MK-677 occupies a strange position in this market. It is the most-studied compound sold for muscle growth outside the approved drugs — more randomised human trials than BPC-157, ipamorelin, CJC-1295 and TB-500 combined — and it is also the one most often described inaccurately, starting with the word peptide. This page reports what the trials measured, including the parts that do not help the sales pitch. It is research journalism and contains no usage guidance; this site publishes no dose figures, and the dosage question is answered by Peptifact's guide to reading a dosing claim rather than here.
It is not a peptide
MK-677, also called ibutamoren, is not a peptide. PubChem records it as a spiro-indole sulfonamide, C27H36N4O5S, 528.7 daltons. It binds the growth hormone secretagogue receptor — the receptor for ghrelin, the body's own hunger and growth-hormone signal — and it does so as a small molecule rather than as a chain of amino acids.
That single structural fact explains the whole commercial position of this compound. Peptides are digested. Ipamorelin, CJC-1295 and the rest of the injectable secretagogues are injected because swallowing them destroys them; the oral peptide category is largely a category error for exactly that reason. MK-677 was designed to survive the gut. When a vendor lists it beside injectable peptides under one heading, the heading is wrong, and the reason it is wrong is the reason MK-677 sells.
The count
Searched on PubMed, 2026-09-07.
| Query | Records |
|---|---|
| MK-677 or ibutamoren, all records | 138 |
| …restricted to randomised controlled trials | 18 |
| …restricted to human records | 91 |
| Capromorelin, all records | 52 |
| …restricted to randomised controlled trials | 4 |
| Anamorelin (the same class, oncology) | 11 RCTs |
Set that against this site's evidence ledger, where BPC-157 returns zero clinical trials and CJC-1295 returns 33 records of any kind, and MK-677 is not in the same evidential category as the compounds it is sold alongside. It is better evidenced. What follows is what that evidence says.
The muscle trial, in full
The paper everyone cites is Nass and colleagues, Annals of Internal Medicine, 2008: a two-year, double-blind, randomised, placebo-controlled trial in 65 healthy adults aged 60 to 81, with fat-free mass and abdominal visceral fat as the primary endpoints at 12 months.
It is cited because of one result. It should be cited for five.
| What was measured | MK-677 | Placebo | |
|---|---|---|---|
| Fat-free mass | +1.1 kg (CI 0.7 to 1.5) | −0.5 kg (CI −1.1 to 0.2) | p < 0.001 |
| Abdominal visceral fat | no significant difference | primary endpoint, missed | |
| Total fat mass | no significant difference | ||
| Limb fat | +1.1 kg | +0.24 kg | p = 0.001 |
| Body weight | +2.7 kg (CI 2.0 to 3.5) | +0.8 kg (CI −0.3 to 1.8) | p = 0.003 |
| Fasting blood glucose | +5 mg/dL (0.3 mmol/L) | p = 0.015 | |
| Cortisol | +47 nmol/L | p = 0.020 |
Three things in that table rarely travel with the headline.
One of the two primary endpoints was missed. The trial set out to show that MK-677 both preserved fat-free mass and reduced abdominal visceral fat. It did the first and not the second. Visceral fat did not move, total fat mass did not move, and limb fat rose more than four times as much on drug as on placebo. A compound marketed for body recomposition produced, in its own registration-grade trial, more limb fat than placebo.
The weight gain is larger than the lean gain. Body weight rose 2.7 kg while fat-free mass rose 1.1 kg. Fat-free mass in this trial was measured by dual-energy X-ray absorptiometry, which counts water as lean; the authors tracked body cell mass separately through intracellular water and saw a smaller change. Some of the gain is muscle, some of it is fluid, and the trial is honest about not being able to separate them cleanly.
The added mass did not show up as strength or function, and the authors say the trial could not settle why. Their own sentence is "Increased fat-free mass did not result in changes in strength or function", immediately qualified: the study's stated limitation is that "Study power (duration and participant number) was insufficient to evaluate functional end points in healthy elderly persons." Both halves belong together. Strength and function were measured and did not move; the trial was also not built to interpret them. Anyone quoting the first half as proof that MK-677 does nothing, or the second half as proof that it would have worked in a bigger study, is taking one half of a sentence the authors wrote whole.
The study was funded by the NIH; Merck supplied the drug and the placebo.
The largest trial was negative, and it was not about muscle
In the same year, Sevigny and colleagues published in Neurology the largest randomised trial MK-677 has ever had: 563 patients with mild-to-moderate Alzheimer's disease, randomised to MK-677 or placebo for 12 months, on the hypothesis that raising IGF-1 would slow the disease.
