Peptides: MK-677 (Ibutamoren) — Not a Peptide, Not a SARM
Nass et al. 2008 (PMID 18981487): 25mg/day MK-677 for 24 months in older adults increased IGF-1 by 40% vs placebo but also elevated fasting glucose.
| Measure | Value | Unit | Notes |
|---|---|---|---|
| Evidence Grade | A | grade | MK-677 — multiple human RCTs including 24-month trial; rare among GH secretagogues |
| Molecular Weight | 624 | Da | Small molecule, not a peptide; MW distinguishes it from peptide GH secretagogues (~500–3000 Da range) |
| Oral Bioavailability | 60–80 | % | Clinically meaningful oral absorption; peptide GH secretagogues require injection |
| IGF-1 Increase (25mg/day) | 40 | % over baseline | Nass et al. 2008 (PMID 18981487): 24-month study in adults 65+ years old |
| Half-Life | 24 | hours | Supports once-daily oral dosing; injectable peptide GH secretagogues typically 15–30 min |
| Typical Dose Range | 12.5–25 | mg/day orally | Higher end (25mg) used in clinical trials; 12.5mg used to minimize side effects |
| GH Pulse Amplitude Increase | 2–3× | fold | Svensson 1998 (PMID 9467542): GH pulsatile amplitude significantly increased; baseline GH also slightly elevated |
The Myth: “MK-677 Is a Peptide (or a SARM)”
MK-677 (ibutamoren mesylate) is frequently mislabeled in supplement retail, forum posts, and even some fitness media. To be direct: MK-677 is neither a peptide nor a SARM. It is a non-peptide small molecule growth hormone secretagogue with a molecular weight of 624 daltons. This distinction matters for how it works, how it is taken, and how it is regulated.
The SARM label is a marketing artifact. MK-677 binds the ghrelin receptor (GHSR-1a) — not androgen receptors. SARMs are androgen receptor modulators. These are different receptor systems entirely.
Classification and Mechanism
The correct classification is: small molecule non-peptide ghrelin receptor agonist, also called a small molecule growth hormone secretagogue mimic (SGHM). MK-677 occupies the same receptor as ghrelin and the injectable GH-releasing peptides (GHRP-2, GHRP-6), but does so through a structurally distinct, orally bioavailable scaffold.
Mechanism:
- Binds GHSR-1a (ghrelin receptor) in the pituitary and hypothalamus
- Stimulates pulsatile GH release from somatotroph cells
- Also slightly elevates GH baseline between pulses (unlike true GHRP pulsatile-only release)
- Elevated GH then stimulates hepatic IGF-1 production
Oral Bioavailability: The Defining Advantage
Oral bioavailability of 60–80% is the clinical differentiator. All peptide GH secretagogues (GHRP-2, GHRP-6, hexarelin, ipamorelin) require subcutaneous injection because they are destroyed by gastrointestinal proteases before absorption. MK-677’s non-peptide backbone is enzymatically stable. Its half-life of approximately 24 hours supports once-daily oral dosing.
Clinical Trial Evidence
| Characteristic | MK-677 (Oral) | Injectable GH Peptides (GHRPs) |
|---|---|---|
| Route | Oral (tablet/capsule) | Subcutaneous injection |
| Bioavailability | 60–80% | ~100% (injected, bypasses GI) |
| Mechanism | GHSR-1a agonist (small molecule) | GHSR-1a agonist (peptide) |
| Half-life | ~24 hours | 15–30 minutes |
| GH pattern | Pulsatile + slightly elevated baseline | Pulsatile spikes only |
| IGF-1 increase | 40–80% (clinical trials) | 20–50% (estimated, less data) |
| Water retention | Moderate to significant | Mild to moderate |
| Insulin resistance risk | Elevated fasting glucose documented | Less documented in long trials |
| WADA status | Prohibited (S2) | Prohibited (S2) |
| Cost per dose | Low (oral, no cold chain) | Moderate (injection supplies + cold chain) |
The strongest evidence for MK-677 comes from Nass et al. 2008 (PMID 18981487): a double-blind, placebo-controlled, 24-month study in 65 healthy older adults (mean age ~70). The 25mg/day group achieved a mean 40% increase in IGF-1 compared to placebo. Fat-free mass (lean body mass) increased by approximately 1.6 kg, and physical performance scores improved. Critically, the treatment group also showed increased fasting glucose and insulin resistance — findings that argue against casual long-term use.
