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/