S-23 is a non-steroidal selective androgen receptor modulator developed by GTx, Inc. — the same laboratory that created Ostarine (MK-2866). It represents the furthest end of the SARM potency spectrum, engineered for maximum anabolic efficacy with a binding affinity (Ki ~1.7 nM) that approaches that of dihydrotestosterone itself. S-23 has been investigated in preclinical models as a potential male hormonal contraceptive — a testament to its profound androgenic potency — and delivers results that researchers frequently describe as more comparable to anabolic steroids than to typical SARMs.

S-23 binds the androgen receptor with exceptionally high affinity and acts as a full agonist in muscle tissue, driving aggressive anabolic signaling — maximized protein synthesis, pronounced nitrogen retention, and substantial myonuclear accretion. Its tissue selectivity, while present, is narrower than milder SARMs like Ostarine: S-23 demonstrates significant androgenic activity in prostate tissue (approximately 30–40% of testosterone's prostate stimulation), though still far less than exogenous testosterone or DHT. In preclinical models, S-23 produced dramatic increases in lean body mass and bone mineral density while simultaneously reducing fat mass — a full recomposition effect rare among individual compounds.

S-23 is documented in the research literature for producing hard, dense, dry muscle gains with significant strength increases — an aesthetic and performance profile that closely parallels injectable anabolics like Masteron or Winstrol. The compound's muscle-hardening, vascularity-enhancing, and fat-reducing properties make it particularly noted for physique refinement and contest-preparation research. Additionally, S-23's investigation as a male contraceptive confirms its potent suppression of gonadotropins and spermatogenesis, which researchers must factor into protocol design.

S-23 is strictly for advanced researchers with prior SARM experience and established bloodwork baselines. Its potency and suppression profile are not appropriate for first-time or intermediate researchers. It is best suited for cutting and hardening phases where the objective is maximum muscle definition, density, and vascularity — or for researchers who have reached the ceiling of what milder SARMs can deliver and seek steroid-adjacent results without the injectable route.

S-23 has an elimination half-life of approximately 12 hours, requiring twice-daily dosing for optimal plasma stability. Oral bioavailability is high. Testosterone suppression is significant and comparable to low-dose anabolic steroid cycles — S-23 will meaningfully suppress LH, FSH, and endogenous testosterone production, and has demonstrated reversible suppression of spermatogenesis in preclinical models. Full PCT (typically Nolvadex 20–40mg for 4–6 weeks, optionally with HCG bridge) is mandatory following any S-23 cycle. Some researchers report mild androgenic effects including oily skin and increased aggression. Typical research protocols range from 10–25mg daily (split into two doses) for 8–10 weeks.