Horny goat weed

What it is

Horny goat weed is the common name for several species of the Epimedium plant, used in Chinese and East Asian traditional medicine. Its best-studied constituent is icariin, a flavonoid from Epimedii Folium. On labels it may appear as Epimedium, Epimedium grandiflorum, Epimedium sagittatum or Epimedium brevicornum, often combined with other ingredients such as tongkat ali and Tribulus terrestris. Products containing it are frequently marketed for sexual performance.

What the research says

Almost all of the pharmacology is preclinical. A 2026 review of icariin explicitly separates its laboratory effects from unproven clinical efficacy, and points to poor bioavailability as a translational barrier. A human pharmacokinetic study bears that out: at oral doses up to 1,680 mg per day, blood levels of icariin were very low or undetectable. The one clinical area with pooled randomised data is bone: a 2025 meta-analysis of 10 small Chinese trials reported improved bone mineral density in primary osteoporosis, while calling for higher-quality studies. For sexual performance, the US Department of Defense states there is a lack of supporting human data.

Evidence grade: Limited evidence.

What goes wrong

In the same study two participants on the highest icariin dose stopped the study drug because of gastrointestinal symptoms. (Source 1)

• Blood level study, Low certainty.
• Size: 2 of 24 participants, in the 1,680 mg group.
• Who: healthy adults.
• How long: 5 days.
• Result: 2 discontinuations at 1,680 mg; tolerability described as good except at the highest dose.
• Funding: not stated.

Two participants receiving 1,680 mg of icariin discontinued the study drug due to gastrointestinal symptoms.

A case report describes severe muscle spasms with raised creatine kinase and creatinine in a 33-year-old man taking an Epimedium supplement bought online. (Source 2)

• Case report, Very low certainty.
• Size: 1 patient.
• Who: a 33-year-old man taking Epimedium for mood, energy and anxiety.
• How long: one month of use before presentation; 10 hours of spasms at presentation.
• Result: elevated creatine kinase and creatinine requiring hospital admission; both improved during admission and he was discharged advised to stop Epimedium.
• Funding: not stated.

In the ED, he had elevations of his creatine kinase (CK) and creatinine and required hospital admission.

The authors describe this as the first reported case of severe muscle spasms with raised CK and creatinine attributed to Epimedium. (Source 2)

• Case report, Very low certainty.
• Size: 1 patient.
• Who: a 33-year-old man.
• How long: one month of supplement use.
• Result: single case; no rate can be derived.
• Funding: not stated.

To our knowledge, this is the first specific case report of severe muscle spasms with associated elevations of CK and creatinine caused by Epimedium use.

A cancer centre monograph summarises case reports of vasculitic rash and of tachyarrhythmia with hypomania after Epimedium use. (Source 3)

• Case series, Very low certainty.
• Size: two individual cases summarised.
• Who: a 77-year-old man and a 66-year-old man with congestive heart failure.
• How long: daily for 2 weeks in the arrhythmia case.
• Result: 3-day hospitalisation for vasculitic rash; shortness of breath, chest pain and new-onset symptomatic arrhythmia controlled with olanzapine and lorazepam.
• Funding: not stated.

Tachyarrhythmia and hypomania: A 66-year-old man with congestive heart failure was hospitalized following symptoms of shortness of breath, chest pain, and new-onset symptomatic arrhythmia. He reported taking Epimedium daily for 2 weeks to increase sexual pleasure.

The same monograph states that toxicity and safety evaluations for Epimedium are lacking, and lists interactions with CYP3A4 substrates and aromatase inhibitors. (Source 3)

• Expert review, not systematic, Very low certainty.
• Size: not stated.
• Who: people taking Epimedium with medicines.
• How long: prolonged use for the CYP3A4 effect.
• Result: no rates; interactions described with clinical relevance not known.
• Funding: not stated.

Aromatase Inhibitors: Icariin enhances the action of aromatase. This may reduce the effect of aromatase inhibitors, like anastrozole, exemestane, and letrozole.

The US Department of Defense supplement safety programme lists gastrointestinal, respiratory and behavioural symptoms among side effects reported with horny goat weed supplements, without giving how often any of them occurred. (Source 4)

• Official position, Certainty not rated.
• Size: not stated; no rates given.
• Who: supplement users.
• How long: not stated.
• Result: Listed on the page as a bulleted list of symptom groups under the lead-in line, with no frequencies, denominators or severity given: gastrointestinal, respiratory and behavioural symptoms, plus dizziness, dry mouth, fever, headache, muscle spasm, nosebleeds and painful skin rash; page dated 8 October 2024.
• Funding: independent (US Department of Defense)

Reported side effects from use of horny goat weed supplements include: Gastrointestinal symptoms such as stomach upset, nausea, vomiting, and diarrhea. Respiratory symptoms such as shortness of breath, chest pain, and rapid, irregular heartbeat. Behavioral changes such as irritable mood, feelings of aggression, and inappropriate verbal or sexual behaviors.

