Caffeine is generally safe for most people if used within the recommended amounts (up to 400 mg per day in a healthy adult, or up to 200 mg in a single dose — see the dosage information section.[1] Furthermore, caffeine-containing beverages like tea and coffee are socially acceptable and widely used.[2][3] However, caffeine has several side effects and drawbacks.
The side effects of caffeine include a short-lived rise in heart rate and blood pressure, heart palpitations, headache, increased urine output, nervousness, gastrointestinal problems, etc.[4][5][6][7] Caffeine can also raise heart rate during and following exercise.[8][9] Some of these side effects, particularly those related to heart rate, blood pressure, and urine output, may subside with regular use due to increased tolerance.[4][6][10][11][12] Because caffeine can cause mild drug dependence, some people also experience withdrawal symptoms — headache, drowsiness, and irritability — when they stop using caffeine after regular daily intake.[13]
One of the major drawbacks is that caffeine impairs sleep quality and can reduce sleep duration.[14][15] A recent meta-analysis found that to prevent deleterious effects on subsequent sleep, coffee should be consumed at least 9 hours before bedtime, and a caffeine-containing pre-workout supplement should be consumed at least 13 hours before bedtime.[14]
Sadly, there have been several case reports of caffeine toxicity[16][17][18][19] and caffeine-related death.[20][21][22][23] However, considering the high global prevalence of caffeine intake, these cases are rare.[2][3][24] Such cases also typically involve exceptionally high caffeine intakes (usually unintentional but sometimes purposeful) or caffeine intake combined with other drugs and alcohol.[20] Some instances of toxicity and death involve energy drinks — beverages that contain as much as 300 mg of caffeine plus other stimulants like taurine, in addition to B vitamins and sugar.[25] At the population level, energy drinks are unlikely to cause harm because there is a low prevalence of consumption.[26][27] However, in certain groups (children, teenagers, people with underlying heart conditions), health risks can emerge when people far exceed recommended daily caffeine intake limits by consuming energy drinks with other caffeine-containing foods and beverages.[26][27]
Some epidemiological studies have found a relationship between high daily caffeine intake (e.g. more than 4 to 6 cups of coffee per day) and increased cardiovascular disease risk.[28][29][30][31] However, the evidence from mendelian randomization studies and meta-analyses of epidemiological studies shows that caffeine (or coffee) intake does not increase the risk of cardiovascular disease, including coronary artery disease, stroke, heart failure, and cardiovascular disease mortality,[27][31][32][33] and that moderate intake might even be protective.[34][35][36][37]
Some studies also find a relationship between caffeine intake during pregnancy and poorer health outcomes for mother and baby following birth; however, the entirety of the evidence is equivocal.[38][39] Furthermore, it is unclear whether reducing caffeine intake during pregnancy has any effect on pregnancy outcomes, including infant health and birth weight.[38][39] However, some authorities recommend that pregnant women limit their caffeine intake to 200 milligrams per day (the equivalent of 1–2 cups of coffee per day or less).[40]