Ibutamoren (MK-677)

Overview

Ibutamoren (development codes MK-677, MK-0677, L-163,191; also "Oratrope") is an orally active, non-peptide growth-hormone secretagogue and ghrelin-receptor (GHS-R1a) agonist developed by Merck. It is a spiropiperidine peptidomimetic small molecule — a lab-made compound designed to imitate a peptide's action at the ghrelin receptor, but it is NOT a peptide and it is NOT a SARM.

It appears on this peptide reference because it is constantly discussed and stacked alongside growth-hormone peptides (and mis-sold as a "SARM") in the fitness and "research chemical" community — not because it is one. Mechanistically it acts entirely through the ghrelin / GH / IGF-1 axis and has zero androgen-receptor activity, which is the defining feature that separates it from the true SARMs.

MK-677 does one thing very reliably: it raises pulsatile growth hormone (GH) and downstream IGF-1 to youthful levels on once-daily oral dosing. What it did not do, in its pivotal 1-year human trial, was improve muscle strength — and it worsened glucose metabolism. Merck never brought it to market; its highest development stage was Phase 2.

Chemistry and structure

MK-677 is a small-molecule peptidomimetic (a spiropiperidine), not an amino-acid chain.

Mechanism of action

• Ghrelin-receptor (GHS-R1a) agonism. MK-677 binds and activates the growth-hormone-secretagogue receptor type 1a (GHS-R1a) — the same G-protein-coupled receptor that endogenous ghrelin activates — in the hypothalamus (arcuate nucleus) and the anterior pituitary. GHS-R1a is a Gq/11-coupled GPCR; activation drives phospholipase C, IP3 generation and intracellular Ca2+ mobilization in pituitary somatotrophs, triggering GH vesicle release. [Human]
• Raises pulsatile GH and IGF-1. By mimicking ghrelin it increases pulsatile GH secretion and downstream hepatic IGF-1, while preserving physiological GH pulsatility rather than flooding the system with exogenous hormone. In Nass 2008, 24-h mean GH rose ~1.8-fold and IGF-1 rose ~1.5-fold at 12 months (both P<0.001), with IGF-1 sustained in the young-adult normal range in 22/43 subjects; Sevigny 2008 confirmed IGF-1 target engagement (+60% at 6 weeks, +73% at 12 months). [Human]
• Orally bioavailable, acts upstream. As an oral non-peptide it acts upstream at the neuroendocrine axis, distinct from injected recombinant GH. Oral activity is the defining pharmacological advantage over peptide GH secretagogues and over GH itself. [Human]
• No androgen-receptor activity. Its mechanism is entirely via the ghrelin / GH / IGF-1 axis. It does not act on the androgen receptor — it is not a SARM, and the frequent "SARM" labeling in the grey market is factually incorrect. [Human]

Research and evidence

MK-677 has real, peer-reviewed human data — which is exactly why its limitations are so well documented. The honest summary: it works as a GH/IGF-1 secretagogue but failed to deliver functional benefit and worsened glucose control in its main trials. Its highest development phase was Phase 2.

Human evidence in context. MK-677's GH/IGF-1 effect is well established. But the fat-free-mass gain was modest and did not translate into strength or function, and whether that mass reflects functional muscle versus water/edema is uncertain given the ghrelin-mimetic fluid retention. The Alzheimer's trial failed on every clinical endpoint despite hitting its IGF-1 target. Long-term human safety beyond 12–24 months is not established, and the plasma half-life (reported ~4–6 h) is not firmly pinned across independent PK sources.

Status and regulation

• Developer & phase: Developed by Merck (Merck & Co.). Highest development stage was Phase 2 (a Phase 3-sized 563-patient Alzheimer's trial was also completed, with no clinical benefit). Merck never brought it to market.
• Regulatory approval: None. No regulator (FDA/EMA) has approved MK-677 for physique, performance, aging, or any disease. The FDA and DoD (OPSS) list it as an unapproved drug. An explicit EMA statement was not located, but it is treated as unapproved everywhere.
• FDA enforcement: The FDA has issued warning letters (2025) to companies selling MK-677 — including one marketing it as a children's growth supplement — because it is not legal as a dietary-supplement ingredient or in any consumer product.
• Anti-doping: WADA-prohibited as a growth hormone secretagogue under the S2 Peptide Hormones, Growth Factors, Related Substances and Mimetics grouping (seeded as S2.2.2). WADA renumbers subsections annually, so the exact subsection digit varies by edition; the category (GH secretagogue) is stable and confirmed. WADA status is sport-eligibility, a separate axis from legality.

