Supplements

Horny Goat Weed (Icariin)

Updated · 29 sources · Editorial policy

Horny goat weed is the common name for Epimedium, a genus of plants whose leaves (Herba Epimedii, or yin yang huo) have long been used in traditional Chinese medicine for sexual function and bone health. Its best-studied flavonoid is icariin. Supplements sold for libido and erections usually list “horny goat weed,” “Epimedium extract” or a percentage of icariin on the label.

The single most important fact is the gap between the lab and the clinic. Icariin weakly inhibits phosphodiesterase type 5 (PDE5), the enzyme blocked by Viagra (sildenafil), but only in test tubes and animals. A 2026 evidence review lists no clinical trials of icariin for erectile dysfunction (Gu et al., Front Pharmacol 2026). In people, almost none of the swallowed icariin reaches the blood intact. A second practical problem is that “herbal” sexual enhancement products are often spiked with real PDE5 drugs.

This page summarizes what laboratory studies, animal studies, the few human trials and FDA notices report. It is for education only and is not medical advice.

Property Detail
Type Herbal dietary supplement (dried leaf, extracts, purified icariin)
Source plant Epimedium species (for example E. brevicornum, E. sagittatum, E. grandiflorum), family Berberidaceae
Main constituent Icariin, a prenylated flavonol glycoside; C₃₃H₄₀O₁₅, 676.7 g/mol (PubChem)
PDE5 inhibition (lab) IC50 0.75–5.9 µM in enzyme assays, vs 74 nM for sildenafil in the same 2008 assay
Oral absorption in humans Icariin very low or undetectable in blood; mainly absorbed as metabolites (icariside II, desmethylicaritin)
Half-life Not established; one human study could not calculate it because levels were too low
Doses used in human trials 60 mg/day icariin (in a combination, 24 months); 100–1,680 mg/day icariin (5 days); 370–1,110 mg of a standardized extract
Human ED trials None identified for Epimedium or icariin alone
US status Sold as a dietary supplement; not a controlled substance (21 CFR 1308.13)
WADA status Neither Epimedium nor icariin is named on the WADA 2026 Prohibited List

What it is

Epimedium leaves contain a family of prenylated flavonoids (flavonoids carrying a five-carbon “prenyl” side chain). Icariin is the main one. It carries two sugars: a rhamnose at the C-3 position and a glucose at the C-7 position. Removing them step by step gives the metabolites icariside II and icaritin, and then desmethylicaritin (Wu et al., Molecules 2016).

The sugars matter. They make icariin a large, polar molecule that crosses the gut wall poorly (Gu et al., 2026). They also affect how strongly the molecule fits into PDE5, which is why chemists have built stronger semi-synthetic derivatives (see below).

Proposed mechanism

During sexual arousal, nitric oxide raises cyclic GMP (cGMP) in the smooth muscle of the penis, which relaxes it and lets blood flow in. PDE5 breaks cGMP down. Prescription PDE5 inhibitors such as sildenafil, tadalafil and vardenafil block that breakdown.

Icariin inhibits the same enzyme in lab assays, but far more weakly. Animal studies also report effects on nitric oxide synthase expression, nerve regeneration and penile tissue fibrosis, which reviewers propose as additional mechanisms (Niu et al., Transl Androl Urol 2022). None of these mechanisms has been confirmed in people.

How it behaves in the body

Conversion by gut bacteria

When icariin was incubated with human intestinal bacteria in the lab, the bacteria stripped off the glucose to form icariside II, and some strains went further to icaritin and desmethylicaritin. The authors concluded that most icariin is likely converted to icariside II before absorption (Wu et al., Molecules 2016).

Human pharmacokinetic studies

Three small human studies measured blood levels. All found that intact icariin barely reaches the circulation.

