BACKGROUND The increasing popularity and availability of herbal supplements among patients necessitates a better understanding of their mechanism of action and the effects they have on the body, both intended and unintended. Stinging nettle (Urtica dioica) is an herbaceous shrub found throughout the world that has been used for medicinal purposes for centuries. CASE REPORT A 30-year-old woman with obesity and GERD presented to a primary care clinic with new-onset galactorrhea. A urine pregnancy test was negative. Prolactin, thyroid-stimulating hormone (TSH), and a metabolic panel were all within normal limits. A mammogram demonstrated scattered areas of fibroglandular density and benign-appearing calcifications in the left breast. The breast ultrasound showed no suspicious findings. Her medications included intermittent Echinacea, etonogestrel implant 68 mg subdermal, and the supplement stinging nettle 500 mg, which she had been taking over the past month for environmental allergies. After consultation with a clinical pharmacist, the stinging nettle was discontinued. No additional changes to her medications or supplements were made. One week after discontinuation, she returned to the clinic with complete resolution of the galactorrhea. CONCLUSIONS Stinging nettle (Urtica dioica) is a common supplement and has effects on (1) sex hormone-binding globulin, (2) histamine-induced prolactin release, and (3) serotonin-induced release of thyrotropin-releasing hormone. The local estrogen bioactivity in breast tissue may subsequently lead to gynecomastia and/or galactorrhea. Supplements are an often overlooked but a critical component of medication reconciliation and potential clinical adverse effects.