MK-677 (Ibutamoren)

MK-677 (ibutamoren mesylate) is not actually a peptide. It's a small non-peptide molecule with a molecular weight of 624.77 Da that mimics the action of ghrelin at the GHS-R1a receptor. CAS number: 159634-47-6. This distinction matters because it means MK-677 can be taken orally, unlike injectable peptide secretagogues.

The biggest advantage of MK-677 over peptide-based GH secretagogues is convenience. You swallow a capsule once a day instead of injecting subcutaneously 2-3 times daily. The trade-off is selectivity: MK-677 isn't as clean as ipamorelin. It activates the ghrelin receptor more broadly, which means it increases appetite (sometimes clearally) and can cause mild cortisol elevation.

MK-677 has the best human data of any GH secretagogue that hasn't received FDA approval. The landmark study is a 2-year randomized, double-blind, placebo-controlled trial published in the Annals of Internal Medicine in 2008 (PMID: 18981485). Sixty-five healthy elderly adults (ages 60-81) received either 25 mg MK-677 or placebo daily for two years. The treatment group maintained GH and IGF-1 levels comparable to healthy young adults throughout the entire study period. Body composition improved modestly but consistently.

Another well-designed study in the Journal of Clinical Endocrinology and Metabolism (PMID: 9467534) showed that MK-677 reversed the nitrogen wasting caused by caloric restriction in healthy young men. This anti-catabolic effect is part of why bodybuilders use it during cutting phases.

The standard dose is 25 mg once daily, taken orally. Most practitioners recommend evening dosing because the GH pulse it triggers can cause drowsiness in some users, and timing it with the natural nocturnal GH surge may maximize the effect. Lower doses (10-15 mg) are sometimes used as a starting point to assess tolerance, particularly for the appetite increase.

Side effects are more predictable than most peptides because we have good human trial data. The most common are increased appetite (directly from ghrelin receptor activation), mild water retention (especially in the first few weeks), transient lethargy, and in some cases, elevated fasting blood glucose. The blood glucose effect is dose-dependent and worth monitoring, especially in anyone with prediabetes or diabetes.

MK-677 is frequently stacked with CJC-1295 or ipamorelin by practitioners looking to maximize GH output through multiple pathways simultaneously. Some also combine it with GHRH analogs on an intermittent schedule to prevent receptor desensitization.

Store MK-677 capsules or powder at room temperature in a cool, dry place away from direct light. Liquid formulations should be refrigerated after opening.