YK11 Review: What YK-11 Is, Research, Results, Side Effects and Legality

YK-11 is a lab-made steroid that researchers at Toho University in Japan described in 2011 as a partial activator of the androgen receptor (Kanno 2011). It is sold as a SARM and a “myostatin inhibitor”, but both ideas come from cell and rodent studies: it has never been tested in a human trial, and ClinicalTrials.gov lists none (ClinicalTrials.gov). The human harms on record come from case reports and an FDA warning about a product linked to a stroke (FDA). It is not approved anywhere, and WADA bans it (WADA 2026).

Below: what YK-11 is, every study we found, what it does and does not do, its side effects, how it compares with RAD-140, LGD-4033 and steroids, its legal status, and links to all of our YK-11 guides. This page also replaces our older complete guide to YK-11.

What is YK-11?

YK-11 (also written YK11, and sold as Myostine) has a steroid skeleton: it is a 19-norpregnane derivative, unlike nonsteroidal SARMs such as ostarine or RAD-140 (Kanno 2011; PubChem CID 119058028; UK Anti-Doping). Its discoverers found that it switches on the androgen receptor only partly, without the receptor’s usual N/C-terminal interaction, and that its effect differs from gene to gene, which is why they called it a selective androgen receptor modulator (Kanno 2011). Anti-doping scientists describe it as “the steroidal SARM YK-11” (Thevis 2017), and WADA lists it with the SARMs (WADA 2026).

We found no record of a company developing YK-11 as a medicine. All published research is in cells, rats and mice, plus anti-doping lab work (ClinicalTrials.gov).

YK-11 research at a glance

No study has tested YK-11 for muscle, strength, fat loss or safety in people (ClinicalTrials.gov). Compare its evidence with other compounds in the SARMs database.

What does YK-11 do? Benefits vs evidence

Muscle growth and the “myostatin inhibitor” claim

The claim rests on one cell study. In mouse muscle cells, YK-11 raised follistatin, a protein that blocks myostatin, and that drove muscle-cell differentiation; blocking follistatin reversed the effect (Kanno 2013). No human study has measured myostatin, follistatin or muscle with YK-11. The one animal study that used it as a myostatin inhibitor was in mice with sepsis: it reported less inflammation, less organ damage and fewer deaths, not muscle growth in healthy animals (Lee 2021). “Myostatin inhibitor” describes a lab-dish mechanism, not a measured effect on muscle in people.

Strength and fat loss

Not studied. We found no trial, animal study or cell study of strength or fat loss with YK-11.

Bone

In cells, YK-11 increased bone-forming activity (Yatsu 2018), and in rats a YK-11 gel placed directly into a skull defect sped up repair (Wang 2025). That is local application in an animal, not evidence about swallowing it.

Is YK-11 the strongest SARM?

There is no way to rank it, because it has never been tested in people and no study has compared its effect on muscle with other SARMs. No published study has measured an anabolic-to-androgenic ratio for YK-11 either. “Strongest” claims come from user reports, which cannot separate YK-11 from training, diet, other compounds or a mislabeled product.

YK-11 results and before-and-after reports

There are no measured results. User reports and photos are collected on YK-11 before and after, with the evidence alongside, and the 8-week YK-11 log is one user’s account.

YK-11 side effects

The FDA warns that life-threatening reactions, including liver injuries needing hospital care, have occurred in people taking SARM products, and lists possible risks including heart attack or stroke, liver failure and infertility (FDA). Adverse event reports and single cases cannot prove that YK-11 caused a problem, but they are the only human safety data there are.

Hair loss, gyno and acne

None has been studied with YK-11. Because it activates the androgen receptor, the androgen-related effects seen with other SARMs and steroids are plausible, but they have not been measured. What is known across SARMs: YK-11 gyno, SARMs and hair loss, SARMs and acne and all SARMs side effects.

Does YK-11 suppress testosterone?

Not known. YK-11 activates the androgen receptor in cell tests (Kanno 2011), but we found no study in people or animals that measured testosterone, LH or FSH with it. Users often treat it as suppressive, and our YK-11 suppression page covers those reports. For what is known about recovery after androgen use and the PCT drugs that have been studied, see PCT after SARMs and the PCT and bloodwork planner.

