms in both those with and without bipolar disorder, although whether this is causal is not established, nor is the precise clinical relevance of this finding.

In survey of people with inflammatory bowel disease (IBD) found that those who used cannabis (17.6% of the sample, almost entirely through inhalation) noted that usage of cannabis was associated wtih improvements in abdominal pain and cramping with semifrequent reports of improving joint pain and diarrhea, although usage of cannabis for six months was a predictor (95% CI OR 1.45-17.46) for requiring surgery those with Chohn's disease.^[676] Another retrospective observational study in patients specifically with Crohn's disease found improvement in self-reported symptoms; a significant reduction in the Harvey-Bradshaw index was seen in patients after they began cannabis use as opposed to before use.^[677]

Small prospective trials have also shown some benefit. An uncontrolled pilot study involving 13 individuals showed self-reported improvement in patients with IBD receiving cannabis showed signficant improvements in several areas of well-being and a reduction in Harvey-Bradshaw index.^[678] Additionally, a small placebo-controlled trial in Crohn's patients administering cannabis cigarettes twice daily containing 115mg Δ⁹THC (with the placebo having the THC removed) found a significant clinical response in the treatment group, although the primary endpoint of complete remission was not reached.^[679] An additional, uncontrolled study also found that patients with IBD reported relief from symptoms when using cannabis, but also found that Chrohn's patients who used cannabis for 6 months or more had a higher rate of surgical intervention (OR 5.03, 95% CI 1.45-17.46), although whether this association was causitive was not clear.^[676]

Preliminary evidence suggests that cannabis may help improve symptoms of IBD, although the evidence to date is only in the form of observational data or very small studies.

Alcohol is a drug commonly used alongside cannabis, and both are known to interact with one another.

The dopamine-releasing effects of alcohol appear to be dependent on the CB1 receptor (as blocking it can reduce dopamine activity induced by alcohol^[680]) and blocking this receptor has been noted to reduce voluntary alcohol intake in various rodent strains^{[681][682][683][684]} (likely due to the suppressive effect of blocking CB1 on dopamine release, since dopamine involved in the motivational effects in alcohol-seeking behaviors^[685]).

Alcohol's effects on dopamine release in rats appears to depend on activation of the CB1 receptors, since blocking it can reduce the dopamine release induced by alcohol in the nucleus accumbens and subsequent self-adminstration of alcohol in rats.

Tolerance to caffeine is known to increase density of adenosine A1 receptors in several brain regions,^[157] and activation of this receptor is similar to activation of the CB1 receptor in the sense that they use are coupled to a similar pool of G-proteins^[686] to subadditively suppress adenyl cyclase activity;^[151] the mechanism for crosstolerance may exist downstream of the receptor-G-protein interface of the two signalling pathways.^[151][156]

In mice tolerant to caffeine (which increases A1 receptor density) who were then given a single dose of Δ⁹THC acutely before cognitive testing noted that, relative to Δ⁹THC alone, that the caffeine-tolerant mice exhibited worse spatial memory deficits from the cannabinoid despite not worsening performance in mice which were not tolerant to caffeine.^[152]

A possible explanation for this may be linked back to glutamate, which is released from CB1 activation of astrocytes,^[687] and its activation of glutaminergic signalling and subsequent downregulation and internalization of NMDA and AMPA receptors^[209] via a COX-2-dependent mechanism.^[211] This could underlie memory impairments from cannabis usage since reduced receptor density impairs the ability of glutamate to improve synaptic plasticity, which is a key feature of cannabis-induced memory impairment.^{[208][209][213]} While activation of CB1 on neurons (rather than astrocytes) normally suppresses glutamate release,^[215] an increase in A1 receptor density is associated with less neuronal suppression and a relative increase in synaptic glutamate in hippocampal cells, as A1 activation attenuates CB1-mediated suppression of glutamate release.^[152]

Animal data suggests that tolerance to caffeine may enhance the negative impact of Δ⁹THC on acute spatial memory formation.

NSAID drugs (such as indomethacin, aspirin, and ibuprofen) appear to be able to inhibit some of the neurological effects of cannabis, including the percieved 'high',^[688][689] by inhibiting COX2, which is induced by CB1 receptor activation and leads to downregulation of glutamate receptors.^[211] This COX2 inhibition concurrent with Δ⁹THC administration appears to prevent memory impairment from Δ⁹THC while it preserves the ability of Δ⁹THC to reduce β-amyloid pigments and attenuate neurodegeneration.^[211]

Nicotine is the main stimulatory component of cigarettes, and its signalling in the brain appears to interact with the main cannabinoid receptor (CB1).