Study Design and dose Main finding
Teo et al., Planta Med 2019 30 healthy men; single dose of 370, 740 or 1,110 mg standardized Epimedium prenylflavonoid extract vs placebo Icariin, icariside I and icaritin below detection. Icariside II peaked at 4.1–4.3 h; desmethylicaritin peaked late, at 24.1–24.4 h
Brown et al., Nat Prod Commun 2019 24 healthy adults; purified icariin 100–1,680 mg/day for 5 days vs placebo Icariin “very low or undetectable” at all doses; pharmacokinetic parameters could not be determined
Yong et al., Phytomedicine 2021 58 postmenopausal women; 740 mg/day extract for 6 weeks vs placebo Icariin and icariside I below detection; main circulating metabolite was desmethylicaritin (median peak 60.9 nM)

This matters for the PDE5 story. The enzyme assays below needed micromolar concentrations of icariin to inhibit PDE5, while human blood levels of icariin after oral doses were mostly undetectable. Whether the metabolites inhibit PDE5 meaningfully at the levels reached in people has not been shown.

What the research shows

PDE5 inhibition (lab data)

These are enzyme or cell-culture experiments. They show that icariin can inhibit PDE5, not that a supplement does so in the body.

Study Compound PDE5 IC50 (concentration giving 50% inhibition) Comparator
Ning et al., Urology 2006 Icariin 1.0, 0.75 and 1.1 µM (PDE5A1, A2, A3) Zaprinast 0.33, 0.23, 0.32 µM
Dell’Agli et al., J Nat Prod 2008 Icariin 5.9 µM (human PDE5A1) Sildenafil 74 nM
Dell’Agli et al., 2008 3,7-bis(2-hydroxyethyl)icaritin (synthetic derivative) 75 nM Sildenafil 74 nM

In the 2008 study, only the Epimedium extract was active among four plant extracts traditionally used for impotence (the others included Tribulus terrestris). The synthetic derivative was 80 times more potent than icariin and roughly matched sildenafil (Dell’Agli et al., 2008). Note that 74 nM vs 5.9 µM is a large gap: plain icariin was much weaker than the drug. A later medicinal chemistry study made other icariin analogs that approached commercial PDE5 inhibitors in potency in vitro (Chau et al., PLoS One 2019). These are experimental chemicals, not what supplements contain.

In rat cavernous smooth muscle cells, icariin at 100–200 µM kept cGMP levels higher after a nitric oxide donor (Ning et al., 2006).

Erectile function (animal studies)

Most erectile data come from rats. Results are measured as the rise in pressure inside the penis during electrical stimulation of the cavernous nerve, relative to blood pressure (ICP/MAP).

  • Nerve-injury model. In rats with cavernous nerve injury, rats were given oral icariin at 1, 5 or 10 mg/kg daily for 4 weeks. The low-dose group had higher ICP/MAP ratios than controls and than rats given a single dose. Treated rats also had more neuronal nitric oxide synthase in penile tissue, and icariin increased nerve fiber outgrowth in cultured pelvic ganglia (Shindel et al., J Sex Med 2010).
  • Combined with sildenafil. In 48 nerve-injured rats, icariin (1.5 mg/kg) plus daily sildenafil gave a higher ICP/MAP ratio than either alone (Xu et al., Andrology 2017).
  • Other models. Reviews describe similar findings in diabetic, hypertensive, castrated and irradiated rats, but note the heavy reliance on rat models and the lack of clinical validation (Niu et al., 2022; Gu et al., 2026).

Gu and colleagues also point out that many preclinical studies use doses that cannot be reached in humans given icariin’s low oral bioavailability (Gu et al., 2026).

Erectile function (human data)

We found no randomized trial of Epimedium or icariin alone in men with erectile dysfunction. A 2026 evidence-graded review lists “None” under clinical trials for this indication (Gu et al., 2026). A 2021 review of popular US erectile and testosterone supplements found that horny goat weed was among the most common ingredients, and that no complete supplement product had published randomized trial evidence (Kuchakulla et al., Int J Impot Res 2021).