The same programme states that some products containing horny goat weed marketed for sexual enhancement and energy have been reported by the FDA to be adulterated with prescription erectile dysfunction drugs. (Source 5)

• Official position, Certainty not rated.
• Size: not stated.
• Who: buyers of sexual enhancement products.
• How long: not applicable.
• Result: adulteration with tadalafil, sildenafil and avanafil reported; page dated 8 October 2024.
• Funding: independent (US Department of Defense)

In addition, some products containing horny goat weed marketed for sexual enhancement and energy have been reported by Food and Drug Administration (FDA) to be adulterated with prescription drugs used to treat erectile dysfunction such as tadalafil (Cialis), sildenafil (Viagra), and avanafil (Stendra).

A laboratory survey of sexual enhancement supplements bought from Californian shops found two thirds were adulterated with at least one prescription PDE5 inhibitor. (Source 6)

• Survey study, Low certainty.
• Size: 102 different products from 28 retail locations.
• Who: products marketed as sexual enhancement dietary supplements, the category horny goat weed is sold in.
• How long: samples collected 2016 to 2018.
• Result: 67% adulterated with at least one PDE5 inhibitor; of positives, 40% contained one and 60% two or more; sildenafil most common (74%), then tadalafil (59%)
• Funding: independent (California Department of Public Health, Food and Drug Branch)

Sixty-seven percent were found to be adulterated with at least one PDE5 inhibitor.

What the evidence supports

A 2025 meta-analysis of 10 randomised trials reported that Epimedium improved bone mineral density at several sites in primary osteoporosis, while calling for higher-quality studies. (Source 7)

• Meta-analysis, Low certainty.
• Size: 890 cases across 10 RCTs (448 experimental, 442 control)
• Who: patients with primary osteoporosis, trials largely from Chinese databases.
• How long: not stated in the abstract.
• Result: overall efficacy rate OR 3.80 (95% CI 2.27, 6.37; p = 0.0001); lumbar BMD SMD 1.15 (0.61, 1.70); femoral neck BMD SMD 1.11 (0.58, 1.65); distal radius BMD SMD 1.27 (0.57, 1.98)
• Funding: not stated.

Limit of this finding: The paper prints one of its bone-density results as "Metacarpal BMD (MD = 0.04; 95% CI: 0.04,0.12)" - the estimate (0.04) is exactly the same number as the bottom of its own confidence range, which is not arithmetically possible. The page was read twice and says the same thing both times, so this is an error in the published paper, not in our copying. Read the metacarpal figure as unusable and draw no conclusion from it. The other bone-density results in the same abstract are printed consistently and are not affected, but a paper with an uncorrected arithmetic error in its headline results is a reason to treat the whole analysis cautiously.

Meta-analysis suggests that the overall efficacy rate of the Epimedium group was significantly higher than that of the control group (OR = 3.80; 95% CI: 2.27,6.37; p = 0.0001).

What the evidence does not support

A human pharmacokinetic study found oral icariin produced very low or undetectable blood levels at every dose tested, preventing pharmacokinetic characterisation. (Source 1)

• Blood level study, Low certainty.
• Size: 24 healthy adult participants.
• Who: healthy adults.
• How long: 5 days, with blood sampling over 24 hours.
• Result: very low or undetectable blood icariin at doses of 100 to 1,680 mg/day; bioavailability described as low at all doses tested.
• Funding: not stated.

At all doses, either very low or undetectable blood levels of icariin were observed, demonstrating the low bioavailability of the oral formulation and preventing a determination of pharmacokinetic properties.

A 2026 review distinguishes icariin's preclinical pharmacology from unproven clinical efficacy and names poor bioavailability and flawed trial designs as translational barriers. (Source 8)

• Expert review, not systematic, Certainty not rated.
• Size: not applicable.
• Who: not applicable.
• How long: not applicable.
• Result: no pooled effect size; an evidence-stratification exercise.
• Funding: not stated.

explicitly distinguishing preclinical pharmacological actions from unproven clinical efficacy

The same review names interspecies pharmacokinetic gaps, poor bioavailability and flawed trial designs as the barriers to clinical use of icariin. (Source 8)

• Expert review, not systematic, Certainty not rated.
• Size: not applicable.
• Who: not applicable.
• How long: not applicable.
• Result: no effect size reported.
• Funding: not stated.

We delineate translational barriers including interspecies pharmacokinetic gaps, poor bioavailability, and flawed trial designs

The US Department of Defense supplement safety programme states there is a lack of human research supporting claims for horny goat weed on sexual health, energy, strength or performance. (Source 9)

• Official position, Certainty not rated.
• Size: not applicable.
• Who: supplement users.
• How long: not applicable.
• Result: no quantitative data; page dated 8 October 2024.
• Funding: independent (US Department of Defense)

There is a lack of human research data on horny goat weed to support any claims for promoting sexual health or boosting energy, strength, or performance.

