Horny Goat Weed

Epimedium brevicornu Maxim. was described in 1889 by the Russian botanist Karl Maximovich, according to Kew’s International Plant Names Index, and the World Checklist of Vascular Plants compiled at Kew, the dataset behind Plants of the World Online, accepts the name, with Epimedium rotundatum K.S.Hao as a synonym; the pharmacology literature often spells it brevicornum. The checklist gives a native range confined to China: North-Central, South-Central, and Southeast China and Qinghai. Flora of China records it from Gansu, Henan, Hubei, Qinghai, Shaanxi, Shanxi, and Sichuan, in forests, thickets, and on slopes at 600 to 3,500 m. The genus has about 50 species, which Flora of China places in China, the western Himalaya, Japan, Korea, the Russian Far East, southern Europe, and North Africa, with 41 in China alone. The other pharmacopoeia species, E. sagittatum (Siebold & Zucc.) Maxim., E. pubescens Maxim., and E. koreanum Nakai, are also accepted names, and the Chinese Pharmacopoeia lists a fifth, E. wushanense T.S.Ying, separately. Some supplements and studies name E. grandiflorum instead, and many labels say only “Epimedium” or “horny goat weed.”

Flora of China describes E. brevicornu as a herb 20 to 60 cm tall with a short, clumped rhizome 3 to 5 mm across. The leaves are usually twice divided into threes (biternate), giving nine leaflets, which fits the old Chinese name “nine leaves on three stems” recorded by Cui and colleagues (Frontiers in Pharmacology 2024). Each leaflet is ovate, 1.5 to 7.5 cm long, deeply heart-shaped at the base, lopsided on the side leaflets, usually shiny above and bluish-white beneath, with a spiny-toothed margin. The flowering stem carries two opposite leaves and a loose panicle 10 to 35 cm long of 20 to 50 white or pale yellow flowers about 1.5 cm across. As throughout the genus, what look like petals are the inner sepals; the true petals are much shorter, with blunt spurs 2 to 3 mm long, and the four stamens protrude beyond them. Flowers open in May and June, and capsules about 1 cm long ripen from June to August. Cui and colleagues describe the medicinal species as preferring shaded, humid places: forest understory, scrub along ditches, and damp mountain slopes.

The medicine is the dried leaf. Wang and colleagues (Frontiers in Pharmacology 2023) summarize the pharmacopoeia’s processing: the leaves are cut into slivers and dried, and a prepared form is stir-baked with refined suet (rendered animal fat), 20 kg of suet per 100 kg of leaf; in TCM theory the prepared form is preferred for kidney yang and the raw form for tendons and bones. Ma and colleagues (Journal of Ethnopharmacology 2011) counted more than 260 compounds isolated from the genus, with prenylated flavonoids (flavonoids carrying a five-carbon side chain) as the main group. The best known are icariin and epimedins A, B, and C, which the 2020 Chinese Pharmacopoeia uses together as quality markers; the 2015 edition required at least 0.5% icariin in the dried leaf (Li and colleagues, Frontiers in Pharmacology 2021). In the body, icariin is broken down to icariside II, icaritin, and desmethylicaritin. Content varies with species, origin, and processing, and a 2014 survey of 104 commercial batches cited by Li and colleagues found only 0.01 to 0.17% icariin, all below the standard.

The name comes from a story. Niu and colleagues (Translational Andrology and Urology 2022) and Cui and colleagues retell a legend from China’s Northern and Southern Dynasties (420–589 AD): shepherds noticed that their male animals had more erections and mated more often after eating a certain wild plant, and told Tao Hongjing, a famous physician of the time, who observed the effect for himself and recorded the plant as an aphrodisiac that strengthened yang. English-language accounts usually tell the tale with goats, while Chinese reviews translate the animals as sheep, and the English name horny goat weed comes from this story; Ma and colleagues also list “rowdy lamb herb” among the plant’s names. Cui and colleagues write that the herb was first documented in the Shennong Materia Medica Classic (Shennong Bencao Jing), and that the Tang-era poet Liu Zongyuan used it for rheumatic leg trouble. Reviews describe more than 2,000 years of medicinal use, Cui and colleagues report that the leaf is used in China, Japan, and South Korea, and MSKCC lists inyokaku among its common names.