By contrast, sildenafil, tadalafil and vardenafil are approved drugs with large placebo-controlled trials. For another plant marketed for libido with somewhat more human data, see maca.

Bone density in postmenopausal women

Bone is the one area with a long, placebo-controlled human trial. In Hong Kong, 100 late-postmenopausal women with low bone density (lumbar T-score between −2 and −2.5) took either Epimedium-derived flavonoids (60 mg icariin, 15 mg daidzein and 3 mg genistein daily) or placebo for 24 months; all took 300 mg calcium (Zhang et al., J Bone Miner Res 2007). Eighty-five completed it.

Outcome at 24 months Epimedium flavonoids Placebo
Lumbar spine BMD change +1.3% (p = 0.091 within group) −2.4% (p = 0.002)
Femoral neck BMD change +1.6% (p = 0.148) −1.8% (p = 0.048)
Deoxypyridinoline (bone resorption marker) −39% No change
Serum estradiol, endometrial thickness No change No change

The between-group difference was significant at the lumbar spine (p = 0.006) and femoral neck (p = 0.008) at 24 months. The product also contained the soy isoflavones daidzein and genistein, so the effect cannot be attributed to icariin alone.

A 6-week trial in 58 postmenopausal women found that 740 mg/day of a purified Epimedium extract raised bone-specific alkaline phosphatase, a marker of bone formation (p under 0.05), with only a trend for the osteoclast marker TRAF6 (p = 0.068). It did not measure bone density (Yong et al., 2021).

A 2025 meta-analysis pooled 10 randomized trials (890 patients) comparing Epimedium preparations, used alone or added to standard drugs, with conventional treatment for primary osteoporosis. It reported higher lumbar BMD (standardized mean difference 1.15, 95% CI 0.61–1.70), but the comparisons were against conventional drugs rather than placebo, and the authors call for higher-quality studies (Zhang et al., Front Med 2025). In ovariectomized rats (a model of menopause), a meta-analysis found that icariin raised femur and spine BMD (Liu et al., Cell Mol Biol 2017).

Mood (pilot data)

In an open-label, uncontrolled pilot study, 10 people with bipolar disorder and alcohol use disorder took icariin up to 300 mg/day for 8 weeks. Depression and anxiety scores and drinking fell, and no one withdrew for side effects (Xiao et al., Am J Drug Alcohol Abuse 2016). Without a control group, these results cannot separate a drug effect from placebo or natural change.

Side effects and risks

Tolerability in trials

The small controlled trials reported few problems:

  • Single doses of 370–1,110 mg standardized extract in 30 men: no adverse effects observed (Teo et al., 2019).
  • 740 mg/day extract for 6 weeks: no significant adverse symptoms and no changes in liver, blood or kidney tests (Yong et al., 2021).
  • Icariin 100–1,680 mg/day for 5 days: generally well tolerated, but two participants on 1,680 mg/day stopped because of gastrointestinal symptoms. Self-reported depressive symptoms rose slightly vs placebo, which the authors judged not clinically significant (Brown et al., 2019).

These studies were short and small, so rare or long-term effects would not show up.

Liver

LiverTox states that oral horny goat weed has not been linked to serum aminotransferase elevations or to clinically apparent liver injury (LiverTox).

Case reports: heart rhythm, mood and others

Case reports cannot prove cause, and in most of them the exact product contents were never tested. Reported events include:

  • Tachyarrhythmia and hypomania. A 2004 letter described a fast heart rhythm and hypomania (a milder form of mania) with horny goat weed (Partin and Pushkin, Psychosomatics 2004). A systematic review of herbal mania reports notes that the authors suspected other unidentified herbs or drugs in the product, which were not tested for (Bostock et al., Front Psychiatry 2018).
  • Psychosis in Parkinson’s disease. A 59-year-old man taking carbidopa/levodopa and ropinirole developed new paranoid psychosis after a month of daily horny goat weed; an ECG showed new atrial fibrillation. The supplement’s contents were unknown. The authors raised a possible interaction through inhibition of the liver enzyme CYP1A2, which breaks down ropinirole (Deliz et al., Mov Disord Clin Pract 2026).
  • Muscle spasms with raised CK. A 33-year-old man had severe muscle spasms with raised creatine kinase and creatinine after a month of an Epimedium supplement bought online; both improved in hospital after stopping (Ledo et al., Cureus 2025).
  • Vasculitic rash attributed to ginkgo and/or horny goat weed (Metz et al., BMJ Case Rep 2009).

