Horny Goat Weed Side Effects: What's Documented, What's Myth, and How Much Is Too Much

Horny goat weed has a reputation as a gentle herb, and for most people who take a normal amount for a few weeks, that reputation holds. The documented side effects are mostly minor: dry mouth, an unsettled stomach, feeling dizzy or hot.

But "most people" is doing some work in that sentence. There are published case reports of a racing heart, of severe muscle spasms that needed a hospital stay, and of liver injury linked to herbal formulas that contain it. There's also a quieter problem that has nothing to do with the plant: some products sold as horny goat weed have been found to contain hidden prescription drug ingredients.

This guide sorts what's documented from what's repeated, explains how much has actually been studied, and covers the ingredients that usually come with horny goat weed in a capsule, because in a blend those can matter as much as the herb.

The Side Effects People Actually Report

Horny goat weed is the common name for Epimedium, a leafy plant used in traditional Chinese medicine. Its best-known compound is icariin, a flavonoid that relaxes blood vessels in lab studies by blocking an enzyme called PDE5. That mechanism explains a good part of the side effect list: when blood vessels relax, some people feel flushed, warm, dizzy or lightheaded.

The U.S. military's supplement safety program, Operation Supplement Safety, groups the reported effects into several buckets (OPSS, 2024):

• Stomach upset, nausea, vomiting and diarrhea.
• Shortness of breath, chest pain, and a rapid, irregular heartbeat.
• Behavioral changes such as irritability and feelings of aggression.
• Dizziness, dry mouth, fever, headache, muscle spasm, nosebleeds and a painful skin rash.

WebMD's list of the common ones is shorter: dry mouth, nausea, upset stomach and vertigo, with a warning not to drive if you feel dizzy (WebMD). Memorial Sloan Kettering adds that high doses can cause sweating or a feeling of heat, and that formal toxicity and safety evaluations are lacking (MSKCC).

That last point is worth sitting with. Nobody has run a large safety trial that counts how often each of these happens. The lists are built from case reports, poison-control data and traditional-use monographs, so they tell you what can happen, not how likely it is.

The Case Reports Behind the Scary Headlines

Three published cases come up again and again when horny goat weed safety is discussed. They're rare, but each one points at a specific risk.

A racing heart in a man with heart failure. A 66-year-old with congestive heart failure developed shortness of breath, chest pain and a new, symptomatic arrhythmia after two weeks of daily horny goat weed. He was diagnosed with tachyarrhythmia and hypomania, an elevated, agitated mood (Partin & Pushkin, Psychosomatics, 2004). This case is the main reason Memorial Sloan Kettering lists cardiovascular disease as a reason not to take it.

Severe muscle spasms in a healthy 33-year-old. In 2025, emergency physicians reported a man who came in after 10 hours of severe muscle spasms. He had started an Epimedium supplement bought online a month earlier, to improve his mood and energy, and the cramps began around the same time. His creatine kinase (a marker of muscle breakdown) and creatinine (a kidney marker) were both raised, and he was admitted. Both improved in hospital, and he was told to stop the supplement. The authors described it as the first specific report of this reaction (Ledo et al., Cureus, 2025).

A skin reaction. A 77-year-old developed a vasculitic rash with pain and burning after taking horny goat weed together with ginkgo; it cleared when both were stopped (MSKCC).

Individual case reports can't prove cause. They can show a pattern worth respecting, and the pattern here is fairly consistent: problems tended to appear within weeks of daily use, in products whose exact contents and dose weren't verified.

What About the Liver?

This is the part most English-language articles skip. In China, where Epimedium is used in prescribed herbal formulas, researchers have described liver injury as the most commonly reported adverse reaction to the herb in recent years (Wang et al., Biomedical Chromatography, 2024).

The context matters, though. The best-documented human cases involve multi-herb products such as Xianling Gubao, a bone-health formula that combines Epimedium with other herbs, including Psoralea, which has its own liver concerns. One carefully assessed case of severe liver injury from that formula was rated "highly probable" using a standard causality scale (Li et al., Chinese Herbal Medicines, 2024). Lab work has pointed at icaritin, a compound your gut bacteria make from icariin, as one possible culprit, but that comes from cell studies.

