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MK-677 (Ibutamoren): Not a Peptide but a Pill That Mimics Ghrelin — What It Is and What Its Human Trials Show

MK-677, or ibutamoren, is not a peptide: it is a small molecule of 528.7 Da that Merck developed in the 1990s to copy ghrelin's action on the growth-hormone secretagogue receptor, taken by mouth. It was never approved. What it is, what its trials in older adults and hip-fracture patients found, and why one was stopped early.

Not approved anywhere in the records checkedNot a peptide — a small-molecule ghrelin mimeticTaken by mouth in trials

Caroline S · Published 2026-10-22

Length

Not a peptide: a spiro-indoline-piperidine small molecule, C27H36N4O5S, 528.7 Da (PubChem CID 178024); trials used the mesylate salt

Sequence

None — it has no amino-acid chain. It is listed here because it is sold and discussed beside the peptide growth-hormone secretagogues

Origin

Merck (development code MK-0677), 1990s. Never approved

Last reviewed

2026-10-22

What is it good for?

Raising growth hormone and IGF-1 levels by acting like ghrelin, which its trials showed it does. Whether that rise turns into anything people care about — strength, function, recovery after a fracture — is what the trials tested, and on those measures the results were mostly negative.

Illustration: A single white pill on a white surface with subtle green and copper reflections.
Illustration

What it is

MK-677 is not a peptide. Its generic name is ibutamoren; PubChem lists it as C27H36N4O5S, molecular weight 528.7 Da, with a spiro-indoline-piperidine skeleton and no amino-acid chain (CID 178024). Merck developed it in the 1990s under the code MK-0677; the trials used ibutamoren mesylate.

It is in this library because it is sold, searched and argued about beside the peptide growth-hormone secretagogues — ipamorelin, GHRP-6, the six-residue hexarelin — and does the same job by the same receptor. The difference that matters to the chemistry: most peptides are broken down in the gut; MK-677 is not a chain of amino acids and survives being swallowed, which is why it was developed as a pill.

How it works

Ghrelin, a 28-residue hormone from the stomach, triggers growth-hormone release through the growth-hormone secretagogue receptor (GHS-R1a) in the pituitary and hypothalamus, and also drives hunger. MK-677 is a ghrelin mimetic — the 2008 trial calls it "an oral ghrelin mimetic" — so it does both: growth hormone and IGF-1 rise, and appetite increases.

What the human trials show

Three trials carry most of the outcome evidence that gets quoted about it:

Trial Who 25 mg/day vs placebo Finding
Nass 2008, Ann Intern Med (PMID 18981485) 65 healthy adults, 60–81, 2 years fat-free mass +1.1 vs −0.5 kg at 1 year GH and IGF-1 to young-adult range; weight +2.7 vs +0.8 kg; no change in visceral fat, strength or function; fasting glucose +0.3 mmol/L, insulin sensitivity down, cortisol up
Adunsky 2011 (PMID 21067829) 123 elderly hip-fracture patients, phase IIb IGF-1 +51.4 ng/mL gait speed better, most functional measures not; stopped early for a congestive heart failure signal
Svensson 1999 (PMID 10468903) 24 obese men, 19–49, 8 weeks — leptin up at 2 weeks only; free T3 up at 8 weeks; total testosterone down

The pattern across them: the hormone numbers move as designed; the outcomes people take it for — strength, function, recovery — mostly do not. No randomised trial of sleep, skin or hair outcomes turned up in the census.

Safety

In the older-adult trial the most frequent side effects were increased appetite that faded over a few months, and transient, mild leg swelling and muscle pain. The metabolic signal is the one to read carefully: fasting glucose rose and insulin sensitivity fell. The hip-fracture trial was stopped because of congestive heart failure in a limited number of patients, and its authors called the safety profile "unfavorable" in that population. Products sold online are unregulated; what is in them is not covered by any of these trials. Questions about any individual's use belong with a clinician.

Where it stands

  • Not approved: openFDA's Drugs@FDA data has no application for ibutamoren or ibutamoren mesylate (searched 2026-10-22).
  • Research record: PubMed 162 records, 19 randomised controlled trials; ClinicalTrials.gov 8 studies (2026-10-22).
  • Questions about it in a training context are MuscleLedger's subject: see its MK-677 page.

The hormone MK-677 imitates is ghrelin. The peptide drugs that act on the same receptor include ipamorelin and GHRP-6; the ones that act on the neighbouring GHRH receptor include sermorelin and tesamorelin.

What the research shows

Raises growth hormone and IGF-1

Two-year RCT in 65 healthy adults aged 60–81 (Nass 2008, PMID 18981485); IGF-1 +51.4 ng/mL vs placebo in 123 hip-fracture patients (Adunsky 2011, PMID 21067829)

Builds lean mass

Fat-free mass +1.1 vs −0.5 kg at one year in healthy older adults, without any change in strength or function (PMID 18981485)

Improves recovery after hip fracture

Not shown: no improvement in most functional measures; the trial was stopped early for a congestive heart failure signal (PMID 21067829)

Improves sleep, skin or hair

No randomised trial of these outcomes found in the PubMed census of 2026-10-22

Bars show how much of the evidence is in humans, not how well anything works.

Where it stands

United States

Not approved: openFDA's Drugs@FDA data holds no application for ibutamoren or its mesylate salt (searched 2026-10-22).

Sold as

A research chemical and on supplement-style sites, outside any approval; the trials gave 25 mg a day by mouth.

Published record

PubMed: 162 records for ibutamoren, MK-677 or MK-0677, 19 tagged as randomised controlled trials; ClinicalTrials.gov: 8 studies (2026-10-22).

Frequently asked questions

Is MK-677 a peptide?

No. MK-677, also called ibutamoren, is a small organic molecule of 528.7 Da with no amino-acid chain. It is often grouped with peptides because it does the same job as the peptide growth-hormone secretagogues such as ipamorelin and GHRP-6: it switches on the ghrelin receptor. Unlike them, it works when swallowed.

What does MK-677 do?

It mimics ghrelin, the stomach hormone that triggers growth-hormone release, so growth hormone and IGF-1 rise. In a two-year trial in healthy older adults it raised both into the young-adult range and added about 1.1 kg of fat-free mass, but strength and function did not change, and fasting blood sugar rose.

Is MK-677 approved?

No. Merck developed it in the 1990s as MK-0677, and openFDA's Drugs@FDA data holds no application for it (searched 2026-10-22). What is sold online is sold outside any approval.

Why was an MK-677 trial stopped?

A phase IIb trial in elderly patients recovering from hip fracture was terminated early because of a safety signal of congestive heart failure in a limited number of patients. The authors concluded MK-677 had an unfavourable safety profile in that population (PMID 21067829).

Does MK-677 raise blood sugar?

In the two-year trial in older adults, fasting glucose rose by an average 0.3 mmol/L (5 mg/dL) and insulin sensitivity decreased on MK-677 (PMID 18981485). What that means for any one person is a question for a clinician.