MK-677 (oral)

MK-677 (ibutamoren, ibutamoren mesylate, MK-0677) is a non-peptidic, orally bioavailable ghrelin mimetic developed by Merck in the 1990s. It is the only ghrelin-pathway growth hormone secretagogue that does not require injection, since it is not degraded in the gut.

Despite persistent vendor labelling, it is not a SARM: it has no androgen receptor interaction and acts exclusively at GHS-R1a to drive pituitary GH release, with downstream hepatic IGF-1 generation.

The defining liability is selectivity. Unlike ipamorelin, MK-677's receptor occupancy carries substantial off-target load: marked appetite stimulation, prolactin elevation, possible cortisol elevation, and impaired insulin sensitivity with prolonged exposure. A half-life of approximately 24 hours means sustained rather than pulsatile exposure, so those effects persist across the dosing interval.

Regulatory status: no FDA approval for any indication, classified as an unapproved drug in the US, not legal for sale as a drug, food or dietary supplement there, and prohibited by WADA under S2 (peptide hormones, growth factors, related substances and mimetics).

Interaction considerations documented across sources: insulin and antidiabetic medication, since MK-677 raises fasting glucose and impairs insulin sensitivity and glycaemic management will need adjustment; GLP-1 agonists such as retatrutide, semaglutide and tirzepatide, which create a pharmacological conflict by suppressing the appetite MK-677 drives; additive GH/IGF-1 elevation with other secretagogues producing oedema, joint pain and carpal tunnel-like symptoms; prednisone, where hyperglycaemia and severe oedema can compound; other appetite-stimulating or CNS-active agents that add to its hunger drive; levothyroxine efficacy and thyroid hormone conversion; and diuretics, antihypertensives and other agents affecting fluid and electrolyte balance. Human interaction data beyond the metabolic picture is limited.

MK-677 is an agonist at GHS-R1a, the ghrelin receptor. Two afferent signals govern somatotroph output: the GHRH pathway, which builds and releases GH from the hypothalamic side, and the ghrelin pathway, which triggers immediate release of stored GH while simultaneously suppressing somatostatin, the inhibitory brake. Ghrelin-pathway activation therefore both stimulates release and removes inhibition, and co-stimulation of both pathways produces a response substantially larger than either alone — the rationale behind cross-pathway stacking.

Where ipamorelin is highly selective for GH release at GHS-R1a, MK-677 is less selective downstream: research shows it also elevates prolactin, can increase cortisol, and drives significant appetite stimulation. These are the core trade-off for oral administration.

Pulsatility is the second structural difference. Endogenous GH is secreted in pulses, the dominant one slow-wave-sleep-linked, and that pattern matters for receptor sensitivity and for downstream IGF-1 responsiveness. With a half-life of approximately 24 hours, MK-677 produces sustained elevation rather than pulse-and-clear kinetics. Chapman et al., 1996 showed the mechanism precisely: GH pulse amplitude increased without change in pulse frequency, meaning existing pulses are amplified rather than new ones created.

Hepatic conversion of GH to IGF-1 supplies the effector for muscle protein synthesis, cell repair, recovery and collagen production. At 25 mg daily, IGF-1 rose approximately 55% at 2 weeks and 88% at 4 weeks in trial subjects, reaching the range of healthy young adults.

Exogenous GH (HGH 191aa) or direct IGF-1 trigger somatostatin release via negative feedback and blunt ghrelin-pathway responsiveness; after a single injection of exogenous GH, ghrelin pathway compounds were blunted by about 32%.

GH/IGF-1 axis. The best-evidenced effect. Multiple RCTs show 25 mg daily reliably raising GH secretion and IGF-1. In healthy elderly subjects mean 24-hour GH concentration rose 97% and IGF-1 reached the healthy young adult range.

Body composition. Nass et al., 2008 (2-year RCT) recorded fat-free mass +1.1 kg versus -0.5 kg on placebo. Svensson et al., 1998 in obese males found sustained fat-free mass increase with a transient rise in basal metabolic rate, significant at 2 weeks but absent by 8 weeks — evidence of adaptation to sustained elevation. Critically, Nass found no improvement in strength or physical function, so the accrued mass did not yield functional gain.

Anti-catabolic. Murphy et al., 1998 showed reversal of diet-induced nitrogen wasting under caloric restriction: nitrogen balance +0.31 g/day on MK-677 versus -1.48 g/day on placebo.

Sleep architecture. Commonly reported improvement in depth and restfulness, consistent with GHS-R involvement in sleep regulation and with the slow-wave-sleep-linked GH pulse; reported effects include increased time in deep restorative stages, REM duration increased by 50%, and increased stage 2 sleep. No RCT has used sleep quality as a primary endpoint for MK-677.

Skeletal. Increased markers of bone formation and resorption over 8 weeks in obese young males, with osteocalcin rising as a turnover marker of improving BMD; osteoblast stimulation and maintenance of bone mass are described, but long-term BMD data specific to MK-677 is limited.

Regenerative, cardiometabolic and behavioural. Accelerated wound healing and tissue regeneration; reported cardiac and immune support and improved cholesterol profile; mood stabilisation; improved libido and sexual function. These sit against a consistent trial signal of rising fasting glucose and falling insulin sensitivity, so glycaemic claims should be treated as dose- and duration-dependent in both directions.