In TCM, Epimedii Folium is described as spicy and sweet, and its actions are summarized as “tonifying kidney yang, strengthening muscles and bones, and dispelling wind and dampness” (Li and colleagues). “Kidney yang deficiency” is a traditional pattern diagnosis rather than a disease of the kidney organ, and the complaints grouped under it include impotence and weak tendons and bones. Qian and colleagues (Journal of Ethnopharmacology 2024) list traditional uses of the Korean species for impotence, involuntary ejaculation, infertility, weak tendons and bones, rheumatic pain, and numbness, and Dietz and colleagues (Pharmacological Reviews 2016) note its use to enhance libido in both men and women and for menopausal and premenstrual symptoms. It is rarely prescribed alone: practitioners combine it with other herbs, and modern Chinese patent medicines such as Xianling Gubao, a six-herb bone formula, carry the tradition into capsules and tablets.

In the West, horny goat weed arrived mainly as an ingredient in sexual-enhancement products. Balasubramanian and colleagues (Journal of Sexual Medicine 2019) analyzed the six top-ranked ED supplements on Amazon and found horny goat weed among the four most popular ingredients, with ginseng, L-arginine, and tongkat ali. Kuchakulla and colleagues (International Journal of Impotence Research 2021) found it among the most common ingredients in 32 best-selling US testosterone and ED supplements, and found no published randomized trials of any whole product. Corazza and colleagues (BioMed Research International 2014) surveyed 203 websites and named horny goat weed, yohimbine, maca, and ginkgo as the most common active ingredients of sexual-enhancement products, linking the four with reports of mood changes, anxiety, and hallucinations. In China, meanwhile, research turned to bone: Li and colleagues (Chinese Herbal Medicines 2024) describe Xianling Gubao, with Epimedium leaf as its first ingredient, as a national essential drug used for osteoporosis, osteoarthritis, and fractures for roughly twenty years.

The Viagra comparison rests on one enzyme. During arousal, nerves release nitric oxide, which raises a messenger called cGMP that relaxes the smooth muscle of the penis’s erectile tissue; PDE5 breaks cGMP down, and sildenafil and tadalafil work by blocking PDE5. Dell’Agli and colleagues screened four herbs traditionally used for impotence against human PDE5: only the Epimedium brevicornu extract (80% inhibition at 50 µg/ml) and its icariin were active, while extracts of tribulus, Ferula hermonis, and cassia cinnamon were not. Icariin’s IC50 (the concentration that blocks half the enzyme’s activity) was 5.9 micromolar, against 74 nanomolar for sildenafil in the same test, a roughly 80-fold difference. A laboratory-modified icariin, 3,7-bis(2-hydroxyethyl)icaritin, matched sildenafil at 75 nanomolar, but it does not occur in the plant. Ning and colleagues (Urology 2006) measured icariin IC50 values of 0.75 to 1.1 micromolar against three forms of PDE5, about three times weaker than the research inhibitor zaprinast. Different assays give different numbers, but all place icariin well below the prescription drugs.

Absorption is a second hurdle. Teo and colleagues (Planta Medica 2019) gave 30 healthy men single doses of 370, 740, or 1,110 mg of a standardized Epimedium prenylflavonoid extract: icariin, icariside I, and icaritin stayed below detection in the blood, and the main circulating compounds were icariside II, peaking about four hours after the dose, and desmethylicaritin, peaking at about 24 hours. Yong and colleagues (Phytomedicine 2021), giving 740 mg a day for six weeks, found a median peak desmethylicaritin level of 60.9 nanomolar. Whether such levels affect PDE5 in human erectile tissue has not been tested. Animal results also differ from the drug model: in rats with cavernous nerve injury, Shindel and colleagues (Journal of Sexual Medicine 2010, funded by the US National Institutes of Health) found that four weeks of daily low-dose icariin improved erectile pressure and raised nerve-related proteins, while a single dose two hours before testing did not, the opposite of sildenafil’s on-demand action.

