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MK-677 and ipamorelin both raise growth hormone through the same ghrelin receptor — but one is an oral small molecule that works for about a day, and the other is a selective injectable peptide that acts in short pulses. The receptor is shared; almost everything about how they behave is not.
Research reference only. Not medical advice, prescribing guidance, or a product recommendation.
| Dimension | MK-677 | Ipamorelin |
|---|---|---|
| Chemical class | Non-peptide small molecule (spiroindoline) | Pentapeptide (GHRP) |
| Receptor | GHS-R1a (ghrelin receptor) | GHS-R1a (ghrelin receptor) |
| Route | Oral | Injectable (subcutaneous) |
| Duration of action | ~24 h per dose (sustained) | ~2 h (short, pulsatile) |
| GH release pattern | Sustained GH / IGF-1 elevation | Discrete, physiologic pulse |
| Selectivity | Ghrelin-receptor agonist; can raise appetite, cortisol, glucose | Highly selective — minimal cortisol / prolactin |
| Appetite effect | Increased (ghrelin mimetic) | Minimal |
| Origin | Merck (MK-0677), 1990s | Characterized by Raun et al., 1998 |
| Molecular weight | 528.7 Da | 711.9 Da |
| FDA approval | None (reached late-stage trials) | None |
| WADA status | Prohibited (S2) | Prohibited (S2) |
The defining fact about MK-677 (ibutamoren) is that it is not a peptide. It is a spiroindoline small molecule Merck designed in the 1990s to do orally what the injectable GHRP peptides do — activate the ghrelin receptor to release growth hormone. A single oral dose raises GH and IGF-1 for roughly 24 hours, which is the practical appeal and, simultaneously, the physiologic caveat: sustained ghrelin-receptor tone raises baseline IGF-1 and blunts the natural pulsatility of the GH axis, and — because it mimics ghrelin — it tends to increase appetite and can nudge cortisol, blood glucose, and water retention.
Ipamorelin is the opposite design. It is a selective pentapeptide that produces a short, discrete GH pulse and, unlike earlier GHRPs, releases GH with minimal effect on ACTH, cortisol, or prolactin (Raun et al., 1998). It has to be injected and it acts briefly — but that brevity preserves the pulsatile pattern the body uses, and its clean receptor profile is studied as more sustainable over repeated exposure. So the choice is a genuine trade-off: oral convenience and all-day elevation versus injectable, selective, pulse-preserving release.
These are not interchangeable. MK-677 is the tool when oral dosing and sustained GH/IGF-1 elevation are the point — accepting increased appetite and a blunted pulse; ipamorelin is the tool when selectivity and a physiologic, pulsatile signal matter — accepting the need to inject. One is a non-peptide catalogued alongside the peptides for its shared mechanism; the other is the reference selective GHRP. Neither is FDA-approved, and both are prohibited in sport.
Both activate the same ghrelin receptor (GHS-R1a) to release growth hormone, but MK-677 (ibutamoren) is an oral non-peptide small molecule that elevates GH and IGF-1 for about 24 hours, while ipamorelin is a selective injectable pentapeptide that produces a short GH pulse with minimal cortisol or prolactin.
No. MK-677 is a small molecule (a spiroindoline), not a peptide. It is grouped with the GH peptides because it shares their ghrelin-receptor mechanism and is used the same way — but chemically it belongs to a different class.
Ipamorelin. Its short half-life produces a discrete pulse that preserves the natural GH rhythm, whereas MK-677’s all-day elevation raises baseline IGF-1 and blunts pulsatility. MK-677 also raises appetite because it mimics ghrelin.
No. MK-677 reached late-stage clinical trials (for example in older adults and hip-fracture recovery) without approval; ipamorelin is a research compound. Both are prohibited in sport. This page is a research and educational reference.
Mechanism, evidence, and trade-offs between MK-677 and Ipamorelin — citation-backed answers grounded in PubMed, PubChem, and ClinicalTrials.gov.