MK-2866 (Ostarine)
A selective androgen receptor modulator (SARM) that binds to androgen receptors to promote muscle growth and bone health with fewer side effects than steroids.
Common Dosage
10mg - 25mg daily (oral) for 8-12 weeks
Category
SARM
- Lean muscle gain
- Bone strength
- Fat loss
- Joint support
MK-2866 (Ostarine, enobosarm) was developed by GTx Inc. as one of the first selective androgen receptor modulators (SARMs) to reach clinical trials. Phase 2 trials were conducted in muscle wasting conditions including cancer cachexia, demonstrating lean mass preservation without the androgenic side effects of testosterone. A Phase 3 trial (POWER) for cancer cachexia was completed, though FDA approval was not obtained. It remains the most extensively human-studied SARM.
Ostarine binds selectively to the androgen receptor (AR) in muscle and bone with high affinity but demonstrates tissue selectivity — producing anabolic effects in muscle and bone while minimizing androgenic effects in prostate and skin. The selectivity arises from AR conformational differences in various tissues and co-regulator recruitment patterns. At higher doses, the selectivity breaks down and androgenic side effects become more likely.
- Muscle wasting associated with cancer — clinical trial indication
- Age-related sarcopenia
- Osteoporosis and bone mineral density improvement
- Lean mass preservation during caloric restriction
- Joint health support — anecdotally strong reports of joint pain relief
Cancer Cachexia Trials
Phase 2 trials in cancer patients showed statistically significant preservation of lean body mass and physical function with Ostarine compared to placebo — a meaningful result in a population where muscle loss significantly affects quality of life and treatment tolerability.
Recomposition in Athletes
Recreational athletes and physique competitors consistently report that Ostarine at 10–15mg produces notable improvements in body composition — modest lean mass gain with fat loss — without the water retention or side effect profile of anabolic steroids.
- Testosterone suppression occurs, particularly at higher doses or with extended use — post-cycle therapy (PCT) may be necessary
- Long-term androgenic effects are not characterized beyond clinical trial duration
- Liver stress reported at higher doses in some cases — liver enzymes should be monitored
- Not approved for any use — classified as performance-enhancing drugs by WADA and sports authorities
- Headache and back pain are commonly reported
- SARMs are under increasing regulatory scrutiny — legal status is evolving
- Mild testosterone suppression
- Headache
- Back pain