Svensson et al. 1998 (PMID 9467542) conducted a 2-month crossover trial in obese adults. MK-677 25mg/day increased IGF-1 by 39–89% depending on the analysis period, with corresponding increases in GH pulse amplitude. Copinschi et al. 1997 (PMID 9349662) demonstrated improved slow-wave sleep with MK-677 — a direct neurological effect of ghrelin receptor agonism in hypothalamic sleep centers.
Side Effect Profile
Water and sodium retention is the most commonly reported effect, attributable to IGF-1’s renal tubular sodium reabsorption actions. Increased appetite (ghrelin receptor-mediated hunger stimulation) is also typical at 25mg — a significant liability for users targeting body composition improvement. Elevated cortisol has been reported in some studies. Insulin resistance is documented in the longest human trials and represents the primary long-term safety concern.
Legal and Regulatory Status
| Jurisdiction | Status | Schedule | Notes |
|---|---|---|---|
| USA | Research chemical | Unscheduled | Not a steroid, peptide, or SARM under current law; WADA prohibited |
| UK | Not scheduled | — | No licensed pharmaceutical product; WADA prohibited |
| Australia | Prescription required | Schedule 4 (TGA) | ASADA prohibited |
| Canada | Gray market | Not scheduled | WADA prohibited; no Health Canada approval |
| EU | Generally unscheduled | — | WADA prohibited; no EMA approval |
WADA prohibits MK-677 under S2 (Peptide Hormones, Growth Factors, Related Substances and Mimetics), specifically the “other growth hormone secretagogues” subcategory. Athletes in tested sports should treat MK-677 as a banned substance regardless of local legal status.
Related Pages
Sources
- 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):601-11. PMID 18981487
- Svensson J et al. Two-month treatment of obese subjects with the oral growth hormone secretagogue MK-677 increases IGF-I levels. J Clin Endocrinol Metab. 1998;83(2):362-9. PMID 9467542
- Copinschi G et al. Prolonged oral treatment with MK-677 improves sleep quality in man. Neuroendocrinology. 1997;66(4):278-86. PMID 9349662
Frequently Asked Questions
Is MK-677 a SARM?
No. MK-677 is not a Selective Androgen Receptor Modulator. It does not bind androgen receptors at all. It binds the ghrelin receptor (GHSR-1a) to stimulate growth hormone release. The SARM label is a widespread marketing error — likely originating from vendors who sell both SARMs and MK-677. Chemically and mechanistically, they are unrelated.
Why can MK-677 be taken orally when GHRPs require injection?
MK-677 is a small molecule (MW 624 Da) with a chemically stable, non-peptide backbone. Peptides such as GHRP-2 and GHRP-6 are broken down by digestive enzymes (proteases) before reaching systemic circulation, which is why they require subcutaneous injection. MK-677's small molecule structure resists proteolytic digestion, giving it 60–80% oral bioavailability.
What were the main findings from the 2008 Nass study?
Nass et al. (PMID 18981487) ran a 24-month, double-blind, placebo-controlled trial in adults aged 60–81. Participants taking 25mg/day MK-677 showed a 40% increase in IGF-1, modest increases in fat-free mass, and improved function scores. However, the treatment group also showed increased fasting glucose and worsening insulin resistance — a clinically important side effect noted explicitly in the paper.
Is MK-677 legal to purchase in the United States?
As of 2026, MK-677 is not a scheduled substance under the Controlled Substances Act in the United States. It is not classified as a steroid, peptide drug, or SARM under current federal law. It circulates as a research chemical. However, it is on the World Anti-Doping Agency (WADA) prohibited list, making it banned for competitive athletes regardless of legal status.