S-23 vs YK-11

Mechanism

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.

Standard Dosing

Research indicates 10-25 mg daily orally for 8-12 weeks. No human clinical trials — dosing extrapolated from rat pharmacology and anecdotal reports.

Timing

Split into 2 daily doses due to relatively short half-life (~12 hours). Morning and evening dosing.

Cycle Duration

8 week cycles maximum recommended. PCT is absolutely mandatory due to profound HPG suppression. Allow full recovery before considering subsequent cycles.

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

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

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Mechanism

YK-11 is a synthetic steroidal compound classified as a gene-selective partial agonist of the androgen receptor. Uniquely among SARMs, YK-11 does not induce the N/C terminal interaction of the AR required for full transcriptional activation, instead selectively activating a subset of AR-dependent genes. Its primary distinguishing mechanism is potent induction of follistatin expression, which directly antagonizes myostatin — a key negative regulator of skeletal muscle mass. This dual action (partial AR agonism + myostatin inhibition via follistatin) theoretically provides anabolic stimulus beyond what full AR agonists alone can achieve.

Standard Dosing

Research indicates 5-15 mg daily orally for 6-8 weeks. No human clinical trials exist — all dosing data is anecdotal.

Timing

Split into 2 daily doses (morning and evening) due to presumed short half-life (~6-10 hours based on structural analysis). Consistent timing essential.

Cycle Duration

6-8 week cycles maximum. PCT strongly recommended. Avoid extended use due to unknown long-term safety profile.

Side Effects

• Testosterone suppression (expected — steroidal compound)
• Hepatotoxicity (17-alpha-alkylated structure implies liver toxicity risk)
• Joint pain and tendon issues (anecdotal — possibly from DHT-like drying effects)
• Hair loss (steroidal androgenic effects)
• Aggression
• Unknown long-term effects (no human studies)

Contraindications

• Pre-existing liver disease
• Androgen-sensitive cancers
• Cardiovascular disease
• Pregnancy and breastfeeding
• Individuals under 21
• Anyone unwilling to accept research chemical risk with zero human clinical data

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