Ostarine (MK-2866)

Ostarine (also called MK-2866 or enobosarm) is a selective androgen receptor modulator (a compound designed to act like a muscle-building hormone in some tissues but not others). It is often — and wrongly — sold online as a “safer steroid alternative” for building muscle. It is not a peptide, and it is not a legal supplement ingredient. There is real human evidence that it raises lean body mass (muscle and other non-fat tissue), but it has no approved use anywhere, no solid support for athletic or cosmetic use, and some documented safety warning signs — including liver injury and the shutting-down of your body’s own testosterone. This page is meant to inform and reduce harm; it contains no dosing, cycles, or sourcing information.

What it is

Ostarine is a selective androgen receptor modulator (SARM) — a small, non-steroidal, pill-form synthetic molecule that latches onto the androgen receptor (the docking site in cells that responds to testosterone and similar hormones). It is not a peptide. During development it went by the code names MK-2866, GTx-024, and S-22; its official drug name is enobosarm. It was first developed by GTx, Inc. (later Veru Inc.).

SARMs were designed to be tissue-selective — meaning the goal was to switch on the muscle-and-bone-building effects of androgens while having a weaker effect on the prostate and other tissues. That was the theoretical edge over traditional anabolic steroids. In reality, that selectivity is only partial in humans.

The claims

Online and in “bodybuilding” and biohacking circles, ostarine is marketed (illegally) for muscle gain, fat loss, strength, recovery, and body recomposition, and is often pitched as a “safer steroid alternative” with “fewer side effects.” It is usually sold with “research chemical” or “not for human consumption” labels, and it is sometimes slipped into products that are labeled as dietary supplements. None of these uses are FDA-approved, and none of these claims are backed by adequate safety or effectiveness data.

What the evidence actually shows

No SARM, including ostarine/enobosarm, is approved by the FDA or EMA (the US and European drug regulators) for any use, despite a lot of clinical research.

• Muscle wasting / cancer cachexia (severe weight and muscle loss from illness): A high-quality phase 2 trial (Dobs et al., Lancet Oncology 2013; PMID 23499390) found that enobosarm clearly raised lean body mass compared with a placebo (a dummy treatment) in cancer patients, and also improved a measure of physical function (stair-climb power). Both the main goal (lean body mass) and a secondary goal (function) were met. But the two larger, decisive phase 3 POWER trials in patients with non-small-cell lung cancer (design described in Crawford et al., Curr Oncol Rep 2016; PMID 27138015 / PMC4853438) raised lean body mass but failed their co-primary goals — which required patients to both hold onto lean mass and improve physical function (stair-climb). The cachexia program never led to approval.
• Breast cancer: A phase 2 trial (Study G200802; Palmieri et al., Lancet Oncology 2024;25(3):317–325) reported anti-tumor activity and acceptable tolerability in a specific type of advanced breast cancer (AR-positive, ER-positive, HER2-negative — labels that describe which hormone receptors the tumor carries). Enobosarm is still being studied for this — a legitimate research use that is completely separate from fitness use.

Bottom line on evidence: there is a genuine biological effect on lean body mass, but no solid evidence that it is safe or effective for building muscle for sport or looks, and its long-term safety in healthy people has basically never been studied.

Legal and regulatory status

FDA (US): SARMs are not legal dietary-supplement ingredients. Products that contain them are unapproved, misbranded drugs, not supplements. The FDA has put out public warnings (notably an October/November 2017 statement and warning letters dated Oct 23, 2017 to Infantry Labs LLC, IronMagLabs, and Panther Sports Nutrition) flagging serious safety concerns, including liver damage and the risk of heart attack and stroke.

DEA scheduling: As of June 2026, ostarine and other SARMs are not federally scheduled controlled substances. Bills to add SARMs to Schedule III — the SARMs Control Act (S.2742, 2018; S.2895, 2019) — were introduced but never became law. No federal scheduling of SARMs is currently in place.

Anabolic-steroid law: SARMs are chemically different from anabolic steroids and are not covered by the Anabolic Steroid Control Act — which is exactly why a separate SARMs Control Act was proposed. (One note to avoid confusion: oxandrolone is a true anabolic-androgenic steroid and a Schedule III controlled substance; it has nothing to do with ostarine, which is a SARM.)

