Horny Goat Weed
Epimedium · Yín Yáng Huò 淫羊藿
Contains a weak natural PDE5 inhibitor — like a faint Viagra.
Evidence tier
Preliminary
Research weight
Citations
18 verified / 18
Classification
TCM Herb
❝
What the evidence says. Early or small human trials; promising but not yet conclusive.
What is Horny Goat Weed?
Horny Goat Weed (Epimedium · Yín Yáng Huò 淫羊藿) is a traditional Chinese medicine herb used for erectile / sexual support. NutriDex grades the human evidence as Preliminary. Horny goat weed (Epimedium) contains icariin, which — intriguingly — is a genuine PDE5 inhibitor, the same mechanism as Viagra. The catch is potency: icariin is on the order of ~100–200× weaker than sildenafil in lab assays, so real-world supplement doses are unlikely to match the drug. Most evidence is from cell and animal models; controlled human trials are lacking, so any benefit is plausible but unproven.
Purported Benefits
✓(Claimed) erectile / sexual support
✓(Claimed) libido & 'Yang' tonic
✓(Claimed) bone support
Evidence by outcome
The same supplement can be well-proven for one use and unproven for another — here is the human evidence graded outcome by outcome.
| Outcome | Evidence | Effect | Studies |
|---|---|---|---|
| Erectile / sexual functionIcariin is a genuine but ~100-200x weaker PDE5 inhibitor with poor oral bioavailability; benefit shown only in rodents, no confirmatory human RCTs. | No Evidence | — no effect | 2 |
| Bone mineral density (postmenopausal)A 24-mo RCT plus meta-analyses (10 RCTs, n=890) show Epimedium flavonoids raise/maintain BMD, though trial quality is limited. | Moderate | ↑ benefit · moderate | 3 |
| Liver injury (hepatotoxicity)Rare idiosyncratic hepatotoxicity case reports exist, but NIH LiverTox rates oral use unlikely (score E) to cause clinically apparent liver injury. | Preliminary | ⚠ risk | 2 |
Dosing & Compounds
Typical Dose
Extracts standardized to icariin; an effective human dose is not established.
Active Compounds
Safety & Cautions
Generally well tolerated short-term, but documented case reports include tachyarrhythmia, agitation/hypomania (in an elderly cardiac patient) and severe muscle spasms. Epimedium has also been linked to liver injury, and possible estrogenic activity makes it relevant to hormone-sensitive conditions. Interacts with nitrates and blood thinners; OTC products are sometimes adulterated with PDE5 drugs. Long-term safety unknown. Educational only — always check with your doctor or pharmacist before combining Horny Goat Weed with any medicine.
Key Studies ★ 18 studies
meta-analysis
Osteoporosis meta-analysis (2025)
✓ Full text
Systematic review/meta-analysis of 10 RCTs (890 patients) found Epimedium significantly increased bone mineral density at lumbar spine (SMD 1.15), femoral neck (SMD 1.11) and distal radius (SMD 1.27) and raised overall clinical efficacy (OR 3.80, 95% CI 2.27-6.37) in primary osteoporosis.
Meta-analysis
Zhang 2024 (Front Pharmacol)
✓ PubMed
Systematic review/meta-analysis of 6 RCTs (n=838) found Epimedium total flavonoids may improve bone mineral density in primary osteoporosis, but evidence quality is limited; no sexual-function endpoint assessed.
RCT
Indran IR et al., 2021
✓ PubMed
Randomized, double-blind, placebo-controlled trial in postmenopausal women examining Epimedium prenylflavonoids: characterized safety and pharmacokinetics and assessed effects on bone-specific alkaline phosphatase (bone-formation marker) and the osteoclast adaptor protein TRAF6, supporting a bone-favorable mechanistic signal in humans.
RCT
Brown ES et al., 2019
✓ Full text
Randomized, double-blind, placebo-controlled human safety/PK trial of oral icariin in 24 healthy adults across doses 100-1,680 mg/day: icariin was well tolerated with no significant between-group differences on side-effect scales, but plasma icariin was very low or undetectable at all doses, demonstrating poor oral bioavailability of the parent compound.
