MK-677

MK-677 (Ibutamoren) is an orally active ghrelin mimetic that stimulates sustained GH and IGF-1 increases with once-daily dosing. It is technically not a peptide but is widely used in the peptide community for muscle, sleep, and anti-aging applications.

MK-677 (ibutamoren, ibutamoren mesylate; development code MK-0677; also designated L-163,191) is a non-peptide, orally bioavailable spiroindoline compound that acts as a potent, selective, and long-acting ghrelin receptor agonist. It is classified as a growth hormone secretagogue (GHS) because its primary pharmacological effect is to stimulate pulsatile growth hormone (GH) release from the anterior pituitary — an action it achieves by mimicking the endogenous hunger hormone ghrelin at the growth hormone secretagogue receptor 1a (GHSR-1a).

Unlike injectable GH-releasing peptides (GHRPs) such as GHRP-6 or hexarelin, which are administered subcutaneously multiple times per day, MK-677 is taken orally once daily. Its plasma half-life of approximately 24 hours allows for sustained elevation of GH and insulin-like growth factor 1 (IGF-1) with a single dose, more closely approximating the GH profile of healthy young adults in older populations where GH secretion has declined with age (somatopause). This oral convenience — combined with the combination of GH axis stimulation, improved sleep quality, and perceived improvements in body composition — has made MK-677 one of the most widely self-administered research chemicals in biohacker and fitness communities despite having no FDA approval for any indication.

Structurally, MK-677 was originally discovered and developed by Merck & Co., Inc. through the 1990s and 2000s as a potential treatment for muscle-wasting conditions, osteoporosis, and growth hormone deficiency in elderly populations. A multicenter phase IIb trial in patients recovering from hip fracture was terminated early because of a congestive heart failure safety signal, and its authors concluded MK-677 had an unfavorable safety profile in that population. Merck did not pursue FDA approval. The compound is not a peptide and is therefore not subject to the 2023 FDA final rule restricting peptide compounding, but the FDA has issued warning letters to supplement manufacturers marketing it for human consumption. It is prohibited by the World Anti-Doping Agency (WADA) under S2 (peptide hormones, growth factors, related substances) and is classified as an unapproved new drug when sold for human use in the United States.

Interest in MK-677 persists because no other oral compound has demonstrated comparable GH-axis activation in humans, and because of the sleep architecture changes documented in a small crossover trial: roughly a 50% increase in stage IV sleep and over 20% more REM sleep in young subjects, and nearly 50% more REM sleep in older ones.

MK-677 produces its effects primarily through full agonism at the growth hormone secretagogue receptor 1a (GHSR-1a), the endogenous receptor for ghrelin. GHSR-1a is a class A G-protein-coupled receptor expressed in the hypothalamus (arcuate nucleus, ventromedial nucleus), pituitary somatotrophs, hippocampus, vagal neurons, and multiple peripheral tissues. When MK-677 binds GHSR-1a in hypothalamic neurons, it triggers the release of growth hormone-releasing hormone (GHRH), which then travels via the portal circulation to the anterior pituitary where it binds to the GHRH receptor and stimulates GH secretion. Simultaneously, MK-677 acts directly on pituitary somatotrophs expressing GHSR-1a to amplify GH pulse amplitude.

The net result of this dual hypothalamic and pituitary action is a preservation and amplification of pulsatile GH secretion — the natural pattern by which GH is released in bursts primarily during the first hour of slow-wave sleep. In healthy adults aged 64 to 81, two weeks of MK-677 25 mg daily raised mean 24-hour GH concentration by 97 ± 23% relative to baseline by enhancing pre-existing GH pulses, with a dose-dependent rise in IGF-1 (Chapman et al., JCEM 1996), and two years of daily dosing restored GH and IGF-1 into the healthy young-adult range (Nass et al., Ann Intern Med 2008). This is mechanistically distinct from exogenous GH injection, which suppresses endogenous GH through negative feedback; MK-677 instead preserves or enhances the pulsatile pattern while respecting the hypothalamic-pituitary negative-feedback loop.

The GHSR-1a pathway has important downstream effects beyond GH secretion. In the hypothalamus, GHSR-1a activation mediates orexigenic signaling — stimulating appetite through NPY/AgRP neurons in the arcuate nucleus and by activating reward circuits in the nucleus accumbens. This appetite-stimulating effect is a clinically significant side effect, explaining the characteristic "MK-677 hunger" reported by most users, and has relevance to its investigated use in cachexia and anorexia states. In the CNS more broadly, ghrelin receptor signaling influences memory consolidation, anxiety regulation, and dopaminergic neurotransmission, which may contribute to the cognitive and mood effects some users report.

