YK-11

What is YK-11?

YK-11 is a synthetic steroid that is sold online as a muscle-building compound. It is not a peptide. It is listed in this directory because it is marketed alongside peptides as a "myostatin inhibitor," and buyers often encounter it in the same stores.

It was first described in the early 2010s by researchers in Japan as a steroidal selective androgen receptor modulator, or SARM. Most SARMs, such as ostarine and RAD-140, are non-steroidal molecules designed to build muscle with fewer effects elsewhere in the body. YK-11 is different: it is a steroid, closely related in structure to the androgens testosterone and dihydrotestosterone (DHT), and is commonly described as a DHT derivative. Anti-doping scientists classify it among the steroidal SARMs.¹

YK-11 has never been tested in a human clinical trial and has no approved medical use anywhere. It is widely sold through internet supplement vendors, which is why anti-doping and food safety laboratories have developed methods to detect it.²

How YK-11 works

Testosterone and related hormones build muscle by binding the androgen receptor inside cells, which switches on muscle-building genes. YK-11 binds the same receptor, but only partly activates it. Its marketing rests on a second claim: in laboratory dishes, YK-11 made mouse muscle cells produce more follistatin, a protein that blocks myostatin. Myostatin normally limits how large muscles can grow, so blocking it could, in theory, allow more growth.

That is the basis of the "myostatin inhibitor" label. But YK-11 does not block myostatin directly, and the follistatin finding comes from cell culture. Whether it happens in a living person at doses people actually take is unknown.

In more precise terms, YK-11 is a synthetic steroid carrying an unusual orthoester and methyl ester structure at carbon 17, which sets it apart from classic anabolic steroids. It is described as a partial androgen receptor agonist that does not trigger the full set of receptor interactions that testosterone does. It is chemically unstable. Analysts found that it breaks down within hours under some conditions, which made it hard to detect intact in supplement samples.⁵ In horses given YK-11 by mouth, researchers identified seven metabolites in plasma and eleven in urine, with breakdown products providing the longest detection window.² There are no published human pharmacokinetic studies.

What the research shows on YK-11

The research on YK-11 falls into two groups, and neither shows benefit in people.

The first is laboratory work. Studies in cultured mouse muscle cells and bone cells reported that YK-11 increased markers of muscle cell development and bone formation, and raised follistatin expression. Animal data are very limited. These results show biological activity, not safety or real-world effect.

The second, and larger, group is detection research. Anti-doping scientists have reviewed methods for catching SARMs, including YK-11, in athletes' urine.¹ A 2023 study mapped how horses metabolize YK-11 so racing laboratories could test for it, noting how easily it can be bought online.² YK-11 has been found in an athlete's doping control sample.³

Food safety testing adds a consumer warning. Korean regulators analyzed 60 dietary supplements bought online and from international vendors in 2020. They found SARMs at high concentrations in 20 products, and found YK-11 in some products where it was not listed on the label.⁵ In other words, some people may be taking YK-11 without knowing it.

There are no human trials, no dose-finding studies and no long-term safety data.

Studied for: Muscle growth research.

YK-11 reference data

YK-11 dosage in published research

How YK-11 is used in practice

YK-11 is sold as capsules or liquid labeled "for research only" or, illegally, as a dietary supplement. Users take it by mouth, usually daily for cycles of several weeks, often stacked with other SARMs. Doses circulating online come from forums and vendors, not from studies.

Product quality is poor across the SARM market. Products may contain a different compound, more or less than the labeled amount, or several undeclared drugs.⁵ YK-11's instability adds another question: what is in the bottle may have partly broken down before it is taken.

No legitimate telehealth service prescribes YK-11. It is prohibited in sport at all times, and athletes have tested positive for it.³

YK-11 risks and side effects

Because YK-11 has never been studied in people, its risks are inferred from its structure, from user reports and from the wider SARM class. As a steroid acting on the androgen receptor, it would be expected to suppress the body's own testosterone production, lower HDL cholesterol and cause androgenic effects such as acne and hair loss in susceptible people. Users also report joint and tendon discomfort and mood changes, including irritability.

SARMs as a class have been linked to cases of drug-induced liver injury, and several have shown cholesterol changes and hormone suppression in human trials. There is no reason to assume YK-11 is safer, and its steroidal structure may make it riskier.

It should be avoided by anyone with liver disease, heart disease, a history of hormone-sensitive cancer, or who is pregnant or under 18. Alcohol and other liver-stressing drugs, including oral anabolic steroids, may add to liver risk. Undeclared contents and contamination are additional, unpredictable hazards.

Reported side effects

Alternatives to YK-11

For building muscle, the proven approach is progressive resistance training with adequate protein and calories. Creatine monohydrate is the best-studied supplement for strength and lean mass, with a strong safety record.

For men with diagnosed hypogonadism, testosterone therapy prescribed and monitored by a clinician is an approved option. For medical muscle wasting, treatment targets the underlying disease. No myostatin-blocking drug is approved, and experimental options such as ACE-031 and follistatin gene therapies remain investigational.

For people worried about age-related muscle loss, regular strength training, enough protein spread across meals, and screening for conditions that cause weakness, such as thyroid disease or vitamin D deficiency, are the evidence-based first steps.

Questions for a clinician

• What evidence applies to my situation, and at what level?
• What established alternatives should I consider first?
• What monitoring would be necessary, and how often?
• Can this be prescribed at all where I live, or only sold for research?