It appears that activation of the CB1 receptor can potentiate the reinforcement stimulus for nicotine and lead to nicotine-seeking behavior in mice withdrawn from nicotine.^[690] In contrast, blocking the actions of the CB1 receptor can reduce dopamine release into the nuclear accumbens from nicotine and decrease nicotine self-adminstration and motivation for nicotine-seeking.^[680] CB1 antagonists do not require preadministered CB1 agonists to be effective at reducing motivation for nicotine.^[691][692] Nicotine also seems to enhance some of the acute physiological responses to THC in rodents such as withdrawal effects.^[693]

Activation of cannabinoid receptor appears to be able to enhance some of the addictive features of nicotine in mice. Conversely, rodent experiments have also shown that nicotine potentiates some of the physiological effects of THC, including withdrawal.

In rats, a high fat diet has been noted to reduce the effects of Δ⁹THC thought to be due to a desensitization of the CB1 receptor due to elevated levels of the endogenous cannabinoids (endocannabinoids) anandamide and 2-arachidonoylglycerol.^[694]

The body can become tolerant to cannabis usage and Δ⁹THC^[695][696] which can be attributed to a desensitization of the CB1 receptor which mediates the psychoactive properties of Δ⁹THC.^[153]

The G proteins to which CB1 is coupled (G_i and G_o^[120]), which mediate signalling of the receptor, are activated to a lesser degree in a desensitized receptor due to chronic exposure of tissue to Δ⁹THC,^[153][154] and this desensitization is seen with most cannabinoid agonists of the CB1 receptor rather than solely due to Δ⁹THC (although differences exist between ligands).^{[120][168][697]} The major reason the G proteins are activated to a lesser degree is due to an loss of surface CB1 receptors which occurs in a time- and dose-dependent manner^[153] with more rapid internalization by more potent agonists,^[170] and this decrease in receptor concentration is likely due to receptor internalization (which underlies agonist-induced desensitization for many G-protein coupled receptors).^[170][698]

Exposure of the CB1 receptor to any agonist, including Δ⁹THC, can cause the receptor to eventually be removed from the cell membrane. This results in less activation of the receptor from Δ⁹THC, less signalling within the cell, and tolerance to the drug.

Such a phenomena has been confirmed in chronic cannabis users who reported 10+/-6 joints a day for numerous years (12+/-7) with more suppression in those who reported a longer smoking history.^[699] Abstinence from cannabis for approximately four weeks is sufficient to normalize CB1 receptor activity.^[699]

Reduction in CB1 receptor availability has been noted in numerous brain regions including the angular singulate cortex, the prefrontal cortex, and parahippocampal gyrus among others such as the parietal, posterior cingulate, and occipital cortices.^[699] In general, downregulation affects cortical regions more than subcortical regions^[700] reaching up to a 20% reduction in cortical regions^[699] while subcortical regions more readily recovere after cessation (based on mouse data^[701]).

A reduction in CB1 receptor availability has been noted in chronic cannabis users. This reduction in CB1 availability is normalized after four weeks abstinence.

Withdrawal from cannabis exists but appears to differ from other drugs as it is not associated with any major medical or psychiatric problems that are seen with withdrawal from alcohol, benzodiazepines, or opioids; cannabis withdrawal tends to involve significant physical and mental impairments to well-being with risk of relapse.^[702]

In terms of withdrawal symptoms' role in the risk of relapse, cannabis is comparable to tobacco in severity^[703][704] although withdrawing from both simultaneously has been reported as being more severe than either alone.^[703] Symptoms of withdrawal occur almost immediately and decline over the course of approximately a week to a month.^[705][706] Similarly to nicotine being able to treat tobacco withdrawal, oral supplementation of Δ⁹THC appears to confer some benefit to cannabis withdrawal.^{[707][708][709]}

Symptoms of cannabis withdrawal include difficulties with sleep, restlessness, decreased appetite, depressed mood, nervousness or anxiety, irritability, and physical symptoms/discomfort; at least three of the aforementioned seven would be present for a diagnosis of cannabis withdrawal syndrome (DSM-V).^[710]

Physical symptoms refer to symptoms such as stomach pain, shakiness, sweating, chills, and/or headaches but are generally less frequently reported than other symptoms.^[711] Major psychological withdrawal symptoms include decreased appetite, difficulty sleeping, irratibility and anger, strange dreams, restlessness, and cannabis cravings.^[705][712] The time course of these symptoms can vary significantly.^[705][713]

Cannabis withdrawal lasts for approximately a week to a month. The symptoms are both physical and mental, with physical symptoms being less prevalent. Major symptoms include insomnia and trouble sleeping, restlessness, irratibility, change in appetite, alongside cravings for cannabis.

