Ostarine (Enobosarm, MK-2866)

Ostarine is the market name for enobosarm, a nonsteroidal selective androgen receptor modulator first developed by GTx Inc under the codes GTx-024, MK-2866 and S-22. It was designed to switch on the androgen receptor in muscle and bone while acting only weakly on prostate and other reproductive tissue, and it was taken into clinical development for muscle wasting in cancer, for age-related loss of muscle, for androgen receptor positive breast cancer and for stress urinary incontinence. It has never been approved by the FDA or the EMA for any indication. Development rights now sit with Veru Inc, which is testing it as an add-on to GLP-1 receptor agonists to limit the loss of lean mass that accompanies rapid weight reduction.

The androgen receptor is a nuclear receptor. Testosterone and dihydrotestosterone bind it, move it into the cell nucleus and change which genes are read. Enobosarm binds the same receptor but recruits a different set of co-regulator proteins, which is why its effects on muscle and bone are stronger than its effects on prostate and skin. Work in mice showed that the muscle response does not come only from satellite cells, since enobosarm still restored muscle weight in animals lacking the androgen receptor in that cell lineage (PMID: 26393303).

In animals, enobosarm improved bone healing in ovariectomized rats (PMID: 31531719), improved muscle tissue in ovariectomized rats (PMID: 33042018) and improved bone in orchiectomized rats used as an osteoporosis model (PMID: 37378829). Not every animal result is favorable: in rats it blunted the effect of endurance training on submaximal endurance (PMID: 38451281) and did not add to the metabolic effect of exercise in obese rats (PMID: 37865959). In humans, a 12-week phase 2 trial in 120 healthy older men and postmenopausal women reported gains in total lean body mass and physical function against placebo (PMID: 22031847), and a phase 2 trial in patients with cancer-related weight loss reported gains in lean body mass (PMID: 23499390).

The safety record is the part most often left out of marketing. Cholestatic and hepatocellular liver injury after ostarine use has been published as individual case reports, including a young man who needed albumin dialysis and took six months to return to normal bilirubin (PMID: 37871633), a hepatocellular case that resolved after stopping the supplement (PMID: 35655632) and cases involving ostarine with post-cycle drugs (PMID: 34141767). A systematic review of safety in healthy adults collected fifteen published cases of drug-induced liver injury across the SARM class along with a tendon rupture and a rhabdomyolysis case (PMID: 37218811). The FDA states that these products are unapproved drugs, are not dietary supplements, and lists liver injury and acute liver failure, heart attack, stroke, psychosis, infertility and testicular shrinkage among reported risks (FDA, Certain Bodybuilding Products Put Consumers at Risk, page updated December 2025).

What is sold online is often not what the label says. In an analysis of 44 products marketed as SARMs, only 52 percent contained a SARM at all, 39 percent contained a different unapproved drug, and the labeled amount matched the measured amount in only 41 percent (PMID: 29183075).

Ostarine (Enobosarm, MK-2866) is available in capsule form.

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