YK-11

YK-11 is a lab-made compound sold as a SARM (selective androgen receptor modulator — a drug designed to switch on the body’s muscle-building signals) and pitched as a way to add muscle. The marketing aside, it is not a peptide. It is a steroid-style molecule built on a 19-norandrostane backbone (a steroid framework similar to one of the body’s own hormones). Its reputation as an unusually strong “myostatin-inhibiting SARM” comes almost entirely from two lab-dish studies. There are no human trials testing whether it works or whether it is safe. It is an unapproved drug under FDA rules, and it is banned at all times in sport.

What it is

YK-11 is a synthetic steroidal SARM. Chemically, it is a gem-dimethyl/dienone derivative on a 19-norandrostane (norprogesterone-like) backbone — meaning its core structure resembles a natural steroid hormone, with two specific chemical groups added. Its molecular formula is C25H34O6 and its registry number (CAS) is 1370003-76-1. That structure sets it apart from the better-known non-steroidal SARMs like andarine and ostarine, which are built on a completely different chemical scaffold (the aryl-propionamide family). It is not a peptide. In the original 2011 work, it behaved as a partial, gene-selective agonist of the androgen receptor — meaning it turned the receptor on only partway and only for certain genes. It sped up the receptor’s movement into the cell nucleus (AR nuclear translocation) but did not trigger the internal “folding” step (the N/C-terminal interaction) that a hormone needs to fully activate the receptor.

The “myostatin inhibitor” label comes from a 2013 study done in mouse muscle cells (C2C12 myoblasts) growing in a dish. In that study, YK-11 nudged the cells to mature into muscle (myogenic differentiation) and raised levels of follistatin — a protein the body makes to block myostatin, which is the molecule that normally puts the brakes on muscle growth. When researchers added an antibody that neutralized follistatin, the muscle-building effect disappeared. That tells us the effect is indirect: YK-11 activates the androgen receptor, which raises follistatin, which then dials down myostatin signaling — rather than YK-11 grabbing and blocking the myostatin protein itself. Some second-hand sources loosely call YK-11 a direct “myostatin inhibitor,” but the original data point to the indirect follistatin route. In that same cell model, YK-11 switched on muscle markers more strongly than DHT (a potent natural androgen).

The claims

YK-11 is sold through “research chemical” and SARM channels for muscle growth, strength, and body recomposition (losing fat while gaining muscle). It is often hyped specifically as a myostatin-inhibiting SARM that delivers gains beyond ordinary SARMs or DHT. All of these claims rest almost entirely on cell-dish data.

What the evidence actually shows

There are no human clinical trials of YK-11. No published controlled studies in people have tested whether it works or whether it is safe. The direct biological evidence boils down to two lab-dish papers (Kanno 2011 on the androgen receptor; Kanno 2013 on follistatin and mouse muscle cells), a few follow-up cell studies, and one mouse study in a model of sepsis (a severe bloodstream infection) that reported lower inflammation markers. This is early, mechanism-only research. It does not show that YK-11 builds muscle or is safe in people.

The other sizable body of work is about doping control — that is, detecting the drug, not testing its benefits. A 2018 study mapped out how YK-11 breaks down in urine (its urinary metabolites) so testing labs can spot it, and a 2024 report described YK-11 turning up in a real doping-control sample. That work confirms people do use YK-11 and that it can be detected. It says nothing about whether it helps. Bottom line: the muscle and strength benefits claimed for people are unproven, and jumping from mouse muscle cells in a dish to real human body composition is not backed by any trial.

Legal and regulatory status

In the US, SARMs are not legal ingredients in dietary supplements. The FDA treats products marketed as SARMs as unapproved drugs, not supplements, and it has sent warning letters and pursued criminal cases against sellers. The FDA has tested a product listing YK-11 as part of an adverse-event report involving a consumer who had a stroke.

As of June 2026, SARMs (including YK-11) are not federally scheduled controlled substances. Two “SARMs Control Act” bills (S.2742 in the 115th Congress and S.2895 in the 116th Congress) proposed adding SARMs to Schedule III, but neither became law. Enforcement happens through the FDA’s authority over drugs and mislabeling, not through the Controlled Substances Act. By contrast, oxandrolone is a genuine FDA-approved anabolic-androgenic steroid and a Schedule III controlled substance under the Anabolic Steroid Control Act — but YK-11 does not fall under that anabolic-steroid scheduling.