So the honest summary is this: there's a liver signal around Epimedium-containing formulas, and it isn't clear how much of it belongs to Epimedium alone. If you have liver disease, or you notice dark urine, yellowing skin or eyes, or pain under your right ribs while taking it, stop and get checked.

Interactions and Who Should Skip It

Memorial Sloan Kettering flags two possible drug interactions, both with the caveat that the clinical relevance is unknown (MSKCC). Prolonged use may induce CYP3A4, a liver enzyme that clears a large share of prescription drugs, which could make some of them less effective. And it may reduce the effect of aromatase inhibitors, hormone-blocking medications prescribed after some cancers.

WebMD adds nitrate heart medications and hormone-based medicines, including birth control, to the list of things to discuss first, and names heart problems, hormone-sensitive conditions, menopause and prostate problems as conditions to tell your doctor about (WebMD).

Putting those together, we'd skip horny goat weed, or clear it with a doctor, if you:

• Have heart disease, an arrhythmia, or low blood pressure, or take blood pressure or nitrate medication.
• Have a hormone-sensitive condition, or take hormone-based medicines.
• Take several prescription drugs, because of the possible CYP3A4 effect.
• Have liver disease.
• Are pregnant or breastfeeding. There's no safety data.
• Have PKU and the product contains phenylalanine (more on that below).

The Adulteration Problem

Some side effects blamed on horny goat weed may not come from horny goat weed. Operation Supplement Safety notes that the FDA has reported products containing horny goat weed, marketed for sexual enhancement and energy, that were adulterated with undeclared prescription drug ingredients (OPSS, 2024).

That's a real safety issue, not a marketing point. Hidden prescription ingredients can drop blood pressure dangerously in people who take nitrate heart medication, and the person taking the capsule has no way of knowing. The practical defense is dull but effective: buy products that list every ingredient with an amount, come from a manufacturer that makes them in a GMP-certified facility, and be suspicious of anything promising effects "in minutes."

How Much Horny Goat Weed Is Too Much?

There's no official upper limit, and no regulator has set a recommended dose. What we have is a small amount of trial data and a lot of label directions.

The best reference point comes from women. In a 24-month randomized trial, 100 late-postmenopausal women took either a daily supplement of Epimedium-derived flavonoids (60 mg icariin, 15 mg daidzein and 3 mg genistein) or a placebo. The Epimedium group kept their bone density while the placebo group lost some, and the researchers found no detectable thickening of the uterine lining, which matters because the flavonoids have weak estrogen-like activity (Zhang et al., Journal of Bone and Mineral Research, 2007). Eighty-five women finished. That's the longest controlled exposure on record, at 60 mg of icariin a day.

Most supplements state their dose as extract weight, and that number alone tells you little. A 1,000 mg extract standardized to 10% icariin delivers 100 mg of icariin; the same weight at 5% delivers 50 mg; an unstandardized extract could deliver almost anything. (We go deeper on that in our horny goat weed dosage breakdown.)

A sensible approach: stick to the label, don't combine two Epimedium products, start with half a serving for the first few days, and take it with food. If side effects show up at a normal dose, a bigger dose isn't the fix.

The Other Ingredients in the Capsule

Very few horny goat weed products are just horny goat weed. Ours isn't either, and the add-ins have side effect profiles of their own. In most blends, including ours, they're present in small amounts, well below the doses used in their own studies, so their effect in either direction is likely modest. One is an exception worth knowing about.

L-phenylalanine. This amino acid is harmless for most people and found in ordinary protein foods. For people with phenylketonuria (PKU), it isn't: without treatment, phenylalanine builds up to harmful levels and can damage the brain (MedlinePlus Genetics). PKU affects about 1 in 25,000 U.S. newborns, and people who have it are used to reading labels for exactly this. If that's you, skip any blend that lists phenylalanine.