Trial-derived timelines: IGF-1 rises significantly within 2 weeks and continues climbing through 4 weeks (Chapman et al., 1996). Fat-free mass gains reached statistical significance only at 12 months in the 2-year Nass et al., 2008 trial. Svensson et al., 1998 recorded increased fat-free mass and basal metabolic rate within 2 months in obese males, but the BMR increase was transient and had faded by 8 weeks — a clear adaptation signal to sustained exposure. Nass found no improvement in strength or physical function, so accrued tissue should not be assumed to be performance-relevant.

User-reported timelines, aggregated from external platforms and weaker than published data: appetite stimulation within the first few days, usually the first and most obvious effect; water retention and mild bloating at 1 to 2 weeks, particularly facial and hand; sleep quality improvement at 1 to 2 weeks; body composition change including increased fullness and modest fat loss at weeks 4 to 8; skin and hair quality change after 2 to 3 months.

Much of the early fullness is plausibly water and glycogen rather than contractile tissue. Numbness or tingling in the hands during the first 2 to 4 weeks is consistent with GH-driven fluid retention and carpal tunnel-type compression. Lethargy and daytime drowsiness cluster around morning dosing. Fatigue in the first couple of weeks, headache, vivid dreams, joint stiffness and increased sweating also feature.

Appetite stimulation and water retention are the two near-universal effects and the most common reasons for discontinuation. Glycaemic drift is the slower one: fasting glucose and HbA1c both trend upward with duration, which is why monitoring matters more than symptoms here.

Oral capsule or tablet. No reconstitution, no refrigeration.

Standard protocol: 10 to 25 mg once daily, before bed, for 8 to 16 weeks, then reassess. Titrate by starting at 10 mg for the first 1 to 2 weeks to assess tolerance and increasing to 25 mg if appetite and water retention are manageable. 25 mg is the most studied dose across Chapman et al., 1996 (which tested 2, 10 and 25 mg daily, with 25 mg producing the greatest GH and IGF-1 elevation), Svensson et al., 1998, Murphy et al., 1998 and Nass et al., 2008.

Goal-based tiers: general GH support 10 to 15 mg daily; body composition 25 mg daily; anti-catabolic support during caloric restriction 25 mg daily.

Timing rationale: evening dosing minimises daytime appetite burden and aligns with the sleep-linked GH pulse; morning dosing is the timing most associated with lethargy. Empty-stomach administration optimises absorption and avoids the insulin-mediated blunting of somatotroph response, since postprandial insulin and free fatty acids suppress GH release. The trials did not require fasting.

Cycling: Nass et al., 2008 used continuous daily dosing for up to 2 years with sustained GH and IGF-1 elevation throughout, so there is no efficacy-based requirement to cycle. Receptor desensitisation work on hexarelin supports a 4-week break after approximately 16 weeks of continuous use, and a 4 to 8 week washout is a reasonable metabolic reset given the glycaemic drift.

Monitoring is the non-negotiable part: fasting glucose and HbA1c at baseline, 4 weeks and every 3 months; IGF-1 to confirm response and keep levels in range; prolactin at baseline and 4 to 8 weeks; blood pressure, which fluid retention can elevate.

MK-677 carries more human RCT data than most secretagogues, without FDA approval for any indication.

Chapman et al., 1996 (J Clin Endocrinol Metab): RCT in 32 healthy elderly subjects (64 to 81), placebo or 2, 10 or 25 mg daily over 14 and 28 day periods. The 25 mg arm raised mean 24-hour GH concentration by 97% and normalised IGF-1 to young adult range; pulse amplitude increased without change in pulse frequency. Fasting glucose rose from 5.4 to 6.8 mmol/L at 4 weeks; prolactin rose 23% within normal range; cortisol unchanged. IGF-1 rose approximately 55% at 2 weeks and 88% at 4 weeks.

Svensson et al., 1998 (J Clin Endocrinol Metab): RCT, 24 obese males (BMI over 30), 25 mg or placebo for 8 weeks. Sustained rises in GH, IGF-1 and IGFBP-3; fat-free mass increased; basal metabolic rate significantly elevated at 2 weeks but not at 8, indicating adaptation to sustained exposure.

Murphy et al., 1998 (J Clin Endocrinol Metab): crossover RCT, 8 healthy volunteers (24 to 39) at 18 kcal/kg/day for two 14-day periods, 25 mg or placebo during the second week. MK-677 reversed diet-induced nitrogen wasting: +0.31 g/day versus −1.48 g/day on placebo.

Nass et al., 2008 (Ann Intern Med): RCT, 65 healthy older adults (60 to 81), 25 mg or placebo for up to 2 years — the longest trial available. Fat-free mass +1.1 kg versus −0.5 kg on placebo, significant by 12 months, with no improvement in strength or physical function. Fasting glucose increased, insulin sensitivity decreased, cortisol rose 47 nmol/L.

Bach et al., 2004 (Arch Intern Med): multicentre RCT, 123 elderly hip fracture patients, 24 weeks, terminated early on a CHF signal (6.5% versus 1.7%).

Svensson et al., 1998 (J Bone Miner Res) showed increased bone formation and resorption markers over 8 weeks in obese young males; BMD outcomes remain limited. Copinschi et al., 1997 reported improved sleep quality with prolonged oral treatment, though no RCT has used sleep as a primary endpoint.