MK-677 (Ibutamoren) vs S-23

Mechanism

MK-677 (ibutamoren) is an orally active, non-peptide growth hormone secretagogue that mimics ghrelin by binding the ghrelin receptor (GHSR1a) in the hypothalamus and pituitary. This triggers pulsatile growth hormone release via the same physiological mechanism as endogenous ghrelin, preserving the natural episodic GH secretion pattern. MK-677 increases GH, IGF-1, and IGFBP-3 levels to those observed in young adults without affecting cortisol levels. It also stimulates appetite through ghrelin receptor activation in the hypothalamus. Unlike exogenous GH injection, MK-677 maintains the pulsatile GH pattern and stimulates all five GH isoforms.

Standard Dosing

Research indicates 10-25 mg daily orally. Clinical trials used 25 mg/day. 10-15 mg may provide GH elevation with fewer side effects.

Timing

Take 30-60 minutes before bedtime. This timing leverages the natural nocturnal GH pulse, maximizes sleep quality benefits, and minimizes daytime appetite increase. Half-life ~24 hours ensures once-daily dosing is sufficient.

Cycle Duration

Can be used continuously for months to years — no cycling required as it works through physiological GH release mechanisms. Clinical trials ran for up to 2 years. Reassess IGF-1 and metabolic markers every 3-6 months.

Side Effects

• Increased appetite (ghrelin mimetic — significant and persistent)
• Water retention and bloating (GH-mediated)
• Numbness and tingling (carpal tunnel-like symptoms)
• Elevated fasting blood glucose and insulin resistance
• Lethargy and increased sleep depth
• Vivid dreams
• Joint pain (less common than exogenous GH)

Contraindications

• Active malignancy (GH/IGF-1 promotes cell proliferation)
• Diabetes mellitus (impairs glucose tolerance — use with extreme caution)
• Active diabetic retinopathy
• History of pituitary tumors
• Congestive heart failure (fluid retention)

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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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