MK-677 (Ibutamoren) — Oral Ghrelin-Receptor Agonist (Research Evidence Summary)

Discovery and characterisation

MK-677 (development code L-163,191; INN ibutamoren) is a small-molecule agonist at the growth hormone secretagogue receptor (GHS-R1a), the same receptor activated by the endogenous peptide ghrelin. Unlike the peptide-class ghrelin-receptor agonists (GHRP-2, GHRP-6, hexarelin, ipamorelin), MK-677 is a non-peptide piperidine derivative that is orally bioavailable — a distinct chemical class, and the pharmacological feature that motivated Merck's development programme in the 1990s.

Pharmacologically, MK-677 amplifies pulsatile GH secretion by increasing pulse amplitude (rather than tonically raising basal GH). This mimics physiological GH release more closely than exogenous recombinant GH administration, which produces a sustained non-pulsatile signal.

Evidence base

The clinical evidence dates largely from the late 1990s and early 2000s Merck development programme.

Chapman 1996 (JCEM). Daily oral MK-677 in healthy elderly subjects raised the amplitude of GH pulses and increased serum IGF-1 into the range typical of younger adults. Established the pharmacological effect on the GH/IGF-1 axis in an older-adult population.

Murphy 1998 (JCEM). In a caloric-restriction model in healthy adults, MK-677 attenuated the diet-induced decline in fat-free mass and improved nitrogen balance versus placebo — the "reverses diet-induced catabolism" claim referenced in the title.

Murphy 1999 (J Bone Miner Res). Markers of bone turnover (osteocalcin, propeptide of type I procollagen) increased in healthy and functionally impaired elderly adults on oral MK-677 — establishing the bone-remodelling signal that motivated subsequent osteoporosis trials.

Murphy 2001 (JCEM). In postmenopausal osteoporotic women, MK-677 alone did not produce a clinically meaningful gain in bone mineral density over 12 months. Combination with the bisphosphonate alendronate was additive on bone-turnover markers but not on BMD end-points. This trial's outcome contributed to the discontinuation of the osteoporosis-indication programme.

Sigalos 2018 (Sex Med Rev). Narrative review of the growth-hormone-secretagogue class (including MK-677) covering safety, indications explored, and reasons the class has not achieved regulatory approval. Notes the fluid retention, insulin sensitivity concerns, and case reports of congestive-heart-failure signals in older-adult trials.

Cardaci 2022 (Exp Physiol). Case report of a young adult using MK-677 concurrently with the SARM LGD-4033. Body composition and biomarker documentation in an off-label / grey-market context — descriptive, not a controlled trial.

Regulatory and clinical status

UK status: MK-677 is not a licensed medicine. It has no MHRA marketing authorisation for any indication. It is sold as a "research chemical", primarily through online supplement / grey-market channels — none of which have regulatory sanction for human use.

US and EU status: Not FDA-approved. Not EMA-approved. The Merck development programme was discontinued in the early 2000s.

Anti-doping: MK-677 is on the WADA prohibited list — banned in-competition and out-of-competition as a growth-hormone secretagogue (WADA S2.2 category). Detected in athlete urine via the standard GHS metabolite panels (see also the peptides and sports anti-doping page).

Advertising implications in the UK: promotion of MK-677 as a growth-hormone or muscle-building aid to consumers falls under the same MHRA / ASA framework as any other unlicensed medicine — see the prescription-only medicine advertising page for the framework.

Translational limitations

• Discontinued programme. The Merck-era trials targeted specific approvable indications (GH deficiency, osteoporosis). When those endpoints did not support licensing, development stopped. Modern trial data is sparse.
• Non-peptide. MK-677 is not chemically a peptide. It sits on this site because of its ghrelin-receptor mechanism, which places it in the same functional class as the GHRP peptides — but its pharmacokinetic profile (oral, small-molecule, longer half-life) is qualitatively different.
• Older-adult trial base. The main efficacy trials were in populations aged 60+ (Chapman 1996, Murphy 1999, Murphy 2001). Effects in younger populations are extrapolated from PK/PD studies, not tested at scale.
• No long-term safety data. Trials ran to 12 months at most. Multi-year effects on insulin sensitivity, cancer risk, and cardiac function are not established.

Related pages

• GHRP-2 monograph — peptide ghrelin-receptor agonist for mechanistic comparison
• MK-677 vs GHRP-2 comparison
• Ipamorelin monograph — selective peptide GHS
• GH axis mechanism map
• Peptides and sports anti-doping

Regulatory and clinical status

UK status: No UK MHRA marketing authorisation. Not approved by the FDA or EMA. Development for indications including growth hormone deficiency and osteoporosis was not carried through to marketing. Listed by the World Anti-Doping Agency (WADA) as a prohibited substance in-competition and out-of-competition. Sold as a research chemical; any human-facing framing falls under UK medicines-advertising rules.

Read more about how UK regulation applies to peptides in our UK regulation hub and our explanation of what “research use only” means in the UK. PeptideStacks does not make editorial sourcing recommendations within research content; any vendor links elsewhere on this page appear in a clearly-labelled sponsor block (see our conflict-of-interest disclosure).

References

Peer-reviewed sources for the claims above. Where an editor has verified study type, sample size, outcome and limitation, the citation is rendered as a card; otherwise as a plain reference. Links open PubMed or the journal DOI.

1. Chapman IM, Bach MA, et al.. Stimulation of the growth hormone (GH)-insulin-like growth factor I axis by daily oral administration of a GH secretogogue (MK-677) in healthy elderly subjects. Journal of Clinical Endocrinology and Metabolism. 1996;81(12) :4249-4257 · PMID: 8954023
2. Murphy MG, Plunkett LM, et al.. MK-677, an orally active growth hormone secretagogue, reverses diet-induced catabolism. Journal of Clinical Endocrinology and Metabolism. 1998;83(2) :320-325 · PMID: 9467534
3. Murphy MG, Bach MA, et al. (MK-677 Study Group). Oral administration of the growth hormone secretagogue MK-677 increases markers of bone turnover in healthy and functionally impaired elderly adults. Journal of Bone and Mineral Research. 1999;14(7) :1182-1188 · PMID: 10404019
4. Murphy MG, Weiss S, et al.. Effect of alendronate and MK-677 (a growth hormone secretagogue), individually and in combination, on markers of bone turnover and bone mineral density in postmenopausal osteoporotic women. Journal of Clinical Endocrinology and Metabolism. 2001;86(3) :1116-1125 · PMID: 11238495
5. Sigalos JT, Pastuszak AW. The Safety and Efficacy of Growth Hormone Secretagogues. Sexual Medicine Reviews. 2018;6(1) :45-53 · PMID: 28400207
6. Cardaci TD, Machek SB, et al.. LGD-4033 and MK-677 use impacts body composition, circulating biomarkers, and skeletal muscle androgenic hormone and receptor content: A case report. Experimental Physiology. 2022;107(12) :1467-1476 · PMID: 36303408

Published: 2026-07-25Last updated: 2026-07-25Last reviewed: 2026-07-25

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