The drug worked pharmacologically. Serum IGF-1 rose 60.1% at six weeks and 72.9% at twelve months. On all four efficacy measures — the clinician's impression of change, the cognitive subscale, activities of daily living, and the clinical dementia rating — there was no significant difference from placebo.
The indication is not this site's subject, and the result says nothing directly about muscle. What it does say is worth carrying anyway: raising IGF-1 by nearly three-quarters, for a year, in a sample nearly nine times the size of the muscle trial, did not deliver the downstream outcome the increase was supposed to cause. The IGF-1 rise is the mechanism every MK-677 sales page cites. This is the one large, well-powered test of whether that mechanism reliably produces an endpoint, and it came back null.
Capromorelin: the trial nobody quotes
There is one trial in this drug class that measured function properly and found something. It used a different compound.
White and colleagues, Journal of Clinical Endocrinology & Metabolism, 2009: 395 men and women aged 65 to 84 with mild functional limitation, randomised across four dosing arms of oral capromorelin or placebo, intended to run two years and terminated early under predetermined treatment-effect criteria — 315 participants completed six months and 284 completed twelve.
At six months, pooled across the active arms: body weight rose 1.4 kg against a 0.2 kg fall on placebo (p = 0.006); lean body mass rose 1.4 kg against 0.3 kg (p = 0.001); and tandem walk improved by 0.9 seconds (p = 0.02). By twelve months, stair climb had improved too (p = 0.04). Adverse events were fatigue, insomnia, and small increases in fasting glucose and glycated haemoglobin.
That is the strongest functional result in the growth hormone secretagogue literature, and it is roughly six times the sample size of the MK-677 muscle trial. It is also almost entirely absent from consumer coverage of this category — which matters, because the honest version of the MK-677 argument leans on a trial of a different molecule.
Two cautions belong with it. The participants had existing functional limitation, so there was room to improve that a trained adult does not have. And the compound's development went in an unexpected direction: PubMed carries 19 capromorelin records in dogs and cats, including a daily-administration safety study in cats, and capromorelin is used in veterinary medicine as an appetite stimulant. The oral secretagogue with the best functional data in humans found its market in animals.
The metabolic trade-off is consistent
Across both compounds and both trials, the same two things happen together: appetite goes up, and glucose handling gets worse. MK-677 raised fasting glucose by about 5 mg/dL and reduced insulin sensitivity; capromorelin raised fasting glucose and glycated haemoglobin. Neither effect was severe in a monitored trial population over a year.
The reason it is worth naming rather than filing under minor side effects is that it is mechanistically expected — these compounds work by mimicking ghrelin, and ghrelin's job includes raising blood glucose — and it is therefore unlikely to be avoidable by choosing a better-made vial. It is what the class does.
There is one harder signal on the record, and it is the reason this compound has a regulatory status at all. On FDA's bulk drug substances evaluation, ibutamoren is placed in category 2 under both 503A and 503B — substances that raise significant safety risks — "due to the potential for congestive heart failure", citing a hip-fracture trial that was stopped early on that signal. A trial halted for a cardiac safety finding is a materially different piece of evidence from a side-effect table, and it is missing from most pages that discuss this compound's safety.
Tested athletes
On the 2026 WADA Prohibited List, ibutamoren sits with the growth hormone secretagogues at S2.2.4, prohibited at all times rather than in competition only. MK-677's oral route and its availability as a research chemical do not change that.
The detection window is the practical part. A 2025 study in Clinica Chimica Acta, written specifically because Court of Arbitration for Sport panels had asked for interpretable hair-test data, detected ibutamoren in hair four weeks after a single exposure, and in a second subject four weeks after discontinuing a 90-day course. Hair testing reaches back well past the point where blood and urine have cleared. An athlete reasoning about washout from the pharmacokinetics of the compound is reasoning about the wrong matrix.
What this page concludes
MK-677 is the best-evidenced compound in this market and the evidence still does not say what it is sold as saying. It reliably raises growth hormone and IGF-1. It reliably increases fat-free mass in older adults, alongside water and body weight and limb fat. It missed the fat-loss half of its own primary endpoint. Its muscle trial could not evaluate whether the added mass improved strength or function, and says so. Its largest trial, in a different indication, showed that the IGF-1 rise it produces does not carry an outcome with it. And it comes with an appetite-and-glucose trade-off that appears to be intrinsic to the class.
None of that has been tested in a trained adult under 60. Every human trial of this compound has been run in older or clinical populations, and the gap between that literature and the audience buying it is the largest unexamined assumption in the category.
Related on this site: the evidence ledger for how these compounds are graded, creatine vs peptides for what a well-evidenced comparator actually looks like, and oral peptides for muscle growth for why the oral route matters so much here.