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Is MK-677 (Ibutamoren) a Peptide? No — It Is an Oral Small-Molecule Ghrelin Mimetic. What the Trials Showed

MK-677 (ibutamoren) is sold next to peptides and stacked with them, but it is a small synthetic molecule that activates the ghrelin receptor by mouth. This guide explains what it is, how it differs from GHRH and GHRP peptides, what its clinical trials showed — including the safety signal that stopped one — and its regulatory status.

By The Peptides Codex Editorial TeamReviewed September 8, 2026

The short answer

MK-677, whose non-proprietary name is ibutamoren, is not a peptide. It is a synthetic small molecule — a spiroindoline sulfonamide of about 529 Da — that contains no amino-acid chain and no peptide bond. It sits in peptide catalogues because it does something peptides in the growth-hormone cluster also do: it activates the ghrelin receptor (GHS-R1a) and so raises growth hormone and IGF-1. The difference is that it does it as a stable, orally bioavailable molecule with a half-life of roughly 24 hours, where the peptide secretagogues are injected and last minutes to hours.

What it actually is

Ibutamoren was developed by Merck in the 1990s as an oral growth-hormone secretagogue, mimicking the action of the then newly discovered hormone ghrelin. It binds the same receptor as ghrelin and as the GHRP peptides (GHRP-2, GHRP-6, ipamorelin, hexarelin), triggering pulsatile GH release from the pituitary and, with continued dosing, a sustained rise in IGF-1. It is not a GHRH analogue — sermorelin, CJC-1295 and tesamorelin act at a different receptor — and it is not a hormone itself. Chemically it is closer to a conventional drug tablet than to anything else on this site.

MK-677 vs the peptide secretagogues
MK-677 (ibutamoren)IpamorelinSermorelin
Chemical classSmall molecule (~529 Da)Pentapeptide29-residue GHRH fragment
ReceptorGhrelin receptor (GHS-R1a)Ghrelin receptor (GHS-R1a)GHRH receptor
RouteOral, once dailySubcutaneous injectionSubcutaneous injection
Half-life≈ 24 h≈ 2 h≈ 10–20 min
Approval statusNot approved anywhereNot approvedFormerly approved (Geref), withdrawn from US market 2008

What the trials showed

Ibutamoren has a genuine clinical literature, which is more than most compounds in its catalogue neighbourhood can say. In a two-year randomised trial in 65 healthy adults aged 60 to 81 (Nass et al., Annals of Internal Medicine 2008), 25 mg daily raised GH and IGF-1 to young-adult levels and increased fat-free mass by about 1.1 kg versus placebo, without a significant change in strength or function; fasting glucose and insulin rose and some participants developed impaired glucose tolerance. Earlier trials in older adults and in hip-fracture recovery reported similar hormonal effects with increased appetite and oedema. A phase 2 trial in heart failure was halted because of an excess of congestive heart failure events in the treatment arm, which effectively ended development for that indication.

Side effects the trials recorded

Increased appetite is near-universal and is the mechanism at work — ghrelin is the hunger hormone. Fluid retention and peripheral oedema, transient increases in fasting glucose and insulin resistance, muscle pain, and lethargy were reported. Prolactin and cortisol were not consistently elevated, which distinguishes ghrelin-receptor agonists from older GHRPs. The heart-failure signal is the serious one and the reason no company has pursued approval in adults; a related compound, LUM-201 (ibutamoren mesylate), is in paediatric growth-hormone-deficiency trials under a different sponsor.

Why it shows up in peptide stacks

Because it raises GH and IGF-1 by mouth, ibutamoren is marketed as a convenient substitute for, or addition to, injectable GHRH and GHRP peptides. The pharmacological logic is real — it acts on the same receptor as ipamorelin — but the combination claims are not: no trial has tested ibutamoren with any peptide, and stacking two ghrelin-receptor agonists has no published rationale. Grey-market material is unregulated and, for a small molecule, unusually easy to counterfeit, since capsules give no visual cue.

Status and what this page is not

Ibutamoren is not approved for any use in any jurisdiction and is prohibited in sport by WADA as a growth-hormone secretagogue. It is sold online as a research chemical, which does not change its legal status for human use in the US, Canada or the EU. This page corrects a classification error and summarises the evidence; it is not guidance on obtaining or using anything, and it is not medical advice.

Sources and what they cover

These primary studies, official records and standards cover the topics linked below. The scope notes distinguish laboratory findings, human trials and regulatory records. This is a selected bibliography, not a systematic review or verification of every sentence; absence of a study here does not prove that no study exists.

