MK-677 (Ibutamoren)

First, a classification note: MK-677 is not a peptide. It's a small molecule taken by mouth, chemically unrelated to the injectable peptides elsewhere in this section — we're including it here only because it's commonly discussed alongside them. It is sold online as a research chemical, is not FDA-approved for any use, and little human safety data exists for long-term or fitness use. This page describes what's known — not dosing or usage guidance, and not a recommendation to use it. Talk to a doctor before considering it.

MK-677 (ibutamoren) is an orally active, non-peptide small molecule that acts as a ghrelin receptor agonist — a "growth hormone secretagogue," meaning it stimulates the body's own growth hormone release through the same receptor that the hunger hormone ghrelin activates, rather than supplying GH directly. It was developed by Merck.

Unlike TB-500 or BPC-157, MK-677 does have a real human clinical trial history — Merck ran Phase II trials for growth hormone deficiency, age-related muscle loss, Alzheimer's disease, and recovery after hip fracture. But the program was discontinued after those trials failed to show a clear enough benefit-risk case to continue: an Alzheimer's trial and an aging trial (both published 2008) didn't support continued development, and a hip-fracture trial in older adults was stopped early after a safety signal (below). No New Drug Application was ever filed, and no other sponsor has since advanced it through FDA review.

MK-677 has never been FDA-approved for any use, and its drug-development program was discontinued, not merely paused. It's sold today only as an unregulated "research chemical" and is not a legal dietary-supplement ingredient. Growth hormone secretagogues as a class are also prohibited at all times in competitive sport under the World Anti-Doping Agency's Prohibited List.

In Merck's hip-fracture trial, 6.5% of patients on MK-677 developed congestive heart failure versus 1.7% on placebo — a real safety signal from an actual controlled human trial, not a theoretical concern. Trials also showed increased fasting blood glucose and HbA1c, consistent with reduced insulin sensitivity, along with increased appetite and fluid retention. This is a substance with more documented human risk signal than most others in this tier, precisely because it was studied in real trials that surfaced a real problem.