Ostarine, also designated MK-2866 or Enobosarm, is a non-steroidal selective androgen receptor modulator developed by GTx, Inc. It is the single most extensively researched SARM in existence, with multiple completed Phase II and Phase III human clinical trials — including the landmark POWER 1 and POWER 2 trials evaluating its efficacy in cancer-related muscle wasting. No other compound in the SARM class approaches Ostarine's depth of published human safety and efficacy data.

Ostarine selectively binds the androgen receptor in muscle and bone tissue, inducing anabolic transcriptional activity — increased myofibrillar protein synthesis, enhanced nitrogen retention, and osteoblast activation — while exerting minimal androgenic effects on prostate, skin, and hair follicle tissues. Its mechanism is tissue-selective via differential AR conformational changes and co-regulator recruitment, analogous to how SERMs like Tamoxifen act as agonists in bone but antagonists in breast tissue. This selectivity produces a favorable therapeutic index compared to traditional androgens.

Clinical trial data demonstrates that Ostarine produces statistically significant increases in lean body mass (1.0–1.5 kg in 12 weeks at 3mg/day) and improvements in physical function in clinical populations — elderly patients, cancer patients undergoing chemotherapy, and post-surgical subjects. In research contexts, it is most valued for its capacity to preserve lean tissue during caloric deficit, accelerate recovery from connective tissue injury, and provide mild but meaningful anabolic effects with an exceptionally well-characterized safety profile.

Ostarine is the recommended entry point for researchers new to SARMs. Its mild potency, extensive human data, well-understood suppression profile, and broad therapeutic window make it the most forgiving compound in the class. It is ideally suited for cutting and recomposition protocols where the primary objective is lean tissue preservation during energy restriction, as well as injury recovery research. Researchers frequently run Ostarine as a standalone compound before progressing to more potent SARMs like LGD-4033 or RAD-140.

Ostarine has an elimination half-life of approximately 24 hours, permitting once-daily oral administration. Oral bioavailability is excellent. Testosterone suppression at standard research doses (10–25mg/day) is mild — typically a 10–25% reduction in total testosterone, with recovery to baseline within 4–6 weeks of cessation. PCT is optional for short, low-dose cycles (under 15mg for 6 weeks or less) but recommended for longer or higher-dose protocols. No clinically significant hepatotoxicity has been observed across multiple human trials. Typical research protocols range from 10–25mg daily for 6–12 weeks.