Icariin: The Lab Chemistry Behind Horny Goat Weed

Why "ingredient one" is a labeling fact, not a ranking

SodaPeak's Supplement Facts panel lists a 1,200 mg "Test Boost Proprietary Blend" containing eight ingredients, with no individual milligram amount printed for any of them. Under standard U.S. dietary-supplement labeling practice, ingredients inside a proprietary blend are listed in descending order by weight — the first name is whatever makes up the largest share of that 1,200 mg, and the last name is whatever makes up the smallest. Horny goat weed is first. That tells you something about the recipe. It tells you nothing about which ingredient is best-studied, most effective, or safest — those are separate questions, and this article is about the research question only: what has actually been measured about horny goat weed's active compound, and under what conditions.

What Epimedium sagittatum is

The label names the species precisely: Epimedium sagittatum, leaf. Epimedium is a genus of small flowering plants native to East Asia, and several species within it — E. sagittatum, E. brevicornum, E. koreanum, and others — have a long history in traditional Chinese medicine, where the dried leaf is known as yin yang huo. English-language marketing renamed it "horny goat weed" sometime in the 20th century; the plant itself predates that name by centuries. What all the Epimedium species share is a family of compounds called prenylated flavonoids, and the one that shows up again and again in the research literature is icariin.

Icariin, the compound that actually gets studied

Almost none of the laboratory research on Epimedium tests the whole dried leaf. It tests icariin, or one of its metabolites, isolated and purified, dissolved at a known concentration, and applied directly to cells, tissue, or an animal. That distinction matters for reading any of the studies below: they tell you what the isolated molecule does at a measured dose in a controlled system, not what a capsule containing an unspecified share of a 1,200 mg blend does inside a person.

A 2022 study published in Chinese Medicine isolated six flavonoid compounds directly from processed Epimedium sagittatum leaf — the same species named on SodaPeak's own label — and tested each one's ability to inhibit an enzyme called phosphodiesterase type 5, or PDE5, in the lab.² Icariin was the first of the six compounds isolated, which is one small piece of confirmation that the chemistry described in the wider literature is present in the actual species this label names, not just a cousin species.

What happens to icariin after you swallow it

One detail from a 2022 narrative review in Translational Andrology and Urology is worth sitting with before anything else: 91.2% of ingested icariin is converted into a different compound, icariside II, by enzymes from bacteria living in the intestine, before it ever reaches the bloodstream in its original form.¹ That review — titled, tellingly, "Deciphering the myth of icariin and synthetic derivatives in improving erectile function from a molecular biology perspective" — summarizes two decades of research on icariin and its lab-synthesized derivative YS-11 from a group of urology researchers in China. Its own framing is a useful check on marketing copy: the title calls out "myth" directly, and the paper's conclusion recommends "more clinical and basic researches with high quality and large samples," which is a polite way of saying the case isn't closed.

The PDE5 finding, species-matched to this label

PDE5 is the same enzyme family that prescription drugs like sildenafil work on — inhibiting it allows a signaling molecule called cGMP to accumulate, which relaxes smooth muscle in blood vessel walls. In the 2022 Chinese Medicine study on Epimedium sagittatum, three of the six isolated flavonoids — including icariin itself — showed measurable PDE5 inhibition in a cell-free enzyme assay, with icariin's potency reported as an IC50 of roughly 8.3 micromolar (the concentration needed to inhibit half of the enzyme's activity in that assay).² The same three compounds also increased cGMP and reduced calcium levels inside rat corpus cavernosum smooth muscle cells — the type of tissue involved in erectile physiology — when applied directly to those cells in a dish.

That is a real, specific, peer-reviewed finding. It is also, by its own methods section, an in-vitro enzyme assay and a rat cell-culture experiment. Micromolar concentrations applied directly to isolated cells are not the same thing as milligrams swallowed in a capsule, digested, partly converted to icariside II by gut bacteria, absorbed at some efficiency nobody has published for this exact product, and circulated through a human body. No step in that chain has been measured for SodaPeak specifically, and the label doesn't disclose how many of the blend's 1,200 mg is horny goat weed at all.

The testosterone-synthesis claim, and where it comes from

The same 2022 Translational Andrology and Urology review states that icariin and its derivatives showed "specific inhibition on phosphodiesterase type 5 (PDE5) and the promotion of testosterone synthesis" across the studies it summarized, along with effects on cell-signaling pathways (PI3K/AKT, TGFβ1/Smad2, p38/MAPK, Wnt) linked to tissue repair and regeneration.¹ Read the review's own conclusion, though, and the hedging is explicit: icariin and its derivatives "could be a potential treatment for restoring spontaneous erections," and the authors call for more research, not less, before that potential becomes a clinical recommendation. A separate 2021 review in the Journal of Ethnopharmacology, which screened 97 medicinal plants for PDE5-inhibiting activity, singled out icariin and icariside II — isolated from a related species, Epimedium brevicornum — as "the most effective inhibitor, till date, from natural sources," while noting that the entire 97-plant review "is supported by preclinical experimental evidence."⁵ Preclinical is the operative word again.

