What it is
Enobosarm is not a peptide. It is a small, non-steroidal molecule — nineteen carbon atoms, a molecular weight of 389, no amino-acid sequence at all — that switches on the androgen receptor, the receptor testosterone and its relatives act through. The online market calls it ostarine or MK-2866; its developer, GTx Inc., called it GTx-024; its international non-proprietary name is enobosarm. All four names mean the same molecule.
It belongs to a class called selective androgen receptor modulators, or SARMs. The design goal of the class is in the name: act like an androgen in muscle and bone, and act less like one in the prostate, the skin and the other tissues where androgens cause problems.
It sits in this library for the same reason MK-677 and tesofensine do: people arrive with the name from peptide shops and peptide forums, and the first useful thing to tell them is what kind of substance it is.
What the trials showed
Enobosarm has a real clinical record — larger than almost any compound sold beside it — and that record is the most useful thing to read.
Lean mass went up. In a 159-person, placebo-controlled phase 2 in people with cancer and recent weight loss (Lancet Oncology, 2013), total lean body mass rose by a median 1.5 kg at 1 mg and 1.0 kg at 3 mg over up to 113 days, against 0.02 kg on placebo. One detail matters here: the p-values the paper reports compare each group with its own starting point, not with the placebo group.
Function did not follow. GTx then ran two phase 3 trials, POWER 1 and POWER 2, in about 650 people starting chemotherapy for lung cancer. Each had two goals: keeping lean mass and keeping stair-climb power, a measure of how well a person can actually move. The registry's posted results:
| Stair-climb responders, enobosarm | Placebo | Lean-mass responders, enobosarm | Placebo | |
|---|---|---|---|---|
| POWER 1 (NCT01355484) | 29.4% | 24.2% | 41.9% | 30.4% |
| POWER 2 (NCT01355497) | 19.5% | 24.8% | 46.5% | 37.9% |
In both trials more people on enobosarm held their lean mass. In the second, fewer people on enobosarm held their physical function than on placebo. A drug that adds lean mass without adding function is the central problem of the whole field of muscle-wasting drugs, and enobosarm is its clearest documented case.
Breast cancer was a second programme. In androgen-receptor-positive, oestrogen-receptor-positive advanced breast cancer, a 136-woman phase 2 (Lancet Oncology, 2024) reported clinical benefit at 24 weeks in 32% at 9 mg and 29% at 18 mg, with no placebo arm. The grade 3–4 drug-related events included raised liver enzymes and raised blood calcium. Two phase 3 trials that followed were terminated early, at 52 and at 5 participants.
The GLP-1 turn
The programme now belongs to Veru Inc., and its current question is new: can enobosarm keep muscle during weight loss on GLP-1 drugs? Semaglutide and tirzepatide take off lean mass along with fat, and every developer of a "muscle-sparing" add-on is chasing that problem — the same one bimagrumab and apitegromab are being tested against.
Veru registered a 168-person phase 2 of enobosarm beside semaglutide (NCT06282458), with the change in lean body mass at 112 days as its primary endpoint. It completed on 22 August 2025. On 23 September 2026 the registry still held no posted results, and PubMed held no primary report — only four reviews discussing the idea. A 239-person phase 2b (NCT07446998) began in March 2026 and is expected to run to the end of 2027.
The POWER trials are the reason to read that result, when it arrives, for function and not only for lean mass.
Safety, as recorded
In trials, enobosarm was generally tolerated at the doses tested, and the 2013 phase 2 recorded no serious adverse events judged related to the drug. Outside trials, the published record is different in kind: PubMed indexes case reports of drug-induced liver injury in people taking it on their own, including a 2022 report titled simply Drug-Induced Liver Injury Secondary to Enobosarm. It is also at the centre of published doping cases — enough that two 2024 papers were devoted to whether it can pass between athletes through shared, sweat-soaked sports sleeves.
No proven harm in a supervised trial is not the same as proven safe in an unsupervised market product whose contents nobody has checked.
Where it stands
Not approved anywhere. Drugs@FDA returned no application on 2026-09-23. ClinicalTrials.gov lists 17 records with enobosarm as an intervention; the two completed phase 3s posted their results in 2014, and the four studies started since 2021 are all Veru's.
Related reading
For the other route to the same goal, see myostatin inhibitor peptides, where every candidate with a human result is an antibody. For the drugs enobosarm is now being paired with, see semaglutide and tirzepatide. MuscleLedger's WADA status by compound covers the sport question.