S-23

Mechanism of Action

S-23 is one of the most potent nonsteroidal SARMs developed, originally investigated by GTx, Inc. as a potential male hormonal contraceptive. It binds the androgen receptor with very high affinity, producing near-steroidal anabolic effects in muscle and bone while profoundly suppressing FSH and LH, leading to oligospermia and azoospermia in animal models. S-23 increases lean body mass and bone mineral density while reducing fat mass in a dose-dependent manner. The contraceptive effect was fully reversible in rat studies — spermatogenesis and fertility recovered completely after a 100-day washout period.

Dosing Protocol

Notes

S-23 is among the most potent and suppressive SARMs available, with pharmacology closer to traditional anabolic steroids than milder SARMs like ostarine. The contraceptive application is its most novel feature — 4/6 rats achieved complete azoospermia, which was fully reversible. However, this same potency makes it one of the riskiest SARMs for recreational use. CRITICAL: Full PCT is mandatory. Consider hCG during cycle to prevent testicular atrophy. Required bloodwork: Liver function panel (AST, ALT, GGT, bilirubin, ALP) at baseline and every 3-4 weeks. Total testosterone, free testosterone, LH, FSH, SHBG, estradiol (suppression will be profound). CBC/hematocrit. Lipid panel. Semen analysis if fertility is a concern (before, during, and after). Post-cycle hormonal panels monthly until recovery confirmed. Medical supervision required.

Stacking

• hCG (to maintain testicular function during the profound suppression)
• MK-677 (non-suppressive GH elevation)
• NAC / TUDCA (liver support)
• Enclomiphene (PCT)

Interactions

• Testosterone / AAS [MEDIUM] — S-23 is already profoundly suppressive. Adding exogenous androgens deepens shutdown without clear benefit over testosterone alone.
• Hormonal contraceptives (female partner) [LOW] — S-23 has contraceptive properties in males but is NOT reliable birth control — do not use as sole contraceptive method.
• Hepatotoxic agents [MEDIUM] — Potential additive liver stress. Monitor liver function.

Contraindications

• Desire for fertility in the near term (profound spermatogenic suppression)
• Pre-existing liver disease
• Androgen-sensitive cancers
• Cardiovascular disease
• Pregnancy and breastfeeding
• Individuals under 21

Side Effects

• Profound testosterone suppression (near-complete LH/FSH shutdown)
• Temporary infertility / azoospermia
• Aggression and mood changes (reported frequently)
• Hair loss (strong androgenic binding)
• Testicular atrophy
• Night sweats
• Prostate effects (despite selectivity claims, S-23 had some prostate stimulation in rats)
• HDL suppression

Key Papers

• 10.1210/en.2008-0674
• 10.1021/jm900280m
• 10.1007/s40264-023-01368-3

Source Quality

Not FDA-approved. Research chemical only with no human clinical data. One of the highest-risk SARMs due to potency and suppression profile. Source with extreme caution — third-party analytical verification essential. High adulteration risk.

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