In adipose tissue, MK-677 indirectly increases GH signaling, which promotes lipolysis in the fasted state and inhibits insulin-stimulated glucose uptake (contributing to the insulin resistance and fasting hyperglycemia observed clinically). In bone, the GH/IGF-1 axis stimulated by MK-677 activates bone remodeling through IGF-1-mediated osteoblast proliferation and differentiation. In skeletal muscle, IGF-1 promotes protein synthesis and nitrogen retention, which is responsible for the lean mass increases documented in elderly trial populations — though the proportion of lean mass gain attributable to water versus contractile protein has been debated.

MK-677 has been evaluated in more clinical research than almost any other research chemical in widespread use, yet it remains unapproved due to a combination of modest efficacy for its primary target indications, the cardiac safety signal in elderly populations, and Merck's commercial decision to deprioritize the program.

**GH/IGF-1 activation: Chapman et al. 1996 (JCEM).** A randomized, double-blind, placebo-controlled dose-ranging trial in 32 healthy subjects aged 64 to 81. Two weeks of MK-677 25 mg/day raised mean 24-hour GH concentration by 97 ± 23% versus baseline, and IGF-1 rose dose-dependently. Importantly, the increase came from enhancing pre-existing pulsatile GH secretion rather than creating new pulses. In healthy young men, a separate 7-day crossover study (Copinschi et al., JCEM 1996) found total GH secreted was unchanged while pulse frequency rose, so the profile of the response differs by age group.

**Sleep quality: Copinschi et al. 1997 (Neuroendocrinology).** A polysomnographic crossover study in 8 young subjects (18–30) and 6 older subjects (65–71). In the young group, 25 mg MK-677 at bedtime for 7 days increased stage IV sleep duration by roughly 50% and REM sleep by more than 20% versus placebo, and the frequency of deviations from normal sleep fell from 42% on placebo to 8%. In older adults, REM sleep rose nearly 50% and REM latency fell. These are the strongest objective sleep data for any growth hormone secretagogue, but the sample is 14 people in total.

**Body composition: Nass et al. 2008 (Annals of Internal Medicine).** A 2-year, randomized, double-blind, placebo-controlled modified-crossover trial in 65 healthy adults aged 60–81. Daily MK-677 25 mg raised GH and IGF-1 into the healthy young-adult range. Fat-free mass increased 1.1 kg on MK-677 versus a 0.5 kg decrease on placebo (P<0.001), and body cell mass rose. But abdominal visceral fat and total fat mass did not differ, limb fat increased more on drug than placebo (1.1 vs 0.24 kg), and body weight rose 2.7 kg. No improvement in strength or physical function accompanied the fat-free mass gain. Fasting glucose rose about 5 mg/dL and insulin sensitivity fell. Bone mineral density changes were consistent with increased remodeling rather than a clear density gain.

**Cardiac safety and early termination.** The multicenter randomized placebo-controlled hip-fracture trial (Adunsky et al., Arch Gerontol Geriatr 2011, n=123) was a phase IIb study, not phase 3, and was terminated early because of a congestive heart failure safety signal in a limited number of patients. IGF-1 rose significantly on drug (+51.4 ng/mL vs placebo) but most functional performance measures did not improve, and the authors concluded MK-677 has an unfavorable safety profile in this population. The often-quoted 6.5% versus 1.7% congestive heart failure figures do not appear in that publication's abstract, and we have not verified them against a retrievable source, so they are not stated as fact here.

**Cancer and IGF-1 concern.** MK-677 chronically elevates IGF-1, a mitogenic growth factor that activates the PI3K/Akt/mTOR pathway in both normal and neoplastic cells. Epidemiological studies have associated chronically elevated endogenous IGF-1 with increased risk of prostate, colorectal, and breast cancers. While no human clinical trial has directly documented MK-677-induced cancer, the theoretical risk is considered clinically meaningful, and MK-677 use is generally considered contraindicated in individuals with a personal or family history of hormone-sensitive malignancies.[1][2][3][4][5][6][7][8]

Peptides commonly paired with MK-677 for synergistic effects.