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Evidence Review

YK-11

YK11 · YK 11 · Myostine · (17α,20E)-17,20-[(1-methoxyethylidene)bis(oxy)]-3-oxo-19-norpregna-4,20-diene-21-carboxylic acid methyl ester · CAS 1370003-76-1 · Synthetic steroidal androgen receptor (AR) partial agonist, marketed as a SARM; also acts as a myostatin-pathway modulator via induction of follistatin.

A steroid-derived compound sold as a "SARM" and myostatin inhibitor, with no human trials — its entire evidence base is cell-culture and animal work.

Not a true SARM. YK-11 is not a true (non-steroidal) SARM. It is a synthetic steroid built on a 19-nortestosterone backbone, and it is grouped with SARMs mainly because it is marketed alongside them and behaves as a gene-selective AR partial agonist. Mechanistically it is better described as a steroidal AR partial agonist with a distinctive follistatin/myostatin-pathway action. WADA and the FDA nonetheless classify and regulate it within the SARM group.
01

What it is

YK-11 is a synthetic compound first described by Kanno and colleagues at Toho University in 2011. Chemically it is a steroid derived from 19-nortestosterone (molecular formula C25H34O6, CAS 1370003-76-1), not a non-steroidal small molecule. It is widely sold online as a "SARM" and "myostatin inhibitor" (trade name "Myostine") for muscle building, but it has never been developed as a pharmaceutical, has no approved medical use anywhere, and has never been tested in a registered human clinical trial. A search of ClinicalTrials.gov returned zero registered studies. The published science on YK-11 consists almost entirely of in-vitro (cell-culture) experiments, plus two animal studies (a mouse sepsis model and a rat bone-defect model).

02

How it works

YK-11 binds the androgen receptor and acts as a gene-selective PARTIAL agonist. In reporter-gene assays (Kanno 2011, MDA-MB-453 cells) it promoted AR activity but, unlike dihydrotestosterone (DHT), did NOT induce the amino/carboxyl-terminal (N/C) interaction of the receptor that is required for full transactivation — and it blocked DHT-induced N/C interaction. This partial, N/C-independent activation is proposed to underlie a more "selective" gene-expression profile. Its signature effect is anabolic signalling through the myostatin pathway: in mouse C2C12 myoblasts (Kanno 2013), YK-11 — but not DHT — induced follistatin, an endogenous antagonist of myostatin (GDF-8), and drove myogenic differentiation with stronger induction of MyoD, Myf5 and myogenin than DHT. Blocking follistatin with an antibody reversed the effect, indicating follistatin induction mediates its anabolic action. Because it is structurally a steroid and an AR agonist, it would be expected to share androgenic/anabolic-steroid class effects, including suppression of the hypothalamic-pituitary-gonadal axis, though this has not been measured in humans.

03

Efficacy — what the human evidence shows

The Institute grades each claimed effect by the strength of the human evidence behind it. Grades are assigned independently for benefit and for harm.

Muscle anabolism / myogenic differentiation (marketed use)Insufficient
In cultured mouse muscle cells, YK-11 induced myogenic differentiation and myosin heavy chain expression more strongly than DHT, with greater induction of MyoD, Myf5 and myogenin. There is NO human data of any kind demonstrating muscle growth, strength, or body-composition change.
In-vitro only: Kanno et al., Biol Pharm Bull 2013;36(9):1460-5 (PMID 23995658), C2C12 mouse myoblasts. No human trials exist; ClinicalTrials.gov returned 0 studies.
Myostatin-pathway inhibition via follistatin inductionInsufficient
YK-11 (but not DHT) induced follistatin, an endogenous myostatin antagonist, in mouse myoblasts, and antibody neutralization of follistatin abolished its anabolic effect — establishing the proposed mechanism. This is a cell-culture mechanistic finding, not a demonstrated clinical effect.
In-vitro mechanistic: Kanno 2013 (PMID 23995658). No measurement of circulating myostatin/follistatin in any human.
Gene-selective AR partial agonismInsufficient
Reporter-gene assays established YK-11 as an AR partial agonist that does not induce the receptor N/C interaction required for full transactivation, and blocks DHT-mediated N/C interaction — the basis for calling it 'selective.'
In-vitro: Kanno et al., Biol Pharm Bull 2011;34(3):318-23 (PMID 21372378), MDA-MB-453 cells, ARE-luciferase assays.
Bone formation / osteogenesisInsufficient
YK-11 (0.25-4 µM) promoted osteogenic differentiation of bone-marrow stromal cells in vitro via the AR and BMP2/Smad pathway, and promoted repair of cranial bone defects in rats in vivo. Animal/in-vitro only; not studied in humans.
Animal + in-vitro: 'YK11 promotes osteogenic differentiation of BMSCs and repair of bone defects,' J Mol Endocrinol 2025;74(2), JME-24-0073. Single, unreplicated animal study.
Muscle wasting / survival in sepsisInsufficient
In a mouse E. coli sepsis model, oral YK-11 reduced muscle atrophy, lowered pro-inflammatory cytokines and organ-damage markers, and improved survival. Animal-only, single study, high oral doses; no human relevance established.
Animal only: Lee SJ, Gharbi A, Shin JE, Jung ID, Park YM. Biochem Biophys Res Commun 2021 (S0006291X21000668). Mouse model.
04

Safety signals

Harm signals are reported prominently even where the evidence grade is low. Absence of evidence is not evidence of safety.