Zooming out: what review articles say about herbal ED treatments

It helps to step back from any single compound and ask what the broader research landscape looks like. A 2011 review in Current Urology Reports surveyed every herbal medicine that had been studied for erectile dysfunction at that point and found a short list: Panax ginseng, Butea superba, Epimedium herbs (icariin), Tribulus terrestris, Securidaca longipedunculata, Piper guineense, and yohimbine.³ Of those, the review states plainly, only Panax ginseng, Butea superba, and yohimbine had any published human trials at all — and even those trials were, in the authors' own words, "not robust." Epimedium/icariin was not among the three with human data; its evidence base at that point was animal-model, laboratory work.

A separate 2015 review in the Journal of Sexual Medicine took a different approach: it identified the actual top-selling nutraceutical products marketed for men's sexual health and catalogued which ingredients showed up most often — horny goat weed among them, alongside ginseng, tribulus, zinc, B-complex vitamins, fenugreek, L-arginine, maca, DHEA, ginkgo, and yohimbine.⁴ Its conclusion about the category as a whole, not any one ingredient: "the currently available medical literature evaluating the efficacy of these substances is generally of low quality." That's not a statement specific to SodaPeak. It's a statement, from a peer-reviewed urology journal, about the entire aisle this bottle sits in.

Why researchers keep making synthetic copies of it

One detail that tells you how seriously the pharmacology community takes icariin's chemistry, even without human trials: researchers have synthesized derivative compounds from it specifically to study and improve on its activity. The 2022 Translational Andrology and Urology review discusses icariside II — the metabolite gut bacteria convert most ingested icariin into — and a further lab-made derivative called YS-11, built from icariside II to test whether a modified structure could act more selectively on the target tissue.¹ That kind of medicinal-chemistry work, where a plant compound becomes the starting point for a synthesized analogue, is standard practice across pharmacology — it's how a number of prescription drugs began. It is also, again, lab and animal work. YS-11 is not an ingredient in any dietary supplement, including SodaPeak; it's mentioned here only because its existence shows how much structural chemistry has already been done on icariin's scaffold, well ahead of any clinical proof for the parent compound itself.

It's worth being precise about what "inhibits PDE5" means in context, too. Prescription PDE5 inhibitors such as sildenafil went through the standard drug-development pipeline: isolated compound, animal pharmacology, dose-finding, and multiple phases of controlled human trials before approval, with an established human dose, a defined half-life, and a documented side-effect profile. Icariin's PDE5 data sits at the enzyme-assay and animal-tissue stage of that same pipeline — the first step, not the last one. Sharing a mechanism of action with an approved drug class is chemically interesting; it is not the same as having gone through the rest of that pipeline.

What none of this shows

Put together, here is what the cited research does not establish:

• No study here gave human subjects an oral dose of horny goat weed, icariin, or any Epimedium extract and measured a human outcome — erectile function, muscle, strength, or otherwise. The evidence is enzyme assays, isolated cells, and rat tissue.
• No study establishes what oral dose, if any, would reproduce the micromolar concentrations that worked directly on cells in a dish. Bioavailability, first-pass metabolism, and the 91.2% conversion to icariside II all stand between a capsule and a cell-culture result.
• SodaPeak's own label does not disclose how much of its 1,200 mg blend is horny goat weed. First-listed by descending weight order means "largest of eight," not a known milligram figure that could be compared with any study's dose.
• Nothing here is a claim about muscle. Every citation above concerns erectile physiology research, not the "supports healthy muscle*" claim printed on SodaPeak's front label — those are different physiological systems, and this article does not connect them.

Reading SodaPeak's own label against this research

SodaPeak's Supplement Facts panel prints the 1,200 mg Test Boost Proprietary Blend total, the eight ingredient names in descending order, and the footnote "*Daily Value Not Established." It does not print a milligram figure for horny goat weed, and it does not cite any of the studies above — nor is it required to; U.S. dietary-supplement labels are not required to cite the research literature behind a botanical ingredient. That's precisely why an independent look at what has and hasn't been studied is useful: the label tells you what's in the blend and how it's ordered, and separately, the peer-reviewed literature tells you what's been measured about that ingredient's chemistry, in what kind of study, and how far that evidence actually reaches.

Bottom line

Horny goat weed is the heaviest-weighted of SodaPeak's eight blend ingredients, and its main compound, icariin, is a genuinely studied molecule — inhibiting PDE5 in cell-free assays, altering signaling in rat cavernosal tissue, and drawing attention across multiple peer-reviewed reviews as a natural-source candidate worth further study. All of that sits on a preclinical foundation: enzyme assays, cell cultures, and animal tissue, not human trials, and not a dose anyone has matched to what's actually in this bottle. That's not a reason to dismiss the ingredient. It's a reason to read "horny goat weed" on a label as a name attached to real, specific, still-early-stage science — not as a finished, human-proven result.