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 |
06 Commonly used with
What ostarine is combined with, and why. This is about intent rather than safety — the interactions table below covers what is dangerous. Nothing here is listed without what it costs.
| Compound | Frequency | Purpose & trade-off |
|---|---|---|
| Stacked compounds | ||
| Ligandrolrecomp | Paired to push a lean-bulk/recomp harder, combining ostarine's mild anabolic effect with ligandrol's stronger mass driver.Trade-off: Adding ligandrol meaningfully increases testosterone suppression, so the stack shifts from mild to clearly suppressive and warrants a PCT. | |
| Cardarineendurance | Cardarine is bolted on as a non-hormonal endurance and fat-loss add-on to improve conditioning while ostarine preserves muscle in a cut.Trade-off: Cardarine carries its own concern — rodent studies showed accelerated tumour formation at high doses — and there is no long-term human safety data. | |
| IbutamorenMK-677 | Added for GH-driven recovery, appetite and sleep support during the cycle without adding further hormonal suppression.Trade-off: Ibutamoren commonly causes water retention, increased hunger, lethargy and can worsen insulin sensitivity over time. | |
| Post-cycle | ||
| TamoxifenPCT | Run as a mini-PCT after the cycle to help testosterone recover, since ostarine is suppressive at typical doses and durations.Trade-off: A SERM is not a guaranteed fix, can cause mood and vision side effects, and using it does not make an unproven research chemical safe. | |