  1. Murphy et al. (1998) — Oral MK-677 and diet-induced catabolism

    Small human endocrine study of a nonpeptide secretagogue; not approval or general muscle-building evidence.

    Relevant sections: The short answer · What it actually is · What the trials showed

  2. IUPAC Gold Book — peptides

    Chemical terminology; no evidence of treatment effectiveness.

    Relevant sections: The short answer

  3. Health Canada (2026) — unauthorized injectable peptides bought online

    Canadian safety and authorization notice for the products described. It does not establish approval status in every country.

    Relevant sections: Status and what this page is not

FAQ

Is MK-677 a peptide?+

No. Ibutamoren is a small synthetic molecule (about 529 Da) with no amino-acid chain. It activates the ghrelin receptor orally, which is why it is grouped with growth-hormone peptides.

What is the difference between MK-677 and ipamorelin?+

Both activate the ghrelin receptor, but MK-677 is an oral small molecule with a ~24-hour half-life and ipamorelin is an injected pentapeptide lasting about two hours. Neither is approved.

Is MK-677 approved?+

No, nowhere. A heart-failure trial was halted for safety, and no adult indication has been pursued since. A related compound is in paediatric trials.

What did the MK-677 trials show?+

In older adults, 25 mg daily raised GH and IGF-1 and modestly increased fat-free mass, with increased appetite, fluid retention and worsened glucose tolerance.

Is MK-677 banned in sport?+

Yes. It is on the WADA prohibited list as a growth-hormone secretagogue.

Related peptide profiles

Ipamorelin
Growth hormone secretagogue (research compound)
Ipamorelin (NNC 26-0161) is a synthetic pentapeptide — Aib-His-D-2-Nal-D-Phe-Lys-NH2 (C38H49N9O5, ≈712 Da) — developed by Novo Nordisk as the first growth hormone secretagogue that released growth hormone without stimulating ACTH or cortisol. It acts at the ghrelin receptor. It has never been approved: phase 2 trials in post-operative ileus were completed without a filing, and it is sold today as an unapproved research peptide and prohibited in sport.
Ghrelin
Hormone
Ghrelin is a 28-amino-acid hunger hormone requiring unique post-translational octanoylation for receptor activation — a key example of peptide modification biology.
Sermorelin
GHRH analogue (former prescription medicine)
Sermorelin is a synthetic 29-amino-acid peptide (C149H246N44O42S, ≈3,358 Da) that reproduces the amino-terminal, biologically active segment of human growth hormone-releasing hormone, GHRH(1-29)-NH2. It activates the GHRH receptor on pituitary somatotrophs to release growth hormone. It was approved as Geref for diagnosing and treating growth hormone deficiency in children, but US marketing was discontinued in 2008; it has no current approval in the US and is prohibited in sport.
CJC-1295
Long-acting GHRH analogue (research compound)
CJC-1295 (C165H269N47O46, ≈3,647 Da) is a 30-residue analogue of growth hormone-releasing hormone(1-29) engineered by ConjuChem to bind serum albumin covalently after injection, extending its action from minutes to days. In healthy adults a single injection raised growth hormone 2- to 10-fold for six days or more and IGF-1 1.5- to 3-fold for 9–11 days, with an estimated half-life of 5.8–8.1 days. It was never approved: a phase 2 trial in HIV-associated visceral obesity was terminated, and it is now an unapproved research peptide prohibited in sport.
Tesamorelin
GHRH analogue (approved medicine)
Tesamorelin (Egrifta, TH9507) is a stabilised synthetic 44-amino-acid analogue of human growth hormone-releasing hormone (C221H366N72O67S, ≈5,136 Da). It stimulates pituitary growth hormone release and was approved by the US FDA in November 2010 to reduce excess abdominal fat in HIV-infected patients with lipodystrophy — the only GHRH analogue with a current marketing approval. It is also being studied for liver fat, cognition and physical function in people with HIV.
Hexarelin
Research peptide
Hexarelin (examorelin) is a synthetic hexapeptide growth hormone secretagogue studied for potent GH-releasing activity and related receptor pharmacology.
GHRP-2
Research peptide
GHRP-2 (pralmorelin) is a synthetic growth hormone–releasing peptide that acts at the ghrelin receptor. It is a foundational compound in GH secretagogue research education.

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Disclaimer: Educational content only. Not medical advice. Not instructions for human use. Regulations vary by jurisdiction.
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