Asian ginseng. Its most common side effect is trouble sleeping. It may lower blood sugar and interfere with blood clotting, and experts recommend against it in pregnancy and breastfeeding (NCCIH). Mention it if you take blood sugar or blood-thinning medication.

Saw palmetto. Generally well tolerated, with mild and infrequent digestive symptoms, dizziness or headache, and possibly unsafe during pregnancy or breastfeeding (NCCIH).

Maca, muira puama, L-arginine, longjack and polypody round out our formula at 62.5 mg or less each. None has a notable side effect profile at these amounts, though anyone with a known allergy to a listed botanical should skip the product.

Can Women Take Horny Goat Weed?

Yes, and the longest human trial of Epimedium was run in women. The 24-month bone trial above enrolled late-postmenopausal women, and the flavonoids didn't affect the uterine lining at 60 mg of icariin a day.

Three cautions apply specifically. Epimedium's flavonoids have weak estrogen-like activity, so women with a hormone-sensitive condition, or on hormone-based medicines including birth control, should ask first. Pregnancy and breastfeeding are off the table because there's no safety data, and ginseng and saw palmetto, common in blends, are flagged for both. And many products, ours included, are formulated and labeled with men in mind, so read the full ingredient panel rather than assuming it's the same herb in a different bottle.

Where Soleu's Formula Fits

Our bottle is labeled Men's Vitality Complex. It lists every ingredient with an amount, and the Epimedium extract is standardized, so the icariin dose is the same in every bottle.

That 100 mg of icariin is more than the 60 mg used in the 24-month trial, and it hasn't been tested as a formula in its own clinical trial. If you're new to it, start with one capsule with a meal for the first few days, and stop if you notice a racing heart, chest discomfort, muscle cramps, or dizziness that doesn't pass.

Frequently Asked Questions

The Bottom Line

At label doses, horny goat weed's usual side effects are minor and short-lived. The serious ones, a racing heart, severe muscle spasms, and liver injury, are rare and mostly described in single case reports, but they cluster in people with heart disease, in daily use over weeks, and in products whose contents weren't verified.

If you don't have a heart condition, liver disease or a hormone-sensitive condition, aren't pregnant, and your product lists every ingredient with an amount, it's a reasonable supplement to try carefully. If any of those don't apply, talk to your doctor before the first capsule.

Related Reading

• How Long Horny Goat Weed Takes to Work Depends on Your Gut Bacteria: why icariin works differently from person to person.
• Horny Goat Weed: What an 80x Potency Gap in the Research Actually Means: how the herb works, and how to read an icariin dose.

Sources

• Operation Supplement Safety (U.S. Department of Defense). Horny goat weed in dietary supplements. 2024. opss.org
• WebMD. Horny goat weed: uses, side effects, interactions and warnings. webmd.com
• Memorial Sloan Kettering Cancer Center. Epimedium (About Herbs). Updated 2023. mskcc.org
• Partin JF, Pushkin YR. Tachyarrhythmia and hypomania with horny goat weed. Psychosomatics, 2004. PMID 15546831
• Ledo T, et al. Herba Epimedium (horny goat weed) toxicity with severe muscle spasms and elevated creatine kinase and creatinine: a case report. Cureus, 2025. PMC12182873
• Zhang G, Qin L, Shi Y. Epimedium-derived phytoestrogen flavonoids exert beneficial effect on preventing bone loss in late postmenopausal women: a 24-month randomized, double-blind and placebo-controlled trial. J Bone Miner Res, 2007. PMID 17419678
• Wang J, et al. Integrating metabolomics and bioinformatics to reveal the mechanism of Epimedium-induced liver injury. Biomed Chromatogr, 2024. PMID 38981997
• Li C, et al. Herb induced liver injury by Xianling Gubao Tablets: a case assessed for causality using updated RUCAM and integrated evidence chain. Chinese Herbal Medicines, 2024. PMC11064585
• MedlinePlus Genetics. Phenylketonuria. medlineplus.gov
• National Center for Complementary and Integrative Health. Asian ginseng. nccih.nih.gov
• National Center for Complementary and Integrative Health. Saw palmetto. nccih.nih.gov