Impared sleep is a commonly-reported symptom of cannabis withdrawal^{[705][706][711][712][714][715][716][717]} and the intensity of this impairment is higher shortly after cannabis cessation gradually decreasing with time.^[705][718] The exact time course of intensity is variable, as while two studies using self-reported side-effects noted a decrease in symptom intensity^[705][718] one study assessing sleep quality via polysomnogram noted that sleep quality was impaired (slight worsening) over the first 13 days of cannabis cessation,^[719] although subjective ratings of sleep disturbance in adults have seen to return to baseline in a week, although strange dreams persisted throughout the 45-day course of the study.^[705]

Impaired sleep quality seem with cannabis cessation is one of the more commonly reported withdrawal effects, and while it appears to be most intense in severity for the first week of cessation it declines in intensity but is still present for up to a month of cessation, while strange dreams can persist even longer.

The cluster of symptoms of nervousness, irritability, and anger are commonly reported with cannabis cessation.^{[705][706][711][714][715][716][717][720]} While nervousness appears to follow the same time course of sleep impairment and physical restlessness in returning to baseline in under two weeks,^[705][718] irritability appears to have an earlier onset and may persist for longer than other symptoms.^[705] Anger also lasts longer than usual, although it has a later onset.^[705]

Nervousness, irratibility, and anger often appear as a cluster of symptoms in cannabis withdrawal, and have variable times of onset and cessation.

Animal models have provided some insight into the mechanism leading to some of the motor symptoms of cannabis withdrawal. After ceasing adminstration of THC twice daily for five days and abruptly injecting rats with rimonabant to precipitate withdrawal,impairments in motor control relative to controls were seen which seemed to be dependent on downregulation of CB1 receptors (specifically in parallel fibers), microglial activation, and signalling through the IL-1 receptor.^[177] Stimulation of the IL-1 receptor is able to induce ataxia, as assessed by injections of IL-1β.^[721][722] The underlying mechanism has been hypothesized^[723] to be traced back to how stimulation of CB1 receptors downregulate glutaminergic activity;^[215] a downregulation of these receptors exacerbates glutamate signalling resulting in inflammatory toxicity.

IL-1β is known to directly excite Purkinje cells,^[722] although no overt neuronal damage was noted in rats during withdrawal^[177] suggesting that neurotransmission towards these cells (from parallel fibers, which were implicated in the process^[177]) were altered during withdrawal.

A reduction in motor control is noted during cannabis withdrawal in rats, which is traced back to a short-lived state of increased inflammation and excitotoxicity (from excess glutamate) causing alterations in Purkinje neuron firing.

Cannabis dependence was defined in the DSM IV-TR as having 3 or more of the following symptoms occuring at any time during the same 12-month period: tolerance, taking cannabis in larger amounts or longer than intended, having a desire to or unsuccessful attempts at reducing use, spending a lot of time obtaining the drug or using or recovering from its use, continued use despite being aware of its adverse consequences, and giving up important activities because of use.^[724] By these criteria, approximately 4.3 percent of Americans have been dependent on cannabis at some point in their lives.^[725] However, a newer edition, the DSM V, has eliminated this specific category of dependence and replaced it with a substance use disorder category^[724] as well as a specifically recognized cannabis withdrawal syndrome.^[713]

The occurrence rate of cannabis dependence disorder amongst users is estimated to be 9%, which for comparison is less than the amount of tobacco users who are dependent on the product (32%).^[726]

Cannabis does have habit-forming properties similar to all neuroactive drugs, but the amount of users who meet the criteria for a dependency disorder is less than the other commonly inhaled drug (tobacco) and is said to be an easier dependency to break

Neurobiologically, common occurrence in drug dependency is reduced dopamine receptor availability, specifically the D2/D3 subsets, in a brain region known as the striatum resulting in lower dopamine neurotransmission. Such an occurrence has been noted in dependency to alcohol,^[190] amphetamines,^[191] cocaine,^[192] heroin,^[727] and opioids^[728] but despite this dependency to cannabis has repeatedly failed to demonstrate any impairment to dopaminergic signalling or receptor availability in this brain region.^{[193][194][195][196]}

Neurobiologically, a classical sign of dependency in the brain, a reduction in dopamine signalling in the striatum and local brain regions, does not appear to extend to cannabis despite most other common drugs of abuse sharing this mechanism.

Cannabis has been associated with numerous side-effects in case studies with varying degrees of implied causation.

Case studies include pancreatitis,^{[543][729][730][731]} enlarged gums,^[732] mania,^[729] nausea and vomiting,^[733] transient ischemic attacks,^[734][735] and atrial fibrillation .^[736]

There have been a few case studies of youth with no history of cardiovascular problems experience nonlethal^[737] and lethal^[738][739] heart attacks (from coronary thrombosis) associated with cannabis usage, and other various coronary syndromes,^[740][741] strokes, ^[742]and combine cerebral and myocardial infarctions.^[743] In some cases symptoms have arisen near the time of cannabis ingestion which at least suggests a causal link,^{[737][744][745][746][746]} It has been hypothesized that cannabis inhalation may serve as an acute 'trigger' exacerbating previously existing symptoms (which were initially benign)^[744][746] although at times adverse effects have also been seen in people with no prior known symptoms or complications.^[746]