Epimedium (horny goat weed)

What is epimedium?

Epimedium is a genus of flowering plants used in traditional Chinese medicine, where it is known as yin yang huo. In the West it is marketed as “horny goat weed,” a name drawn from a legend about goats becoming more sexually active after grazing on it. The leaves contain flavonoids, especially icariin, which is often used as a marker compound in extracts.

What the research says

Most research on epimedium is laboratory and animal work on icariin and its metabolites. A 2022 review explained that after oral intake, most icariin is converted by gut bacteria into icariside II. In laboratory and animal studies, icariin and its derivatives inhibited PDE5 (the enzyme targeted by drugs such as sildenafil), promoted testosterone synthesis and affected pathways involved in repairing erectile tissue (Niu et al., 2022). These are mechanistic findings; they do not show that an epimedium supplement improves erections in men.

We found no well-designed randomized trials of epimedium alone for erectile dysfunction, libido or prostate health. A broad meta-analysis of herbal supplements for erectile dysfunction did not identify epimedium monotherapy trials among its included studies, highlighting how thin the human evidence is (Borrelli et al., 2018).

Claims that epimedium supports “your health and your immune system” or the prostate are not based on clinical data.

How it’s used in supplements

Horny goat weed is a staple of male enhancement products, usually as a leaf extract standardized to a percentage of icariin. Many labels list the plant amount without the icariin content, so actual potency is unknown.

Short-term use is generally reported as well tolerated, but safety data in humans are limited. Higher doses have been associated with dizziness, dry mouth, nosebleeds and rapid heartbeat in reports. Because icariin may act on PDE5 and blood vessels, combining epimedium with erectile dysfunction drugs, nitrates or blood pressure medicines could add to their effects. It may also have blood-thinning and hormone-like effects. Avoid it in pregnancy, and talk to a doctor if you have heart disease, a bleeding disorder or a hormone-sensitive condition.

The bottom line: epimedium is popular because its main compound behaves like a weak version of erectile dysfunction drugs in the laboratory, but that has not been confirmed in good human trials, and product potency is rarely disclosed. Erectile problems can be an early warning sign of heart disease or diabetes, so they are worth discussing with a doctor rather than self-treating with herbal blends.

Icariin: the compound behind the horny goat weed claims

Icariin is the flavonoid most often used to standardize epimedium extracts, and labels typically list it as a percentage (for example, “standardized to 10% icariin”). Its reputation comes from its effect on PDE5, the enzyme blocked by erectile dysfunction drugs. In a test-tube study using the human enzyme, icariin did inhibit PDE5, but it needed a concentration of about 5.9 micromolar to block half of the enzyme activity, while sildenafil needed about 74 nanomolar, making icariin roughly 80 times weaker (Dell’Agli et al., 2008). The researchers had to chemically modify icariin to approach sildenafil’s potency, and that modified compound is not what is in a supplement.

In other words, the lab findings are real but modest, and they do not tell us what a capsule of horny goat weed does in a person. For the bigger picture on ingredients marketed for stamina and libido, see our men’s vitality hub and the topic page on low stamina and libido after 40.

How much horny goat weed per day?

There is no established dose for epimedium or icariin. The best-designed human trial we found was not about sexual function at all. In a 24-month randomized, double-blind, placebo-controlled trial of 100 late postmenopausal women with low bone density, a daily dose of epimedium-derived flavonoids (60 mg icariin plus 15 mg daidzein and 3 mg genistein, with 300 mg calcium) helped maintain spine bone density, while the placebo group lost bone. Serum estrogen and the uterine lining did not change (Zhang et al., 2007). That is a single trial with a purified, defined extract, so it cannot be applied to typical “horny goat weed” capsules.

Practical label tips:

• Look for the icariin content in milligrams, not just the leaf or extract weight.
• Be wary of proprietary blends that hide the epimedium amount.
• More is not better: side effects reported with horny goat weed have tended to involve high intake.

Safety concerns beyond the label

Heart rhythm and mood. Doctors have published a case of rapid, irregular heartbeat and hypomania (an elevated, agitated mood state) in a man taking a horny goat weed product (Partin and Pushkin, 2004). A single case does not prove cause, but it is a reason for caution if you have a heart rhythm problem or bipolar disorder.

Hidden drugs. The FDA has repeatedly found “natural” sexual enhancement products that contain undeclared prescription drug ingredients such as sildenafil or tadalafil, which can dangerously lower blood pressure when combined with nitrates (FDA: tainted sexual enhancement products). Products that work “in minutes” are a red flag.

Interactions and pregnancy. Because icariin acts on PDE5 and blood vessels and may have hormone-like effects, avoid combining epimedium with erectile dysfunction drugs, nitrates, blood pressure medicines or blood thinners unless your doctor approves. Do not use it during pregnancy or breastfeeding. Erectile problems can be an early sign of heart disease or diabetes, so talk to your doctor before trying herbal blends that also contain ingredients like tribulus or tongkat ali.

Frequently asked questions

What is horny goat weed good for?

Horny goat weed (epimedium) is mainly marketed for libido and erections. Its main compound, icariin, blocks the PDE5 enzyme in lab studies, but about 80 times more weakly than sildenafil, and there are no well-designed human trials showing it improves erections. One 24-month trial of a purified epimedium flavonoid extract helped maintain bone density in postmenopausal women.

What is icariin?

Icariin is a flavonoid found in epimedium leaves and is the marker compound used to standardize horny goat weed extracts. After you swallow it, gut bacteria convert much of it into related compounds such as icariside II. In test-tube and animal studies it affects PDE5, nitric oxide and testosterone pathways, but human evidence is limited.

How much horny goat weed should I take?

There is no established dose. The longest controlled human trial used 60 mg of icariin daily, combined with small amounts of soy isoflavones, for 24 months in postmenopausal women. Many supplements do not disclose icariin content at all. Follow the label, avoid high doses, and ask your doctor first if you take any heart or blood pressure medicine.

Does horny goat weed work like Viagra?

Not in any proven way. Icariin inhibits the same enzyme as sildenafil (Viagra) in the lab, but roughly 80 times more weakly, and good human trials are missing. If a horny goat weed product works dramatically and fast, be suspicious: the FDA has found many sexual enhancement supplements secretly spiked with prescription drugs.

What are the side effects of horny goat weed?

Reported side effects include dizziness, dry mouth, nosebleeds and rapid heartbeat, mostly at high doses. A published case described a fast irregular heart rhythm and hypomania in a man using horny goat weed. It may add to the effects of blood pressure drugs, nitrates, erectile dysfunction drugs and blood thinners, and it should be avoided in pregnancy.

Is horny goat weed safe for women?

Short-term use appears to be tolerated in studies, and one two-year trial in postmenopausal women found no change in estrogen levels or the uterine lining with a purified extract. Still, epimedium may have hormone-like effects, so avoid it during pregnancy or breastfeeding and if you have a hormone-sensitive condition, and talk to your doctor before use.

Can I take horny goat weed with blood pressure medication?

Check with your doctor first. Icariin acts on PDE5 and blood vessels, so epimedium could add to the blood pressure lowering effect of antihypertensive drugs, nitrates and erectile dysfunction medicines. It may also affect bleeding, so the same caution applies to blood thinners. Hidden drug ingredients in some sexual enhancement products add further risk.