YK-11

YK-11 is a synthetic steroidal selective androgen receptor modulator (SARM). Unlike the more common nonsteroidal SARMs such as ostarine and RAD-140, YK-11 is built on a steroidal backbone; its full chemical name is (17-alpha,20E)-17,20-[(1-methoxyethylidene)bis(oxy)]-3-oxo-19-norpregna-4,20-diene-21-carboxylic acid methyl ester, a 19-norprogesterone-derived molecule ^[1][3]. It was first described in 2011 by Kanno and colleagues at Toho University, who identified it as a partial agonist of the androgen receptor ^[1].

What sets YK-11 apart from other SARMs is its reputation as a myostatin inhibitor. In muscle cell studies it not only activates the androgen receptor but also induces the expression of follistatin, a protein that antagonizes myostatin, the body's principal negative regulator of muscle growth ^[2]. Because raising follistatin can release this brake on muscle, YK-11 is marketed as a potent muscle-building agent, and it is described in the scientific literature as having both SARM-like and myostatin inhibitor-like properties ^[5].

YK-11 has never completed clinical development and is not approved for human use. It surfaced as an ingredient in seized black-market products and is sold on the supplement and research-chemical markets as a powder or oral capsule ^[5]. Like all SARMs, it is prohibited in sport; the World Anti-Doping Agency added SARMs to its Prohibited List in 2008, and doping-control laboratories have developed dedicated methods to detect YK-11 and its urinary metabolites ^[3][4][5]. Human safety and efficacy data are essentially absent, and its evidence base rests on cell and animal studies.

YK-11 is an unapproved research compound, a steroidal SARM that also acts as a myostatin/follistatin modulator, with no human safety studies and no status as a medicine. Based on its androgenic activity and steroidal structure, it is expected to suppress natural testosterone, adversely affect cholesterol, and potentially stress the liver. Its long-term and cardiovascular risks in humans are unknown.

YK-11 is a synthetic, steroidal selective androgen receptor modulator first described by Yuichiro Kanno and colleagues in Japan, whose research around 2011 to 2013 characterized both its partial-agonist behavior at the androgen receptor and, distinctively, its ability to induce follistatin in cultured muscle cells. The follistatin finding, that blocking follistatin reversed its muscle-building effect, established the follistatin-driven, indirect myostatin-inhibiting mechanism that gave YK-11 its reputation. It has never been developed as an approved drug and exists only as an experimental research chemical with no established human safety profile. Because SARMs were added to the World Anti-Doping Agency's Prohibited List in 2008, YK-11 has been prohibited in sport, and analytical laboratories have since developed methods to detect it in doping control.

YK-11 is one of the most talked-about compounds in bodybuilding circles because of its unusual dual action: it activates the androgen receptor and, uniquely among SARMs, drives muscle cells to produce follistatin, which opposes myostatin, the body's built-in brake on muscle size. That myostatin-inhibiting angle is genuinely distinctive and is the source of much of the excitement around it. Honesty requires flagging the serious caveats: it is an unapproved research chemical with no human safety data, it is banned in sport, and rodent work has reported adverse effects including impaired memory consolidation and pro-inflammatory and apoptotic signaling in the brain at anabolic exposures. It is best understood as a mechanistically fascinating but experimental agent.