Bloodwork

No study has tracked lab markers in people taking YK-11. Liver enzymes and bilirubin were raised in the case report where it was used with other SARMs (Lee 2023), and the markers that changed in trials of other SARMs are testosterone, FSH, SHBG and HDL cholesterol (Basaria 2013). Our SARMs bloodwork guide explains each test.

YK-11 dosage: what studies used

No human dose has ever been studied. Cell studies use concentrations in a dish, and the rat studies used doses per kilogram that do not translate to people. The only non-oral research use was a gel placed directly into a bone defect in rats (Wang 2025); there is no research on injecting YK-11. Products are sold by label strength: SwissChems, for example, sells YK-11 as 5 mg capsules. Our YK-11 dosage page explains what users report and what is known.

YK-11 half-life

Not measured in any published study. In an anti-doping study, no unchanged YK-11 appeared in urine after a dose; its breakdown products were traceable for more than 48 hours, and the unconjugated ones were gone within 24 hours (Piper 2018). Half-life figures quoted online do not come from a published study.

Is YK-11 legal?

• United States: not FDA-approved. In a December 2025 warning letter, the FDA called products sold as “YK-11” unapproved new drugs (FDA warning letter, 2025), and it says SARM products are unapproved drugs, not dietary supplements (FDA).
• United Kingdom: UK Anti-Doping lists YK-11 (Myostine) among SARMs, says no SARM is approved for human use, and notes that SARMs are illegal to sell or advertise as food or in supplements (UK Anti-Doping).
• Australia: not named separately, but SARMs as a class are prescription-only and possession without authority is illegal (Poisons Standard 2026).
• Germany: named as an example SARM in the Anti-Doping Act’s annex (AntiDopG annex); trading in it for doping in sport, or possessing a non-small quantity for that purpose, is banned (AntiDopG §2).

More: are SARMs legal?

YK-11 in sport and drug tests

WADA names YK-11 as a SARM, banned at all times (WADA 2026). Labs test for its breakdown products because the intact drug does not show up in urine (Piper 2018), and a Los Angeles anti-doping lab reported finding it in a doping control sample (Sobolevsky 2024). See SARMs drug test.

YK-11 vs RAD-140, LGD-4033 and steroids

The practical difference is evidence: LGD-4033 has placebo-controlled human data showing both its effect and its suppression; YK-11 has none. Head-to-head pages: YK-11 vs RAD-140 and YK-11 vs LGD-4033, or use the compare tool.

Is YK-11 for beginners?

There is no human dosing or safety data for anyone, beginner or experienced. Of all the compounds sold as SARMs, it is one of the least studied: no human trial, no human pharmacokinetics, and human harms known only from case reports and adverse event reports. Our start here page covers the basics of the evidence and the risks.

YK-11 stacks

No study has tested YK-11 with anything. The human case reports all involve combinations: the liver-injury case with LGD-4033 and RAD-140 (Lee 2023) and the FDA product that also held an undeclared steroid (FDA). Stacking makes it impossible to tell which compound caused a problem. Our YK-11 stacks page covers common combinations, and the stack checker shows where documented risks overlap.

Buying YK-11: is it what the label says?

Products sold as SARMs are often mislabeled: in a test of 44 products, only 52% contained a SARM and 25% contained unlisted substances (Van Wagoner 2017), and the FDA’s YK-11 case involved a hidden steroid (FDA). Look for a lot-specific certificate of analysis and learn how to read a lab test. Our current pick is SwissChems, which publishes lot-specific lab reports; see our SwissChems review.

Bottom line

YK-11 is a steroid-based androgen receptor activator with an interesting cell study behind the “myostatin inhibitor” label and nothing behind the muscle claims in people. It has never been tested in a human trial, its half-life and its effect on testosterone are unknown, and the human safety record consists of a stroke linked to an FDA-tested product, a serious liver injury with other SARMs, and adverse event reports. In rats it harmed the hippocampus. It is banned in sport and cannot legally be sold as a supplement in the US or UK.