Safety

Documented and reported signals:

• Impaired glucose metabolism / insulin resistance — raised fasting glucose and HbA1c and reduced insulin sensitivity over 12 months at 25 mg/day. [Human] (Nass 2008, PMID 18981485)
• Fluid retention / edema and increased appetite — common trial-reported effects: increased appetite (ghrelin-mimetic), ankle/leg swelling (edema), and mild muscle/joint aches. [Human] (Nass 2008; OPSS)
• Potential congestive-heart-failure concern — the DoD Operation Supplement Safety program flags a potential for congestive heart failure, particularly in susceptible individuals; one myocardial infarction occurred 7 days after starting MK-677 in the Nass trial (causality not established, small N). [Human] (OPSS; Nass 2008 serious adverse events)
• Unapproved drug — not a legal supplement — the FDA/OPSS classify MK-677 as not approved for human use and not legal as an ingredient in dietary supplements or consumer products; FDA warning letters were issued in 2025. [Human] (FDA 2025; OPSS)
• Mis-sold as a "SARM" — MK-677 is routinely combined with SARMs or labeled as a SARM in grey-market products; it has zero androgen-receptor activity and is not a SARM. Class labeling on these products is unreliable. [Human] (OPSS)

Legal status

How it compares

MK-677 is usually grouped with injectable GH-releasing peptides, but it is a different molecule class — an oral small molecule, not a peptide:

• Ipamorelin and GHRP-2 / GHRP-6 / Hexarelin — injectable peptide ghrelin-receptor agonists that also drive GH release. MK-677 targets the same GHS-R1a receptor but is an oral non-peptide; all are early-evidence and none is an approved physique/performance drug.
• CJC-1295, Sermorelin and Tesamorelin — GHRH-analog peptides that raise GH by a different upstream mechanism (the GHRH receptor). Tesamorelin is the one FDA-approved agent in this cluster (for HIV-associated lipodystrophy), unlike MK-677, which has no approval.

The decisive difference: MK-677 reliably raised GH/IGF-1 but did not improve strength and worsened glucose control in its pivotal trial, and Merck never brought it to market. See the Muscle & growth-hormone overview.

Common misconceptions

• "MK-677 is a SARM." No. SARMs act on the androgen receptor; MK-677 has zero androgen-receptor activity and works entirely via the ghrelin / GH / IGF-1 axis. It is a GH secretagogue, routinely mis-sold as a SARM.
• "MK-677 is a peptide." No. It is a spiropiperidine peptidomimetic small molecule (C27H36N4O5S, 528.7 g/mol) — a compound designed to imitate a peptide, not one. It is covered here only because it is discussed and stacked with peptides.
• "SARMs (and MK-677) are a mild or legal alternative to steroids." No. MK-677 isn't a SARM at all, and nothing in this class is an approved, benchmarked, safe alternative to anything. MK-677 specifically impaired glucose control, didn't improve strength, and is not approved — and true SARMs carry their own liver-injury and hormone-suppression signals. "Milder/safer/legal" is not supported.
• "It builds muscle." It raised fat-free mass modestly, but strength did not improve in the Nass RCT, and part of the mass may be water/edema from ghrelin-mimetic fluid retention — the functional benefit is not established.
• "It boosts GH, so it must be good for anti-aging or Alzheimer's." GH/IGF-1 elevation is real, but it did not translate into clinical benefit: the 563-patient Alzheimer's trial found no effect on any endpoint despite confirmed IGF-1 elevation.

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This entry is educational and summarizes published research and public regulatory information. It is not medical advice, a recommendation, or a guide to obtaining or using any substance.