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Ostarine (MK-2866)

MK-2866 · Enobosarm · Ostarine · SARM

Animal replicated

Research chemicalSARM Selective Androgen Receptor Modulator Small Molecule

RouteOralGray-market only

Evidence grade: Animal replicated

Effect demonstrated in multiple animal studies; human data sparse or extrapolated. Grades summarize evidence quality, not whether a compound is appropriate, legal, or risk-free.

At a glance

What it is: Nonsteroidal selective androgen receptor modulator (SARM); orally active small-molecule androgen receptor ligand.
Why people use it: Used primarily for muscle and performance and hormonal support.

If you only read one thing

Ostarine's central tension is not safety — it's the gap between clinical efficacy at the doses studied and the community doses that exceed them. Phase 2 trials used 1-3 mg/day; Phase 3 used doses in the 1-3 mg range. Community users take 12.5-25 mg/day — roughly 5-25x the clinical trial doses. The Phase 2 efficacy and safety data are the most favorable of any SARM, but they apply to a dose range substantially below community use. The good news: the dose gap between clinical trials and community use is smaller for Ostarine than for RAD-140 or LGD-4033, and the hepatotoxicity signal in community users is the weakest of the three compounds. For a first SARM cycle, particularly for women or older men, Ostarine remains the evidence-based conservative choice — not because it's 'safe' in an absolute sense, but because the human data is more extensive and more favorable than any other SARM.

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Published literature

4human RCTs1human study0animal0in vitro

Evidence reality check

Human evidence

5 human studies

4 randomized; 1 observational.

Preclinical base

0 lab signals

0 animal; 0 in-vitro/mechanistic.

Risk posture

No major flags listed

Review route-specific cautions before use.

Categories

Muscle & Performance Hormonal & HPTA

Properties

Active malignancy: hard stopWADA S1✓ Human RCTHPTA: suppressiveNot injectable

Evidence

CAnimal replicated

The POWER Phase 3 Story

GTx ran two parallel Phase 3 trials (POWER 1 and 2) in non-small cell lung cancer (NSCLC) patients with muscle wasting. Ostarine improved lean body mass in both trials. However, FDA's novel drug approval framework required both lean body mass AND physical function (stair climb power) to be statistically significant simultaneously as co-primary endpoints — a combined endpoint bar that the compound did not clear in both trials simultaneously. One trial met lean mass but not function; one met function but not lean mass. GTx did not submit an NDA. The compound was not approved. Critically: the compound worked in the biology sense — it built lean mass in cancer patients — but it failed to meet an unusually demanding co-primary statistical framework. This is not the same as a safety failure.

Phase 2 Efficacy Data

Dobs AS, et al. (2013, Lancet Oncology): n=159 cancer cachexia patients; 1 and 3 mg/day for 16 weeks; lean mass +1.5 kg (1 mg) and +1.3 kg (3 mg) vs +0.1 kg placebo; chair climb power improved; well-tolerated; ALT elevations in 2 patients per group including placebo. Dalton JT, et al. (2011, JCSM): healthy elderly men and postmenopausal women; lean mass improved and physical function improved. These Phase 2 results are the best evidence for Ostarine's mechanism working and the lowest adverse event rate of any SARM in its class in controlled trials.

Why Ostarine Is the 'Safer' SARM

In the SARM comparative context: RAD-140 has 59% elevated AST in its Phase 1 trial and 6+ DILI case reports. LGD-4033 has one DILI case at 10 mg/day (10-100x clinical dose). Ostarine has the smallest published hepatotoxicity case series and the lowest severity signal of the three. HPTA suppression at 12.5-25 mg/day community doses is milder and more consistently recoverable without PCT than LGD-4033 or RAD-140. The Phase 2 trials at 1-3 mg/day showed minimal liver signal and good overall tolerability in patients already compromised by cancer. The community uses it at 12.5-25 mg — significantly higher than Phase 2 doses — but the dose gap is smaller than RAD-140's or LGD-4033's, and the safety signal at community doses is more modest.

HPTA and Women

Ostarine at community doses (12.5-25 mg/day) produces mild to moderate HPTA suppression in men — less than RAD-140 or LGD-4033 at equivalent doses. Short cycles (4-6 weeks) at 12.5 mg may not require PCT in some users; longer cycles or higher doses (20-25 mg, 8 weeks) do. Women: Ostarine at 5-12.5 mg/day is the most commonly recommended SARM for female users given the lower virilization risk relative to other SARMs; menstrual cycle disruption has been reported at higher doses or longer durations. Unlike anabolic steroids, no permanent virilization is expected at standard community doses.

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Alternatives

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Enclomiphene

Same class · Small Molecule

The Pure Active Isomer in Clomid. The Drug That Raised Testosterone as Well as TRT While Preserving Fertility — and Still Failed FDA Approval. The Zuclomiphene Problem: Why the Racemic Mixture Causes Side Effects Pure Enclomiphene Shouldn't. The NDA That Died in 2021. How It Differs from TRT, HCG, and Gonadorelin for Male Hypogonadism. The 2025 Meta-Analysis: +273 ng/dL Testosterone vs Placebo.

LGD-4033 / Ligandrol

Same class · SARM

The Most Phase-1-Tested SARM in History. The Problem: Phase 1 Used 0.1-1 mg. The Community Uses 5-20 mg. +1.21 kg Lean Mass at 1 mg in 21 Days. Phase 2 Hip Fracture Trial Positive. HPTA Suppression Dose-Dependent and Documented in Clinical Data. The Dose Gap Between Clinical Trials and Community Use Is the Central Risk Issue. Multiple Elite Athlete Doping Violations Including Canelo Álvarez. WADA S1.2.

RAD-140 / Testolone