In sport, YK-11 is banned at all times under the WADA Prohibited List, Section S1.2 “Other Anabolic Agents.” It is one of six SARMs named outright (andarine, enobosarm/ostarine, LGD-4033/ligandrol, RAD140, S-23, and YK-11), and SARMs as a group are banned (as non-Specified Substances) even when not individually listed. For comparison, GW-501516 (cardarine) and SR9009 (stenabolic) work through different pathways (PPARδ/Rev-erb) and are banned under a different section, S4.5, rather than S1.2.

Safety

There are no human safety data specific to YK-11. Direct evidence of risk from the compound itself is essentially nonexistent. The concerns below are inferred from the wider SARM class and from how an androgenic/steroidal molecule is expected to behave.

• Liver injury: SARMs as a group are linked to drug-induced liver injury (damage that shows up as blocked bile flow or harm to liver cells) in published case reports. LiverTox describes a young patient who developed jaundice (yellowing of the skin and eyes) after taking three SARMs, including YK-11, and a systematic review found one YK-11 case among its SARM liver-injury cases; the injuries generally cleared up after people stopped, with no deaths in those reports. The FDA warns that SARM products in general can cause liver injury, including cases that led to hospitalization.
• Testosterone suppression / endocrine effects: Drugs that activate the androgen receptor suppress the HPG axis (the hormone loop between the brain and the testes that controls natural testosterone). The FDA lists testicular shrinkage, infertility, and sexual dysfunction among SARM risks. This is expected for YK-11 based on how it works, but it has not been measured in people.
• Lipids / HDL: Androgen-receptor drugs commonly lower HDL (“good” cholesterol), and this is documented for SARMs as a group. There are no published human cholesterol data for YK-11 specifically — it is plausible but unverified for this compound.
• Cardiovascular and other: The FDA links SARM products to a higher risk of heart attack and stroke, plus psychosis and sleep problems.

For related compounds (not YK-11 itself): andarine (S-4) has well-documented, reversible vision side effects — a yellow tint to vision and trouble adjusting to night or bright light. GW-501516 (cardarine) caused tumors in several different organs during long-term cancer-testing studies in rodents, which ended its drug development around 2006–2007. No reliable published study ties YK-11 itself to QT prolongation (a heart-rhythm change) or blindness.

Bottom line

Almost everything promotional about YK-11’s muscle benefits in people is an extrapolation from cell-dish studies — there are no human trials. Class-level safety risks like lower HDL and suppressed testosterone are biologically expected for YK-11, but they have not been directly measured in humans for this specific compound, so they remain inference, not established fact. YK-11 is an unapproved drug, not a supplement, and it is banned at all times in sport.

Evidence grade: 3/10 · Animal only.

Sources

• Kanno Y, et al. YK11 is a partial agonist of the androgen receptor. Biol Pharm Bull. 2011;34(3):318-23. PMID 21372378
• Kanno Y, et al. YK11 regulates myogenic differentiation of C2C12 myoblasts by follistatin expression. Biol Pharm Bull. 2013;36(9):1460-5. PMID 23995658
• Lee SJ, et al. Myostatin inhibitor YK11 as a preventative health supplement for bacterial sepsis. BBRC. 2021. PMID 33588136
• Piper T, et al. Studies on the in vivo metabolism of the SARM YK11. Drug Test Anal. 2018;10(11-12):1646-1656. PMID 30379415
• Sobolevsky T, et al. Detection of YK-11 in a doping control sample. Drug Test Anal. 2024. PMID 37946705
• FDA. Certain Bodybuilding Products Put Consumers at Risk (SARMs)
• FDA. FDA Warns of Use of SARMs Among Teens, Young Adults
• USADA. Selective Androgen Receptor Modulators (SARMs) — Prohibited Class: Anabolic Agents
• WADA Prohibited List
• SARMs Control Act of 2018 (S.2742, 115th Congress)
• SARMs Control Act of 2019 (S.2895, 116th Congress)
• SARMs-Induced Liver Injury: A Case Report and Review of Literature. PMC10024817
• LiverTox: Selective Androgen Receptor Modulators. NBK619971
• Nash E, et al. Drug-induced liver injury from SARMs, AAS and bodybuilding supplements in Australia. Aliment Pharmacol Ther. 2024;59(8):953-961