Drug interactions

Lab studies cited in a 2026 review report that icaritin (a metabolite) inhibits the drug-metabolizing enzymes CYP1A2, CYP2C9 and CYP3A4, and that icariin affects the drug pump P-glycoprotein. The review flags a possible interaction risk with narrow-therapeutic-index drugs such as tacrolimus and cyclosporine, but this has not been tested in people (Gu et al., 2026). Safety data in pregnancy, children and older adults are sparse.

The more immediate interaction risk comes from adulteration (next section): a product secretly containing sildenafil or tadalafil carries those drugs’ interaction with nitrates.

Quality and contamination

“Natural” sexual enhancement supplements are among the products most often found to contain hidden drugs.

  • NMR analysis of 150 products. Of 150 supplements sold for sexual performance and claimed to be all natural, 61% contained PDE5 inhibitors: 27% the approved drugs sildenafil, tadalafil or vardenafil, and 34% unapproved structural analogs. In 25% of products with the approved drugs, the amount exceeded the maximum recommended dose. Only 2.5% contained plant compounds such as icariin, and only 31% could be considered true herbal products (Gilard et al., J Pharm Biomed Anal 2015).
  • Ghana analysis. Among 40 herbal sexual enhancement supplements used in Ghana, sildenafil was found in 10, vardenafil in 2 and tadalafil in 1; six products gave estimated daily intakes above 100 mg sildenafil equivalents. Only one contained icariin, at a level too low to explain its PDE5 activity (Akuamoa et al., Food Addit Contam 2022).
  • FDA recall. In May 2025, a product sold on Amazon as “Endurance Boost With Horny Goat Weed” was recalled after sampling found undeclared sildenafil and the sildenafil analog propoxyphenylsildenafil. The notice warns that these may interact with nitrates such as nitroglycerin and cause a potentially life-threatening drop in blood pressure (FDA recall notice, 2025).
  • FDA notifications list. FDA keeps a running list of sexual enhancement products found to contain hidden drug ingredients, describes them as falsely advertised as dietary supplements or all-natural treatments, and states that the list covers only a small fraction of contaminated products (FDA).

In practice, a horny goat weed product that produces a strong, fast erectile effect is more consistent with a hidden PDE5 drug than with icariin, given how weak and poorly absorbed icariin is.

Key takeaways

  • Weak PDE5 inhibitor in the lab. Icariin inhibits PDE5 at micromolar concentrations; sildenafil does so at about 74 nM in the same assay. Stronger synthetic derivatives exist but are not in supplements.
  • Barely absorbed. In human studies, intact icariin was very low or undetectable in blood after oral doses up to 1,680 mg/day; gut bacteria convert it to metabolites.
  • No human ED trials. The erectile benefits come from rat studies. No randomized trial of Epimedium or icariin alone in men with erectile dysfunction was identified.
  • Some bone data in women. A 24-month placebo-controlled trial of an Epimedium flavonoid combination (with soy isoflavones) prevented the bone loss seen with placebo in 100 postmenopausal women.
  • Mostly tolerated, with case reports. Short trials found few side effects, but case reports describe arrhythmia, hypomania, psychosis and muscle injury, usually with untested products.
  • Adulteration is the main hazard. Sexual enhancement “herbal” products frequently contain hidden sildenafil, tadalafil, vardenafil or their analogs, which can interact dangerously with nitrates.

Sources

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