Hepatotoxicity / cholestatic liver injury (SARM class)Probable
SARMs as a class cause a well-documented pattern of cholestatic liver injury resembling anabolic-steroid injury — fatigue, jaundice, pruritus, dark urine, marked hyperbilirubinemia (often peaking 30-40+ mg/dL) with only modest aminotransferase elevation, typically 2-3 months after starting, usually self-limited but occasionally requiring hospitalization. NIH LiverTox rates the SARM class likelihood 'B (likely cause)' and names YK-11 as a recreationally used steroidal SARM. The FDA reports liver injuries requiring hospitalization in SARM users. YK-11-specific human case data were not identified, so this is a strong class-level signal extrapolated to YK-11 — absence of YK-11-specific reports is not evidence of safety.
Androgenic effects and HPG-axis / testosterone suppressionInsufficient
As a steroidal AR agonist, YK-11 would be expected to suppress endogenous testosterone and cause androgenic side effects (as seen with other SARMs and anabolic steroids: testicular shrinkage, infertility, sexual dysfunction, acne). This is a mechanistic/class expectation; it has NOT been measured for YK-11 in humans.
Cardiovascular and psychiatric risk (SARM class)Insufficient
The FDA links SARM products to increased risk of heart attack and stroke, plus psychosis, sleep disturbance and other effects. These are class-level warnings; no YK-11-specific human cardiovascular or psychiatric data exist.
Product mislabeling, contamination and unknown human toxicologyPreliminary
YK-11 is sold on an unregulated grey market; analytical surveys of SARM products repeatedly find mislabeling, wrong doses, and undeclared ingredients, so real-world exposure is unpredictable. Combined with the complete absence of any human toxicology or dose-finding study, the human safety profile of YK-11 is genuinely unknown.
06

Legal & regulatory status

United States
Not approved by the FDA for any use. The FDA states that SARM-containing bodybuilding products are NOT dietary supplements but unapproved drugs not reviewed for safety or effectiveness; it has issued warning letters and pursued criminal actions against distributors, and warns of life-threatening reactions including liver injury requiring hospitalization. Sold illegally as a 'research chemical.'
Anti-doping (WADA)
Prohibited at all times (in- and out-of-competition) under Section S1.2 'Other Anabolic Agents' (SARMs) of the WADA Prohibited List. YK-11 has been confirmed in an actual doping-control sample (Sobolevsky 2024).
Other jurisdictions
Health Canada has warned it is not authorized for any use. It is not a licensed medicine in any jurisdiction; in the UK/EU it is sold illegally as an unlicensed research chemical.
07

The bottom line

YK-11 is a synthetic steroid marketed as a "SARM" and myostatin inhibitor. Its entire evidence base is cell-culture work (Kanno 2011/2013, showing AR partial agonism and follistatin-driven myogenic differentiation) plus two single animal studies (mouse sepsis, rat bone). There are ZERO human clinical trials — ClinicalTrials.gov lists none — so there is no human evidence that it builds muscle or is safe. Every efficacy claim grades Insufficient. On the harm side, the SARM class is prohibited by WADA, classified by the FDA as unapproved drugs (not supplements), and carries a documented, likelihood-B pattern of cholestatic liver injury (some requiring hospitalization) plus expected testosterone suppression and FDA-flagged cardiovascular/psychiatric risks. YK-11-specific human safety data do not exist, and that absence should not be read as safety. This is an experimental, unapproved, banned compound with real class-level liver and hormonal risks and no proof of benefit in humans.

08

References

  1. (17α,20E)-...(YK11) is a partial agonist of the androgen receptorKanno Y, et al. Biological & Pharmaceutical Bulletin, 2011;34(3):318-323 (PMID 21372378)
  2. Selective androgen receptor modulator, YK11, regulates myogenic differentiation of C2C12 myoblasts by follistatin expressionKanno Y, et al. Biological & Pharmaceutical Bulletin, 2013;36(9):1460-1465 (PMID 23995658)
  3. Myostatin inhibitor YK11 as a preventative health supplement for bacterial sepsisLee SJ, et al. Biochemical and Biophysical Research Communications, 2021
  4. YK11 promotes osteogenic differentiation of BMSCs and repair of bone defectsJournal of Molecular Endocrinology, 2025;74(2), JME-24-0073
  5. Detection of selective androgen receptor modulator YK-11 in a doping control sampleSobolevsky T, et al. Drug Testing and Analysis, 2024;16(6):655-660 (PMID 37946705)
  6. Selective Androgen Receptor Modulators (SARMs) — drug-induced liver injuryHoofnagle JH. LiverTox, NIDDK/NIH; updated Sep 20, 2025 (NBK619971)
  7. Certain bodybuilding products put consumers at risk for heart attack, stroke, serious liver damage and moreU.S. Food & Drug Administration (FDA) consumer warning
  8. Selective Androgen Receptor Modulators (SARMs): Prohibited Class of Anabolic AgentsU.S. Anti-Doping Agency (USADA)
  9. The Prohibited List (S1.2 Other Anabolic Agents — SARMs)World Anti-Doping Agency (WADA), 2025
  10. YK-11 — chemical identity, structure (19-nortestosterone-derived steroid), CAS 1370003-76-1Wikipedia (secondary; primary identity confirmed via Kanno 2011)
SARMS Institute Research Desk. Compiled from primary sources. Last updated 20 July 2026.
This review is for education and does not provide medical or legal advice. No SARM is approved for human use.