Adverse effects of stinging nettle supplements are uncommonly reported in literature. Minor gastrointestinal upset, including diarrhea, abdominal pain, and nausea, have been reported.[1][2]
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.[3] In another case, stinging nettle is thought to have caused gynecomastia (i.e., breast growth in a male person),[4] and in another case it is likely to have caused galactorrhea (i.e., spontaneous lactation).[5] 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.[6]
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.[7]