Horny goat weed is the common name for the plant known as Epimedium, which is used in traditional Chinese herbal medicine as a tonic, an aphrodisiac, and an antirheumatic agent. It also goes by the names Herba epimdii, yin yang huo, fairy wings, and rowdy lamb herb. While over 200 compounds have been identified in horny goat weed, the main bioactive constituents appear to be flavonoids, of which icariin is the most well-studied.^[2] Icariin is also the main active ingredient in horny goat weed supplements.

Horny goat weed appears to have anti-atherogenic, anti-inflammatory, antiviral, and immune-modulating effects, though most of these mechanisms have only been investigated in research animals and in vitro.

Although horny goat weed is one of the most common ingredients in male testosterone-boosting and erectile dysfunction (ED) supplements,^[3] there is a lack of human evidence to support its effectiveness for these outcomes. Horny goat weed (icariin) does have pro-erectile properties in rodent models of ED.^[4][5][6] One study found that horny goat weed elevated estrogen and reduced cholesterol and triglyceride levels in postmenopausal women.^[7]

Some studies showed that horny goat weed appears to improve bone mineral density (BMD) and pain intensity in people with osteoporosis, both in combination with standard use medications and as an alternative treatment.^[8]

In vitro and in vivo studies have suggested that horny goat weed is useful for treating cardiovascular diseases, including atherosclerosis and coronary heart disease.^[9] Furthermore, recent evidence supports the neuroprotective properties of horny goat weed,^[10] though human studies are needed to confirm these effects.

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^[11] and skin rashes^[12] 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.^[7]

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.^[13]

Horny goat weed might benefit ED through its inhibition of phosphodiesterase-5 (PDE-5). This is the same mechanism exerted by pharmaceuticals like Viagra.^[14] PDE-5 inhibition increases levels of cyclic GMP, which enhances smooth muscle relaxation and blood flow to the penis and promotes erection. Horny goat weed may also increase the activity of nitric oxide synthase enzymes and elevate the production of nitric oxide, which also promotes smooth muscle relaxation.^[15][16]

The potential cardioprotective qualities of horny goat weed may be related to a decrease in vascular smooth muscle cell proliferation, reduced platelet activation, lipid modulation, and improvements in endothelial function.^[9] The suspected neuroprotective mechanisms of horny goat weed could include the upregulation of antioxidant activity^[17] and the inhibition of inflammation and oxidative stress in the brain.^[18][19]