Epimedium compounds also act on hormones, platelets, and drug-handling enzymes, so far mainly in the laboratory. Dietz and colleagues explain that icariin is converted to icaritin and desmethylicaritin, which likely account for mostly estrogen receptor alpha-selective, estrogen-like activity, and MSKCC cites cell studies in which icariin increased production of aromatase, the enzyme that makes estrogen. In the bone trial by Zhang and colleagues, however, neither blood estradiol nor the thickness of the womb lining changed over two years. In the six-week trial by Yong and colleagues, the extract raised a marker of bone formation (bone-specific alkaline phosphatase), with a borderline drop in a protein that drives bone-resorbing cells. Irfan and colleagues (Evidence-Based Complementary and Alternative Medicine 2021) found that an icariin-rich fraction of E. koreanum inhibited aggregation of human and rat platelets and reduced clot formation in rats. On drug metabolism the lab evidence conflicts: MSKCC cites activation of the pregnane X receptor, which can induce the drug-metabolizing enzyme CYP3A4, while Zhong and colleagues (Journal of Pharmacy and Pharmacology 2017) found no CYP3A4 induction in human liver cells but inhibition of CYP1A2 and CYP2B6.

Erectile dysfunction is the use people search for, and it is the least supported by trials of the herb itself. Borrelli and colleagues (Drugs 2018) systematically reviewed 24 randomized trials of herbal supplements in 2,080 men with ED; the 12 single-herb trials tested ginseng, saffron, tribulus, maritime pine bark, and maca, and none tested Epimedium. We found no randomized trial of Epimedium alone for ED. Blends have been tested: Punyawudho and colleagues (African Journal of Traditional, Complementary and Alternative Medicines 2013) gave 63 Thai men with mild to moderate ED Cappra, a five-ingredient traditional formula that LiverTox notes includes Epimedium, or placebo for two weeks each in a crossover design, and erectile function scores rose by 4.87 points on Cappra and 3.44 on placebo. Nishimatsu and colleagues (Aging Male 2014) found that a six-ingredient Japanese preparation containing Epimedium, ginseng, aged garlic, and other materials raised erection scores more than a Kampo formula over six months in an open-label trial of 49 older men. These results belong to the mixtures. Balasubramanian and colleagues concluded that human studies of ED supplement ingredients have yielded no definitive findings. Verdict: unproven, and a doctor can offer ED treatments with known effects.

Bone health in postmenopausal women is the most promising use. Zhang and colleagues randomized 100 healthy women 10 to 18 years past menopause, with bone density just above the osteoporosis threshold (a T-score of −2 to −2.5), to a daily capsule of 60 mg icariin, 15 mg daidzein, and 3 mg genistein, which the authors called Epimedium-derived phytoestrogen flavonoids, or to placebo; everyone took 300 mg of calcium, and 85 finished. After 24 months, lumbar spine density had fallen 2.4% and femoral neck density 1.8% on placebo, but held steady on the capsule (+1.3% and +1.6%), with significant differences between groups, and a marker of bone breakdown (deoxypyridinoline) fell 39%. A 2022 meta-analysis by Shi and colleagues (Frontiers in Pharmacology) pooled 12 randomized trials in 1,017 people, mostly from China, and found better bone density when Epimedium was added to or replaced conventional treatment, but one main outcome was a loosely defined “effective rate,” and the authors called for more rigorous trials. MSKCC describes the bone evidence as a small trial needing confirmation. Verdict: promising but unconfirmed, with no fracture data, and no replacement for proven osteoporosis treatment.

Libido, testosterone, and menopause symptoms are traditional uses without trials of the herb on its own. Dietz and colleagues note traditional use for desire in both sexes and for menopausal and premenstrual symptoms, and MSKCC says that more studies are needed to verify any effect on sexual dysfunction. In rats, Liu and colleagues (Asian Journal of Andrology 2005) found that four weeks of icariin improved erections after castration without raising testosterone, which argues against the “testosterone booster” framing of many products. We found no published trial measuring testosterone, sexual desire, or hot flashes in people taking Epimedium alone. In the bone trial, neither estradiol nor the womb lining changed, so that capsule did not behave like hormone replacement. Verdict: traditional use only; people with low desire are better served by a check-up that looks at medicines, mood, hormones, and relationships, and by herbs with at least some trial evidence.

