MK-677 (Ibutamoren / Nutrobal)
Orally active small-molecule ghrelin receptor (GHS-R) agonist — not a peptide, but discussed alongside the peptide GH secretagogues.
At a glance
What it is: Orally active small-molecule ghrelin receptor (GHS-R) agonist — not a peptide, but discussed alongside the peptide GH secretagogues..
Primary research applications:
- GH and IGF-1 elevation research
- Sleep and recovery (off-label use)
- Investigated in elderly and frailty populations
Editorial summary: MK-677 is technically not a peptide — it's a small-molecule ghrelin receptor agonist taken orally. We include it because it dominates discussion alongside the peptide GH secretagogues. Its pharmacology raises GH and IGF-1 reliably; clinical efficacy for body composition or frailty in older adults has been mixed, and its appetite stimulation is significant.
- Class / structure
- Spiropiperidine small molecule (not a peptide)
- Half-life
- ≈ 6 hours
- First described
- 1990s (Merck research program)
- Regulatory status
- Not FDA-approved; never reached approval despite multiple trial programs
What is MK-677?
MK-677, also called ibutamoren, is an orally active spiropiperidine small molecule that mimics ghrelin's action at the GHS-R1a receptor — the same receptor activated by hexarelin and ipamorelin. Despite being commonly grouped with peptide GH secretagogues, MK-677 itself is not a peptide.
Discovery and development
MK-677 (developmental name) was developed by Merck in the 1990s as part of an extensive ghrelin-receptor program aimed at producing an oral GH secretagogue. Multiple Phase 2/3 trials were conducted in older adults with sarcopenia, healthy elderly subjects, and patients with hip fracture. Despite reliable GH and IGF-1 elevation, clinical efficacy endpoints did not consistently support approval, and Merck eventually discontinued development.
Mechanism of action
MK-677 binds and activates GHS-R1a, producing pulsatile GH release from the pituitary that mimics the natural pattern. This drives downstream IGF-1 elevation. Because it is a small molecule rather than a peptide, MK-677 is orally active — unusual in this class.[1]
Pharmacokinetics
Oral bioavailability is high; half-life of approximately 6 hours supports once-daily dosing. Peak plasma concentrations occur within ~2 hours of oral administration.
What the research shows
The peer-reviewed literature on MK-677 is summarized below across two tiers: human research (the highest standard), and preclinical / emerging research (animal models and early-stage human work).
Claims and the evidence behind them
This table summarizes commonly discussed claims and how the published evidence weighs in. The aim is clarity — supported claims, claims that look promising but need more data, and claims that outrun the science.
| Claim | What the evidence shows | Verdict |
|---|---|---|
| Reliably elevates GH and IGF-1 in humans | Multiple controlled studies | Supported |
| Increases lean body mass | Modest in trials; not typically translating to functional outcomes | Mixed |
| Improves sleep quality | Some controlled data on slow-wave sleep | Supported |
| Significantly stimulates appetite | Consistent finding in trials and user reports | Supported |
| Treats sarcopenia or frailty | Phase 3 hip fracture program did not meet primary endpoints | Mixed |
| Causes water retention and possible insulin resistance | Documented in trials | Supported |
Reported user experiences
How the research describes administration
Oral, typically once daily at evening dosing in user communities to align with sleep-architecture effects. No FDA-approved formulation exists; products in the marketplace vary widely in identity and purity.
Editorial note
Administration details above describe how the peptide is given in published studies. We summarize this for educational completeness — these descriptions are not protocols, dosing recommendations, or instructions for personal use. Decisions about treatment require an appropriately licensed clinician.
Safety considerations and open questions
The takeaway
MK-677 reliably raises GH and IGF-1 — that part of its biology is well established. Whether those biochemical changes translate to the body-composition, recovery, or longevity outcomes that drive online interest is more nuanced: modest in trial data, with significant trade-offs around water retention and insulin sensitivity. It deserves the prominent place in the conversation, but with realistic expectations about the gap between hormone-level effects and functional outcomes.
Frequently asked questions
Is MK-677 a peptide?
No. MK-677 is a small-molecule spiropiperidine that activates the same receptor as ghrelin and ghrelin-mimicking peptides (GHS-R1a). We cover it because it dominates discussion alongside true peptide GH secretagogues.
How does MK-677 compare to CJC-1295/Ipamorelin?
All three elevate GH and IGF-1 by activating GH secretion. MK-677 is oral and longer-acting (half-life ~6 hours), producing more sustained baseline elevation; CJC-1295/ipamorelin combinations produce more discrete GH pulses.
Why was MK-677 never approved?
Phase 3 trials in elderly and hip-fracture populations did not meet their functional primary endpoints despite reliable GH/IGF-1 elevation. Merck discontinued the program.
Is MK-677 banned in sports?
Yes. WADA prohibits MK-677 (and the GH secretagogue class generally) at all times, both in and out of competition.
References
- Patchett AA, et al. Design and biological activities of L-163,191 (MK-0677): a potent, orally active growth hormone secretagogue. Proc Natl Acad Sci U S A. 1995;92(15):7001-7005. https://pubmed.ncbi.nlm.nih.gov/7624358/
- Nass R, et al. Effects of an oral ghrelin mimetic on body composition and clinical outcomes in healthy older adults. Ann Intern Med. 2008;149(9):601-611. https://pubmed.ncbi.nlm.nih.gov/18981488/