History

YK-11 was first characterized by Yuichiro Kanno and colleagues at Toho University. A 2011 paper (Biol Pharm Bull 2011;34(3):318-23) described it as a partial, gene-selective agonist of the androgen receptor that accelerates AR nuclear translocation without inducing the N/C-terminal interaction needed for full transactivation. A follow-up 2013 study reported that in mouse C2C12 myoblasts YK-11 induced myogenic differentiation and increased follistatin, an endogenous antagonist of myostatin — the basis for its later "myostatin inhibitor" marketing.

YK-11 is a synthetic compound sold as a SARM (selective androgen receptor modulator) and promoted for muscle growth. Despite the marketing, it is not a peptide — it is a steroidal molecule built on a 19-norandrostane backbone. Its reputation as a uniquely powerful “myostatin-inhibiting SARM” rests almost entirely on two in-vitro studies; there are no human clinical trials of its efficacy or safety. It is an unapproved drug under FDA rules and 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, molecular formula C25H34O6, CAS 1370003-76-1 — distinct from the non-steroidal aryl-propionamide SARMs such as andarine and ostarine. It is not a peptide. In the original 2011 work it acted as a partial (gene-selective) agonist of the androgen receptor: it accelerated AR nuclear translocation but did not drive the N/C-terminal interaction required for full transactivation.

The “myostatin inhibitor” framing comes from a 2013 in-vitro study in mouse C2C12 myoblasts, where YK-11 induced myogenic differentiation and raised follistatin, an endogenous antagonist of myostatin. The anabolic effect was blocked by an anti-follistatin antibody, implying the mechanism is indirect (AR activation → follistatin → reduced myostatin signaling), not direct binding or inhibition of the myostatin protein. Some secondary sources loosely call YK-11 a direct “myostatin inhibitor,” but the primary data support the indirect follistatin pathway. In that cell model, YK-11 induced myogenic markers more strongly than DHT.

The claims

YK-11 is sold through “research chemical” and SARM channels for muscle growth, strength, and body recomposition, often hyped specifically as a myostatin-inhibiting SARM producing gains beyond ordinary SARMs or DHT. These claims rest almost entirely on cell-culture data.

What the evidence actually shows

There are no human clinical trials of YK-11. No published controlled human efficacy or safety studies exist. The direct biological evidence is essentially two in-vitro papers (Kanno 2011 on AR partial agonism; Kanno 2013 on follistatin and C2C12 myogenesis), follow-on cell work, and a mouse sepsis-model paper reporting reduced inflammatory markers. This is preclinical and mechanistic only and does not establish efficacy or safety for muscle-building use in people.

The other substantial literature is analytical/doping-control work — a 2018 study characterizing YK-11 urinary metabolites for detection, and a 2024 report of YK-11 detected in a real doping-control sample. That work confirms YK-11 is used and detectable; it says nothing about benefit. Bottom line: the claimed muscle and strength benefits in humans are unproven, and extrapolating from C2C12 cells to human body composition is not supported by any trial data.

Legal and regulatory status

In the US, SARMs are not lawful dietary-supplement ingredients. The FDA classifies products marketed as SARMs as unapproved drugs, not supplements, and has issued warning letters and pursued criminal actions against sellers. FDA has tested a product listing YK-11 in connection with an adverse-event report involving a consumer stroke.

As of June 2026, SARMs (including YK-11) are not federally scheduled controlled substances. “SARMs Control Act” bills (S.2742, 115th Congress; S.2895, 116th Congress) proposed adding SARMs to Schedule III, but neither was enacted. Enforcement runs through FDA drug and misbranding authority, not the Controlled Substances Act. For contrast, oxandrolone is a genuine FDA-approved anabolic-androgenic steroid and a Schedule III controlled substance under the Anabolic Steroid Control Act; YK-11 does not fall under that anabolic-steroid scheduling.

In sport, YK-11 is prohibited at all times under the WADA Prohibited List, Section S1.2 “Other Anabolic Agents.” It is one of six SARMs explicitly named (andarine, enobosarm/ostarine, LGD-4033/ligandrol, RAD140, S-23, YK-11), and SARMs as a class are non-Specified Substances banned even if not individually listed. For context, GW-501516 (cardarine) and SR9009 (stenabolic) are PPARδ/Rev-erb agonists banned under S4.5, not S1.2.

Safety

There are no human safety data specific to YK-11. Direct human risk evidence for the compound itself is essentially absent; concerns are inferred from the SARM class and its androgenic/steroidal mechanism.

Liver injury: SARMs as a class are linked to drug-induced liver injury (cholestatic/hepatocellular) in published case reports. LiverTox documents a young patient who developed jaundice after taking three SARMs including YK-11, and a systematic review identified one YK-11 case among SARM-DILI series; injury generally resolved after stopping, with no deaths in those series. FDA warns of liver injury, including hospitalizations, from SARM products generally.
Testosterone suppression / endocrine effects: AR agonists suppress the HPG axis; FDA lists testicular shrinkage, infertility, and sexual dysfunction among SARM risks. Expected mechanistically for YK-11 but not quantified in humans.
Lipids / HDL: AR agonists commonly lower HDL, documented for SARMs as a class. No published YK-11-specific human lipid data — plausible but unverified for this compound.
Cardiovascular and other: FDA associates SARM products with increased risk of heart attack and stroke, psychosis, and sleep disturbance.

For related compounds (not YK-11): andarine (S-4) has well-documented, reversible visual side effects — yellow-tinged vision and trouble with night/light adaptation; and GW-501516 (cardarine) showed tumors across multiple organ types in long-term rodent carcinogenicity studies, ending its pharmaceutical development around 2006–2007. No reliable published finding ties YK-11 itself to QT prolongation or blindness.

Bottom line

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

Evidence grade: Animal only.