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Not medical advice. The Atlas is not a substitute for a clinician who can examine you.

Horny Goat Weed

Epimedium spp. (Yin Yang Huo, icariin)

Evidence grade 2 of 5, Thin. Read how the Atlas grades evidence.

The written rationale for this grade is still being prepared.

FDA
Not FDA evaluated (dietary supplement)

Reference only. Not medical advice.

What can go wrong

Generally tolerated; can cause dizziness, dry mouth, and, in theory, the same caution as other PDE5-active agents regarding nitrates and blood-pressure medications. Adulteration of 'natural' sexual-health products with undeclared synthetic PDE5 inhibitors is a documented industry problem.

This is something to take to a clinician. The Atlas can tell you what is known about horny goat weed in general. It cannot tell you what is happening in your body, and the difference matters most for exactly the symptoms that bring people to this page.

A Traditional Chinese Medicine herb whose active compound, icariin, is a natural phosphodiesterase-5 (PDE5) inhibitor, the same enzyme target as sildenafil, though far weaker. Its plausible mechanism is therefore for erectile function (grade 2) rather than for raising testosterone. Human data are limited, and product icariin content varies widely.

Contains icariin, a weak natural PDE5 inhibitor, more about erections than testosterone.

What people take it for

These are the reasons people give for taking it. They are not effects it has been shown to have.

  • Erectile function (mechanistic basis)
  • Libido
  • Reported vitality/energy
  • Bone-health claims (separate, preliminary)

How it is supposed to work

Icariin inhibits PDE5, slowing the breakdown of cyclic GMP in penile smooth muscle and supporting erections, mechanistically parallel to pharmaceutical PDE5 inhibitors but with much lower potency. Testosterone effects in humans are not well established.

A real mechanism, weak in practice

The PDE5 mechanism is genuine, which is why the herb is plausibly active for erections, but icariin is a far weaker inhibitor than sildenafil, oral bioavailability is poor, and finished products rarely disclose standardized icariin content, so real-world potency is uncertain.

What people get wrong

  • Often saidHorny goat weed boosts testosterone

    ActuallyIts established mechanism is weak PDE5 inhibition, relevant to erections, not a testosterone increase.

  • Often saidIt's a natural Viagra of equal strength

    ActuallyIcariin is far weaker than sildenafil and poorly absorbed; equivalence claims are unfounded.

Who profits, and who pushes back

  • Sexual-health supplement marketers

    Sells this. Has a commercial interest in a product sold as this compound, as of July 2026.

    Promotes itBroad

    Heavily promoted; note recurring adulteration concerns in this product class

How we got here

Epimedium (Yin Yang Huo) has been used in Traditional Chinese Medicine for centuries for sexual and kidney-yang complaints; folklore attributes its name to a goat herder observing increased mating activity in animals grazing on it. Modern chemistry identified icariin as the principal active flavonoid.

  1. AntiquityTCM use as Yin Yang Huo
  2. ModernIcariin identified as a PDE5 inhibitor
  3. 2000s-nowMass-market sexual-health supplement

At a glance

Tradition
Traditional Chinese Medicine
Active compound
Icariin (flavonoid)
Mechanism
Weak natural PDE5 inhibition
Best-supported angle
Erectile function, not testosterone

What this entry does not yet cover

What follows is unfinished work on our side, not a finding about Horny Goat Weed. A short entry is not a safe entry.

  • Cost and market. We have not documented what it sells for or who sells it.
  • Supply chain. We have not mapped where it is made.
  • Who uses it. We have not documented the user groups.
  • What could change. No forward-looking summary yet.
  • How it compares. We have not set it against the alternatives.
  • Children and teenagers. We have not checked what is known about younger people.
  • Where we land. We have not taken a position on this yet.

Sources

The studies below are named and described, but not linked. You cannot open them from here and check our reading against them, which means this page is our summary of the literature rather than a document you can verify. Weigh it accordingly until the links exist.

The studies that matter

  • Icariin pharmacology (preclinical)various

    Icariin demonstrated as a PDE5 inhibitor in laboratory studies, supporting a mechanistic basis for erectile benefit

  • Human RCTslimited

    Robust human efficacy trials are sparse; most support is mechanistic and traditional