Fatigue, mood, and other conditions have only small or negative studies. MSKCC notes that Epimedium is traditionally used for fatigue but has not been studied in clinical trials. Younger and colleagues (International Journal of Environmental Research and Public Health 2021) tested Epimedium sagittatum in 29 men with Gulf War Illness, a chronic multisymptom condition in veterans, using a placebo-controlled crossover design, and found no improvement in symptom severity at either dose. Xiao and colleagues (American Journal of Drug and Alcohol Abuse 2016) gave icariin, up to 300 mg a day for eight weeks, to 10 people with bipolar disorder and alcohol misuse in an open-label pilot study; depression and anxiety scores and drinking fell, and no one withdrew because of side effects, but without a control group the results cannot be separated from natural change or expectation. That finding sits awkwardly beside a published case of hypomania linked to horny goat weed. Verdict: not established for energy or mood.

Many other claims rest on laboratory and animal work. MSKCC lists bone-protective, nerve-protective, immune-modulating, and anticancer effects of Epimedium compounds in preclinical studies, and notes that most published human trials combine the herb in a multi-herb recipe, which makes its own contribution impossible to judge. LiverTox notes that clinical studies of Epimedium extracts have suggested effects on high blood pressure, coronary artery disease, and sexual satisfaction, but that none has been proven in rigorous prospective controlled trials. We found no controlled trial for joint or rheumatic pain, the other classic TCM use. Friș and colleagues (Plants 2026), reviewing five medicinal species, concluded that robust clinical evidence is still lacking for all of them and that the species are not interchangeable. For erectile dysfunction, ginseng has more trial evidence than any other herb, and the ginseng page covers it.

In Chinese medicine, horny goat weed is the dried, cut leaf, used raw or stir-baked with suet and usually decocted (simmered) with other herbs. Wang and colleagues based their animal doses on a human dose of 9 g of leaf, taken from the 2015 Chinese Pharmacopoeia. In the trials pooled by Shi and colleagues, Epimedium was given as decoctions, granules, extract tablets, or capsules, and the decoctions were often prepared by hospitals rather than standardized manufacturers. TCM practitioners also use it in fixed patent medicines; Xianling Gubao, for example, combines Epimedium leaf with Psoralea fruit (bu gu zhi), Dipsacus root, salvia root (dan shen), Anemarrhena rhizome, and prepared Rehmannia root. Those products are licensed medicines in China, but they carry their own liver warnings (see Side effects), and their results cannot be transferred to Western horny goat weed capsules.

Western supplements are a different product. LiverTox describes horny goat weed tablets and capsules of 250 or 500 mg, usually taken once a day, and notes that it is often combined with other botanicals. Doses used in human research were specific extracts: 60 mg of icariin with 15 mg daidzein and 3 mg genistein daily for two years in the bone trial by Zhang and colleagues; 740 mg a day of a standardized Epimedium prenylflavonoid extract for six weeks in 58 postmenopausal women (Yong and colleagues, funded by Singapore’s National Medical Research Council); single doses up to 1,110 mg in 30 healthy men (Teo and colleagues); and purified icariin up to 300 mg a day for eight weeks in the bipolar pilot study. None of these established an effective dose for sexual function. Outside China we found no official dosing standard, and in the United States horny goat weed is sold as a dietary supplement, which does not require FDA approval before sale.

Quality is the central practical problem. Icariin content varies widely: in the 2014 survey cited by Li and colleagues, every one of 104 commercial batches fell below the pharmacopoeia minimum, and Friș and colleagues found that the five medicinal species differ in chemistry and should not be treated as interchangeable. Many labels name no species, give no icariin content, or hide the herb in a “proprietary blend.” Worse, some products contain drugs. FDA recall records list Class I recalls, the most serious category, of RAM IT and To the Moon capsules, each labeled “Horny Goat Weed, 1000 mg” and found to contain undeclared sildenafil (2024), and of Endurance Boost with Horny Goat Weed, which contained sildenafil and a sildenafil analogue (2025). FDA warns that many sexual enhancement products are likely to be contaminated with hidden drug ingredients and that its public list covers only a small fraction of them, so a product’s absence from the list does not mean it is safe.