Where the evidence is mixed

The same study found a statistically significant but not clinically significant rise in self-reported depressive symptom severity on icariin relative to placebo, and no cognitive differences. (Source 1)

• Blood level study, Low certainty.
• Size: 24 healthy adult participants.
• Who: healthy adults.
• How long: 5 days.
• Result: no significant between-group differences on side effect scales or cognitive assessments; a statistically but not clinically significant increase in self-reported depressive symptom severity.
• Funding: not stated.

A statistically significant, but not clinically significant, increase in self-reported depressive symptom severity was observed with icariin relative to placebo.

The authors of that meta-analysis state more high-quality studies are needed to verify long-term efficacy. (Source 7)

• Meta-analysis, Low certainty.
• Size: 890 cases across 10 RCTs.
• Who: patients with primary osteoporosis.
• How long: not stated in the abstract.
• Result: adverse reactions described as low and mainly mild gastrointestinal reactions or skin allergies.
• Funding: not stated.

Limit of this finding: The paper prints one of its bone-density results as "Metacarpal BMD (MD = 0.04; 95% CI: 0.04,0.12)" - the estimate (0.04) is exactly the same number as the bottom of its own confidence range, which is not arithmetically possible. The page was read twice and says the same thing both times, so this is an error in the published paper, not in our copying. Read the metacarpal figure as unusable and draw no conclusion from it. The other bone-density results in the same abstract are printed consistently and are not affected, but a paper with an uncorrected arithmetic error in its headline results is a reason to treat the whole analysis cautiously.

but more high-quality studies are needed to verify long-term efficacy

Where the research disagrees

Whether the clinical evidence for Epimedium and icariin supports use

• Authors of the 2025 Frontiers in Medicine meta-analysis of Epimedium in primary osteoporosis, meta-analysis of 10 randomised trials, largely from Chinese databases: Epimedium can effectively improve bone density and clinical symptoms in patients with POP, with good safety, making it a potential alternative or adjunctive treatment option (Source 7)
• Authors of the 2026 Frontiers in Pharmacology review of icariin, narrative review with an evidence-stratification framework: explicitly distinguishing preclinical pharmacological actions from unproven clinical efficacy (Source 8)

How much

• Reference intake: No reference intake such as an RDA or AI exists for horny goat weed. It is a herbal ingredient, not an essential nutrient, and the US Department of Defense states there is a lack of human research supporting any of its marketed claims. (Source 9)
• Upper limit: We found no tolerable upper intake level or acceptable daily intake set by any body. A cancer centre monograph states instead that toxicity and safety evaluations for Epimedium are lacking (page last updated 5 April 2023). (Source 3)
• Studied: A randomised, double-blind, placebo-controlled human study gave oral icariin at 100 to 1,680 mg per day to 24 healthy adults over 5 days. (Source 1)
• Studied: A 2025 meta-analysis pooled 10 randomised trials of Epimedium in primary osteoporosis with 890 patients; the abstract does not state the doses used. (Source 7)

A common belief, and what the research shows

The belief: Horny goat weed is a natural Viagra.

What the research shows: Two things undercut that. First, the human data are missing: the US Department of Defense states "There is a lack of human research data on horny goat weed to support any claims for promoting sexual health or boosting energy, strength, or performance." Second, when sexual enhancement supplements do work like Viagra it is sometimes because they contain it: in a laboratory survey of 102 products bought from Californian shops, "Sixty-seven percent were found to be adulterated with at least one PDE5 inhibitor." The Department of Defense notes that "In addition, some products containing horny goat weed marketed for sexual enhancement and energy have been reported by Food and Drug Administration (FDA) to be adulterated with prescription drugs used to treat erectile dysfunction such as tadalafil (Cialis), sildenafil (Viagra), and avanafil (Stendra)."

Questions and answers

What is it?

Horny goat weed is the common name for several species of the Epimedium plant, also called barrenwort or yin yang huo. It has a long history in Chinese and East Asian traditional medicine. On supplement labels it may be listed as Epimedium or by species name, and it is often blended with other ingredients sold for sexual and athletic performance. (Source 9)

What does it do in the body?

Its main studied compound is icariin, a flavonoid. Most of what is known about how it acts comes from laboratory and animal work rather than people, and a 2026 review of icariin sets that preclinical pharmacology apart from clinical efficacy, which it describes as unproven. In humans the practical problem is absorption: oral icariin barely reaches the bloodstream. (Source 8)

Is it good or bad for you?