Anti-doping (WADA): Ostarine and other SARMs (andarine, LGD-4033/ligandrol, RAD140/testolone, S-23, YK-11) are banned at all times under category S1.2 (“Other Anabolic Agents”). Ostarine is one of the most commonly detected agents in this group in anti-doping samples. Worth knowing: GW-501516 (cardarine) and SR9009 are NOT SARMs — GW1516 is a PPARδ agonist and SR9009 is a Rev-erbα agonist (these target different cell pathways), and both are banned under S4.5 (Hormone and Metabolic Modulators).

Safety

• Testosterone / gonadal-axis suppression: Because it acts on the androgen receptor, ostarine lowers your body’s own testosterone and the hormones that signal it (gonadotropins) — an effect seen across the SARM class in clinical trials. Bouncing back afterward is not guaranteed, and recovery is poorly understood after gray-market use.
• Liver toxicity (hepatotoxicity): Several published case reports of drug-induced liver injury (DILI — liver damage caused by a drug) are on record — mostly a cholestatic pattern (a backup of bile flow in the liver) — for example Bedi et al., ACG Case Reports Journal 2021 (PMID 34368386 / PMC8337042), plus an Australian case series (Nash et al., Aliment Pharmacol Ther 2024; 23 cases, 14 involving SARMs). The reported SARM-DILI numbers were small (in the low tens) as of the early 2020s, but the signal is real.
• Cholesterol (HDL): SARMs as a group lower HDL cholesterol (the “good” cholesterol); drops in HDL showed up in enobosarm trials — a change in the wrong direction for heart health.
• A note on related compounds: Andarine (S-4) — a different SARM, not ostarine — is linked to yellow-tinted or disturbed vision; that is specific to andarine and should not be blamed on ostarine. GW-501516 (cardarine), often sold alongside ostarine but actually a separate PPARδ agonist, had its development abandoned (GSK, ~2007) after long-term rodent studies showed dose-related cancers in multiple organs (liver, stomach, skin, bladder).
• Product-quality risk: An independent lab analysis published in JAMA (Van Wagoner et al., 2017;318(20):2004–2010; PMID 29183075) tested 44 products sold online as SARMs and found that only 52% actually contained a SARM, 39% contained a different unapproved drug, 9% contained no active compound, and 59% had an amount of substance that didn’t match the label. Gray-market “SARM” products are often mislabeled, under-dosed, over-dosed, or contaminated with something else.

No specific blindness or QT-prolongation (a heart-rhythm concern) claim for ostarine is included here — no solid source confirmed one.

Bottom line

Ostarine is a SARM, not a peptide, and not a supplement. It clearly raises lean body mass in humans, which is why it is still being studied for legitimate cancer research — but it has no approval anywhere, failed its decisive phase 3 functional goals in cachexia, and carries real safety warning signs (testosterone suppression, cholestatic liver injury, lower HDL). Products sold online are routinely mislabeled or contaminated, and it is banned in sport at all times. There is no solid evidence that it is safe or effective for athletic or cosmetic use.

Evidence grade: 7/10 · Moderate.

Sources

• Dobs AS et al., Lancet Oncology 2013 (PMID 23499390)
• Crawford J et al., POWER trials design, Current Oncology Reports 2016 (PMID 27138015)
• POWER trials design (PMC4853438)
• Palmieri C et al., Enobosarm in AR+/ER+/HER2− advanced breast cancer, Lancet Oncology 2024;25(3):317–325
• Bedi H et al., Drug-Induced Liver Injury From Enobosarm (Ostarine), ACG Case Reports Journal 2021 (PMID 34368386)
• Bedi et al. (PMC8337042)
• Nash E et al., DILI from SARMs/AAS/bodybuilding supplements, Aliment Pharmacol Ther 2024
• Van Wagoner RM et al., Chemical Composition and Labeling of Substances Marketed as SARMs, JAMA 2017 (PMID 29183075)
• FDA, “Certain Bodybuilding Products Put Consumers at Risk…”
• FDA In Brief, “FDA warns against using SARMs in body-building products”
• SARMs Control Act of 2019 (S.2895, 116th Congress)
• SARMs Control Act of 2018 (S.2742, 115th Congress)
• USADA, “Selective Androgen Receptor Modulators (SARMs)”
• USADA, “What Should Tested Athletes Know About GW1516?”
• WADA Prohibited List
• GW501516 overview (Wikipedia)
• Enobosarm overview (Wikipedia)