Horny goat weed appears to be safe and well-tolerated when taken orally, and only minor side effects, including abdominal discomfort and nausea, have been observed. In some cases, abnormal heart rhythms[1] and skin rashes[2] have been reported, but in both instances, horny goat weed was coingested with other herbal supplements or pharmaceuticals.
Since horny goat weed may have antihypertensive and anticoagulation effects in the body, taking it alongside medications that reduce blood pressure or prevent blood clotting (e.g., platelet inhibitors or anticoagulants) may cause unwanted side effects. Horny goat weed may also increase estrogen in women.[3]
In vitro, icariin and some of its metabolites have demonstrated inhibitory effects on specific isoforms of UDP-glucuronosyltransferase (UGT) in the gastrointestinal tract. These enzymes play a crucial role in the metabolism of orally administered drugs like opioids, naproxen, acetaminophen, and amitriptyline by facilitating their first-pass metabolic clearance. In vivo studies are required to assess the inhibitory effect of icariin’s metabolites on UGT enzymes in human participants, and to understand the potential herb-drug interactions that may occur when icariin is taken alongside medications.[4]