Ostarine (MK-2866, Enobosarm)
Ostarine (enobosarm, MK-2866) is a non-steroidal selective androgen receptor modulator investigated for muscle wasting and cachexia. It is the most clinically studied SARM, having reached Phase II/III trials, yet it remains unapproved for any indication and is banned in sport. Anecdotal recreational use targets lean-mass gains at low doses, with testosterone suppression and lipid changes as the main documented downsides.
01 Overview
Enobosarm was developed by GTx as an oral agent to build or preserve lean mass without the prostate and virilising effects of testosterone. Human trials in cancer cachexia, muscle wasting and stress urinary incontinence showed modest increases in lean body mass and stair-climbing power, but the POWER trials failed to meet co-primary functional endpoints, and no regulatory approval followed.
Despite being sold as a research chemical, ostarine is not a licensed medicine anywhere. It is prohibited at all times by WADA and is a frequent cause of positive doping tests, sometimes from contaminated supplements. Recreational dosing data are anecdotal; even low doses reliably suppress endogenous testosterone and lower HDL cholesterol.
02 Mechanism
Binds the androgen receptor as a tissue-selective partial agonist, favouring anabolic activity in muscle and bone over androgenic activity in prostate and skin.
03 Dosing
| Tier | Dose | Route | Notes |
|---|---|---|---|
| Light | 10–15 mg/day | Oral | Common anecdotal starting range; not a validated therapeutic dose. |
| Common | 20–25 mg/day | Oral | Doses studied for lean-mass gain in trials were around 1-3 mg; recreational doses run higher. |
Ester comparison
| Ester | Half-life | Injection freq. | Testosterone by mass |
|---|
04 Effects
| Effect | Magnitude | Evidence | |
|---|---|---|---|
| Increased lean body massPhase II trials showed dose-dependent lean-mass gains versus placebo in older adults and cancer patients. | +1-1.5 kg over 3-4 months at 3 mg | Clinical | |
| Improved physical functionSome improvement in stair-climb power, though pivotal cachexia trials missed functional co-primary endpoints. | modest | Clinical | |
| Recomposition at low dosesUsers report simultaneous fat loss and muscle retention on a calorie deficit; not established in controlled trials at recreational doses. | Anecdotal |
05 Side effects
| Effect | Severity | Frequency | Evidence | Countermeasures |
|---|---|---|---|---|
| Testosterone suppressionDose-dependent suppression of total and free testosterone; typically recovers within weeks of cessation but recovery is not guaranteed. | Moderate | Near-universal at recreational doses | Clinical | |
| HDL cholesterol reductionLowered HDL and shifts in lipid profile observed in trials, raising theoretical cardiovascular risk with prolonged use. | Moderate | Common | Clinical | |
| Elevated liver enzymesCase reports of drug-induced liver injury associated with ostarine-containing products. | Moderate | Uncommon | Observational |