For the uses it is mostly sold for, the human evidence is not there. The clearest pooled randomised data are for bone density in primary osteoporosis, from 10 small trials whose authors still call for higher-quality studies. Against that sit case reports of muscle injury with raised creatine kinase, of arrhythmia with hypomania, and a well-documented problem of products in this category being spiked with prescription erectile dysfunction drugs. (Source 9)

How do you get more of it?

It is taken only as a supplement, typically capsules or tablets of Epimedium extract, sometimes standardised to icariin. The controlled human study we found used oral icariin at 100 to 1,680 mg per day for five days in 24 healthy adults, and found blood levels very low or undetectable, so what was swallowed was largely not absorbed. This describes what a study gave people, not a recommendation. (Source 1)

If it is harmful, what reduces it?

Epimedium is not made by the body, so exposure ends when the supplement is stopped. In the published case of muscle spasms with raised creatine kinase and creatinine, the patient recovered in hospital and was discharged with instructions to stop taking it. (Source 2)

Why might someone be low in it or missing it?

This question does not apply. Epimedium is a plant sold as a supplement ingredient, not something the body makes or needs, so there is no deficiency to explain. We found no description of a horny goat weed requirement or deficiency state in the reviews, the meta-analysis or the Department of Defense summary we read. (Source 9)

We searched: WebSearch for Epimedium and icariin systematic reviews and meta-analyses; the 2026 Frontiers in Pharmacology icariin review; the 2025 Frontiers in Medicine meta-analysis; the Operation Supplement Safety monograph; the Memorial Sloan Kettering Epimedium monograph

Which whole foods contain it or feed it?

No whole food contains it. Epimedium is an ornamental and medicinal plant, not a food crop, and it reaches people only as a prepared supplement or traditional herbal preparation. It is commonly sold blended with other non-food botanicals such as tongkat ali and Tribulus terrestris. (Source 9)

What happens if you do not have it?

Nothing happens. Horny goat weed is not essential and there is no consequence described in the literature for never taking it. The open question in the research is whether taking it adds anything, and for sexual health, energy, strength and performance the Department of Defense states the supporting human data are lacking. (Source 9)

We searched: the Operation Supplement Safety horny goat weed monograph, the 2026 Frontiers in Pharmacology icariin review, the 2025 Frontiers in Medicine Epimedium meta-analysis and the 2019 human icariin study; none describes an absence or deficiency state

How can you test for it?

There is no clinical test for horny goat weed status, and measuring its marker compound in blood has proved impractical. In a human study of oral icariin at doses up to 1,680 mg per day, blood levels were very low or undetectable, which prevented the researchers from even characterising its pharmacokinetics. What can be tested is the product: laboratory analysis is the only way to know whether a sexual enhancement supplement has been spiked with prescription drugs. (Source 1)

We searched: the 2019 human icariin pharmacokinetic study, the 2026 Frontiers in Pharmacology icariin review and the Operation Supplement Safety monograph; no validated test of Epimedium status in a person was described

References

1. Natural Product Communications. Human Safety and Pharmacokinetics Study of Orally Administered Icariin: Randomized, Double-Blind, Placebo-Controlled Trial. 2019. DOI 10.1177/1934578X19856789. Read the source
2. Cureus. Herba Epimedium (Horny Goat Weed) Toxicity With Severe Muscle Spasms and Elevated Creatine Kinase and Creatinine: A Case Report. 2025. PMID 40546602, DOI 10.7759/cureus.84608. Read the source
3. Memorial Sloan Kettering Cancer Center, About Herbs. Epimedium (Adverse Reactions, Case Reports and Herb-Drug Interactions sections). 2023. Read the source
4. Operation Supplement Safety, Consortium for Health and Military Performance, US Department of Defense. Horny Goat Weed in Dietary Supplements (reported side effects list). 2024. Read the source
5. Operation Supplement Safety, Consortium for Health and Military Performance, US Department of Defense. Horny Goat Weed in Dietary Supplements (adulteration with prescription erectile dysfunction drugs). 2024. Read the source
6. Nutrition and Dietary Supplements. Cross-Sectional Survey of Adulterated Sexual Enhancement Products Sold in the Sacramento Area of California. 2023. DOI 10.2147/NDS.S409032. Read the source
7. Frontiers in Medicine. The efficacy and safety of Epimedium in the treatment of primary osteoporosis: a systematic review and meta-analysis. 2025. PMID 41048931, DOI 10.3389/fmed.2025.1675160. Read the source
8. Frontiers in Pharmacology. Icariin: from preclinical promise to clinical reality—a critical evidence-based assessment. 2026. DOI 10.3389/fphar.2026.1832264. Read the source
9. Operation Supplement Safety, Consortium for Health and Military Performance, US Department of Defense. Horny Goat Weed in Dietary Supplements (opening sections). 2024. Read the source