Ingredient deep-dive

Icariin Is Not Sildenafil, And The Difference Is Four Orders Of Magnitude

Horny goat weed genuinely does contain a compound that inhibits the same enzyme the prescription drugs inhibit. That finding is real, it is published, and it is the reason the comparison keeps being made. Here is what happens to it on the way from a test tube to a person.

By VigorHorse Team · Last updated November 4, 2026 · 8 min read · Ingredient deep-dive · Reviewed by Dr. Edmund Ko, MD

The comparison that keeps being made

Horny goat weed, Epimedium, is the most scientifically interesting ingredient in this aisle and the most consistently oversold, and both of those come from the same piece of real science.

The claim you will meet, in various phrasings, is that it is a natural version of the prescription drugs used for erectile dysfunction. Unlike most claims in this category, that one has an actual paper behind it rather than a tradition.

It is worth walking the argument step by step, because it is one of the few in this aisle where the first step is sound and every subsequent step is where the trouble lives.

What the enzyme study actually found

Dell'Agli and colleagues tested icariin against human phosphodiesterase-5 and found potent inhibition. Phosphodiesterase-5 is the enzyme that breaks down cyclic GMP, the signalling molecule that keeps vascular smooth muscle relaxed. Inhibit it and the signal persists longer.

That is the same target the prescription drugs act on, and it is the same mechanism. The paper is not a fringe finding and it is not being misrepresented when someone says icariin inhibits PDE-5. It does.

The word doing the work in that sentence is potent, which in an enzyme assay is a relative term with a number behind it. The number is where the argument turns.

What micromolar means next to nanomolar

Enzyme inhibition is quoted as a half-maximal inhibitory concentration: the concentration at which the compound shuts down half the enzyme's activity. A lower figure means a more potent compound.

Icariin's figure in that work was 5.9 micromolar. Sildenafil's is in the low nanomolar range. A micromole is a thousand nanomoles, so the gap is roughly four orders of magnitude, a factor of around ten thousand.

Four orders of magnitude is not a technicality to be waved through. It means you would need something in the region of ten thousand times the concentration of icariin to achieve what the prescription compound achieves at the same site, in glassware, before any question of getting it there has been raised.

An order of magnitude is a factor of ten. Four of them is a factor of ten thousand, and that gap is before absorption is considered at all.

The compounds that did match were not icariin

Follow-up work explored the same activity further and produced the detail most often left out of the retelling.

The compounds that approached the potency of a prescription drug in those experiments were synthetic derivatives of icariin: molecules built in a laboratory by modifying the natural structure specifically to improve its activity. That is medicinal chemistry working exactly as intended, and it is a genuinely promising line of research.

It is also not what is in any jar. A derivative synthesised to be more potent than its parent compound is a different substance from the parent compound, and the results belonging to it do not belong to the plant.

The step that settles it

Even granting the enzyme result, one question decides whether it can matter to somebody swallowing a capsule: does the compound reach the tissue at a concentration anywhere near the assay's?

The pharmacokinetic study measured prenylflavonoid concentrations following oral administration and found them low. Icariin is poorly absorbed and heavily metabolised, which is a common fate for flavonoid glycosides and the reason so many promising in vitro results never become anything.

So the chain has two breaks rather than one. The compound is roughly ten thousand times weaker than the comparison at the target, and an oral dose delivers only a small fraction of what is swallowed to the bloodstream. Those two gaps multiply.

The first row is solid and interesting. Each row below it is a step the marketing takes and the literature has not.

The one human trial, and what it studied

There is a placebo-controlled human trial of the compound, which is more than muira puama or catuaba can claim. It is worth reading and it is worth being precise about.

It was a pseudo-randomised crossover trial, and it ran in a different condition entirely, not in male sexual function. Its existence tells you the compound has been given to people under controlled conditions and studied. It does not tell you anything about the use this ingredient is sold for.

A systematic review of the genus gathers traditional uses, phytochemistry, pharmacology and toxicology together, and is the best single place to start for the whole picture rather than the headline. What none of this literature contains is a controlled trial of horny goat weed for erectile function at any dose.

Why that gap has not been filled

It is a fair question, given how much of this ingredient is sold every year. Part of the answer is that nobody owns it: a plant extract cannot be patented in the way a synthesised derivative can, so the commercial case for funding an expensive trial sits with whoever made the derivative rather than whoever sells the powder.

The rest of the answer is the absorption data. An investigator reading the pharmacokinetic work before designing a trial would have to explain how an oral dose was expected to reach the concentration the assay needed, and that question does not currently have a good answer.

Where that leaves a name on a blend line

Horny goat weed extract is the ninth name on this jar’s blend line, sharing 82 mg with eight others and carrying no figure of its own.

Stack the three gaps and the position is clear enough. There is no human trial for this use at any dose, so there is no dose to fall short of. The enzyme result is four orders of magnitude weaker than its comparison. Oral absorption is poor. And the amount present is a share of 82 mg.

The fair statement is that this ingredient's presence is traditional and its science is interesting, and that neither of those is a prediction about a gummy. That is how it is described on the ingredients page too.

Why this matters more in this aisle than most

There is a sharper reason to be careful with the natural-sildenafil framing than mere accuracy.

Published work on adulteration has repeatedly found prescription PDE-5 inhibitors and their analogues inside products sold as herbal and natural sexual performance supplements. A product that produces a drug-like effect on a drug-like timescale from a plant extract is, on the published record, more likely to contain a drug than to be an unusually good plant extract.

So when a botanical is marketed as working like the prescription drug, the claim is either false or alarming, and there is no third option that favours the buyer. Anybody avoiding a prescription because of a nitrate they take for their heart has a concrete reason to care which it is.

• Icariin inhibiting PDE-5 in an assay is established and interesting.
• The potency gap to the prescription comparison is roughly ten thousandfold.
• The compounds that closed that gap were synthetic derivatives, not the plant.
• Oral absorption of prenylflavonoids is poor, which breaks the chain a second time.
• No controlled human trial exists for this use, so no dose can be said to be adequate.

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