RCT
Fan Y et al., 2019
✓ PubMed
First-in-class phase I single-arm trial of oral icaritin (600 or 800 mg b.i.d.) in 20 advanced HCC patients: no drug-related adverse events >= grade 3; 46.7% achieved clinical benefit (1 partial response, 6 stable disease), median time-to-progression 141 days and median OS 192 days (488 days in PR/SD responders), with survival linked to immune-cell dynamics.
rct
Postmenopausal bone RCT (2007)
✓ PubMed
24-month double-blind RCT in 100 late-postmenopausal women (lumbar T-score -2 to -2.5) found Epimedium-derived phytoestrogen flavonoids maintained bone mineral density versus a placebo decline (lumbar spine +1.3% vs -2.4%, p=0.006; femoral neck +1.6% vs -1.8%, p=0.008) without endometrial hyperplasia.
review
Mechanistic review (2024)
✓ PubMed
Review concludes icariin and icariside II improve erectile function in preclinical models via PDE5 inhibition, eNOS/NO upregulation (PI3K/Akt) and stem-cell-mediated cavernous repair, with some animal data comparable to sildenafil but no confirmatory human RCTs.
review
Liver effects review (2023)
✓ PubMed
Review of icariin and its metabolites reports predominantly hepatoprotective and anticancer effects at typical doses (icaritin developed as an anticancer agent), contextualizing the otherwise idiosyncratic, high-dose liver-injury signal.
Observational
Ledo 2025 (Cureus)
✓ PubMed
First case report of Epimedium (horny goat weed) toxicity: a 33-year-old man developed severe muscle spasms with elevated creatine kinase (1,210 U/L) and creatinine (1.23 mg/dL) after one month of use, resolving with supportive care.
Observational
Herb-Induced Liver Injury 2025 (PMC)
✓ Full text
Case of acute hepatotoxicity in a 53-year-old man after >1 year of daily horny goat weed use, with markedly elevated liver enzymes (ALT 516, AST 185, total bilirubin 4.94), improving after cessation and supportive care.
review
Narrative review (2022)
✓ PubMed
Narrative review of pharmacokinetics and mechanisms finds icariin and derivatives could potentially restore spontaneous erections via PDE5 inhibition and testosterone promotion, but stresses absence of high-quality, large-sample human clinical data.
Cohort
Qin SK et al., 2020
✓ PubMed
Clinical study of the icariin-derived small molecule icaritin in advanced HBV-related hepatocellular carcinoma with poor prognosis: no grade III/IV treatment-related adverse events; median overall survival of 329-565 days, with longer survival correlated to baseline HBsAg positivity and Th1/Th2 cytokine and immune-checkpoint dynamics, supporting immune-modulatory activity in humans.
lab
Hepatotoxicity mechanism (2022)
✓ PubMed
Metabolomics study shows Epimedium koreanum Nakai extract causes liver injury in mice, with icariside I/II triggering idiosyncratic injury via NLRP3 inflammasome activation plus GPX4/GSH depletion-mediated ferroptosis.
authority
LiverTox safety assessment (NIH)
✓ Full text
NIH LiverTox assigns horny goat weed a likelihood score of E (unlikely cause of clinically apparent liver injury): oral forms have not been linked to serum aminotransferase elevations or clinically apparent injury, and it is not listed in large drug/herbal-induced liver injury registries, with only rare poorly-attributed reports from multi-ingredient products.
authority
Supplement adulteration warning (DoD OPSS)
✓ Source
U.S. DoD Operation Supplement Safety warns there is no human evidence supporting sexual-health/performance claims and that FDA has found horny goat weed products adulterated with hidden prescription ED drugs (tadalafil, sildenafil, avanafil), posing risks of serious side effects and drug interactions.
Mechanism
Reviews
✓ Full text
Plausible mild effect, but not demonstrated in quality human trials.
Preclinical
Icariin mechanism
✓ Full text
Icariin is a PDE5 inhibitor, roughly 100–200× less potent than sildenafil in vitro.
Preclinical
Animal models
✓ Full text
Improved erectile responses in rodent ED models; human RCTs lacking.