Stinging nettle, or Urtica dioica, is the most common member of the plant family Urticaceae that grows worldwide . The plant is tall and green, with green flowers and black seeds. The leaves and stems of most subspecies of U. dioica have trichomes, small needle-like projections that inject chemicals into the skin when touched. These chemicals, a combination of histamine, acetylcholine, and leukotrienes, cause pain, skin irritation, and rashes. Once the plant is processed, by heating it or drying it out, these toxins are no longer active. All parts of the plant have been used for medicinal purposes, including the roots, the leaves, the flowers, and the stems.^[4][5]

Stinging nettle (particularly the roots) has been used to treat BPH. It is thought to decrease the size of the prostate gland, and to improve the urinary symptoms associated with an enlarged prostate. ^[6][7] It is also sometimes used to treat other urinary tract disorders, and some studies have supported the use of stinging nettle to prevent and treat kidney stones, as the flavonoids and saponins found in the nettle are able to disintegrate oxalate stones.^[8][9]

Stinging nettle has been used in arthritis in general, as it is thought to have some analgesic and anti-inflammatory properties. While some observational studies have shown the potential use of stinging nettle supplements for arthritis, there are no formal clinical trials to support these claims. Through similar mechanisms, stinging nettle could be useful in other diseases that cause musculoskeletal pain. Observed decreases in joint pain, improved mobility, and stiffness are positive findings that warrant further research.^[10][11][12]

Stinging nettle has been studied in diabetes, and has been shown to lower fasting blood sugar levels.^[13][14] While this could be helpful, the nettle extracts studied did not improve glycated hemoglobin, also referred to as HbA1c, which reflects average blood sugar over a period of months. Although a rat study found that stinging nettle improved pancreatic function, and therefore insulin levels,^[10] human clinical trials have not found that stinging nettle supplementation improved insulin levels.^[13][14]

In allergic rhinitis, a handful of trials found that stinging nettle supplements decreased symptoms more than placebo. Although this is being a popular use of the supplement, there is not a lot of evidence to support it.^[15][1][10]

Due to its antioxidant and antiproliferative (i.e., cell-growth-suppressing) effects, stinging nettle has been studied for cancer prevention and/or treatment. In vitro studies have shown that it induces apoptosis (the programmed death of unwanted cells) in prostate cancer cells, colon cancer cells, and gastric cancer cells.^[16][17][18] In breast cancer cells, stinging nettle also induced apoptosis and slowed proliferation in certain types of breast cancers in vitro.^[19]PMID29506637^[20][21][22] Stinging nettle might also be helpful at increasing sensitivity to some chemotherapeutic agents for breast cancer, namely paclitaxel^[23] and cisplatin.^[24] In acute myeloid leukemia (AML), stinging nettle leaf tea had antiproliferative effects on the leukemic cells in vitro and induced apoptosis in them; some of the phenolic compounds in the tea were also found to have antitumor effects.^[25] While these results are all promising, clinical trials in humans will need to be done to confirm these effects.

Stinging nettle has also been used topically to prevent hair loss, and in combination medications as an antihemorrhagic agent (i.e., one that stops bleeding), but there is minimal evidence to support these uses.^[26][27][28]

Adverse effects of stinging nettle supplements are uncommonly reported in literature. Minor gastrointestinal upset, including diarrhea, abdominal pain, and nausea, have been reported.^[14][3]

While these are minor side effects, a few case reports of more concerning negative effects from stinging nettle supplementation have been published. One elderly diabetic is reported to have had a severe hypoglycemic episode.^[29] In another case, stinging nettle is thought to have caused gynecomastia (i.e., breast growth in a male person),^[30] and in another case it is likely to have caused galactorrhea (i.e., spontaneous lactation).^[31] The latter two cases could be because stinging nettle increases estrogen levels by binding to sex hormone binding globulin (SHBG), or could be related to the impact stinging nettle has on both histamine and serotonin levels.^[32]

Stinging nettle supplements may also influence the cytochrome P450 (CYP450) enzyme system, which is a crucial part of drug metabolism in the body. One rat study found that stinging nettle supplements lowered the activity of CYP450. This could be a problem for people taking certain medications, such as diabetic medication, certain antimicrobials, and hormone therapy.^[33]

Stinging nettle contains a wide range of phenols and polyphenols in the roots, stems, and leaves. Phenolic compounds, such as flavonoids, tannins, and lignans, are all present in stinging nettle and are thought to be largely responsible for the effects of stinging nettle supplements.^[5]

In BPH, stinging nettle, particularly the root, disrupts the cells that need to proliferate to cause prostatic hyperplasia. This is as a result of nettle’s impact on androgens and their transport proteins, SHBG. Stinging nettle’s phenolic compounds may also lower the activity of enzymes in the cell walls of the prostate.^[32][6]

In order to lower blood sugar, stinging nettle binds to intestinal receptors, lowering the absorption of glucose. It may also stimulate insulin secretion.^[14][32] Further, stinging nettle may lower the synthesis of cyclooxygenases and prostaglandins, which are involved in inflammation and pain responses. It may also inhibit interleukin-1, and interleukin-2, and tumor necrosis factors, which are important for the inflammatory response.^[32][12]

Nettle extract, especially from the leaves, may also have antioxidant properties. This is because of the phenolic compounds in the leaves, which help clear out reactive oxygen species.^[32][5]

Stinging nettle may improve asthma and allergy symptoms as it blocks histamine receptors and lowers the release of cytokines involved in inflammation.^[15][32]