Macimorelin vs Ibutamoren

Macimorelin and ibutamoren are both oral ghrelin-receptor agonists, but they were developed for opposite purposes. Macimorelin is approved as a one-off test for adult GH deficiency; ibutamoren was studied as a daily agent to raise GH and IGF-1.

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Macimorelin

An oral ghrelin-receptor agonist approved in the US and EU as a diagnostic test for adult growth-hormone deficiency.

BEST STUDIED FOR

GH secretagogues

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Ibutamoren

An orally active, non-peptide growth-hormone secretagogue that mimics ghrelin at the GHS-R1a receptor.

BEST STUDIED FOR

GH secretagogues

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SIDE BY SIDE

	Macimorelin	Ibutamoren
The key difference	Approved as a single-dose diagnostic test	Investigational daily oral agonist
Class	GH secretagogues	GH secretagogues
Mechanism	Macimorelin stimulates GH release through GHS-R1a after a single oral dose; a low GH peak after stimulation indicates GH deficiency, offering an alternative to the insulin tolerance test.	MK-677 is a small molecule (not a peptide) that activates the ghrelin receptor, producing sustained increases in GH and IGF-1.
Status	Approved as a diagnostic agent (adult GHD); also explored in a pilot cancer-cachexia study.	Studied in clinical trials from the 1990s (catabolism, ageing, frailty); never approved. Widely sold as a "research chemical" and banned in sport.
Reported safety signals	In diagnostic use it is a single dose; the phase 3 trial compared its accuracy with the insulin tolerance test, which carries hypoglycaemia risk.	Reported effects include oedema, increased appetite and muscle pain; a 2025 case report described hepatotoxicity, and a 2024 case linked an adulterated performance supplement to gynaecomastia.
A key study	A phase 3 study compared the oral macimorelin test against the insulin tolerance test for diagnosing adult GHD. (J Clin Endocrinol Metab 2018, PMID 29860473)	A randomised, placebo-controlled crossover study measured MK-677’s effect on diet-induced negative nitrogen balance in healthy volunteers. (J Clin Endocrinol Metab 1998, PMID 9467534)

WHEN TO RESEARCH WHICH

Macimorelin is relevant to diagnostic-testing research; ibutamoren to sustained-exposure models. Both are discussed here strictly as research compounds; nothing on this page is medical advice or a dosing guide.

REFERENCES (PUBMED)

[Macimorelin] Macimorelin as a Diagnostic Test for Adult GH Deficiency — J Clin Endocrinol Metab 2018. PMID 29860473
[Macimorelin] Pilot clinical trial of macimorelin to assess safety and efficacy in patients with cancer cachexia — J Cachexia Sarcopenia Muscle 2023. PMID 36860137
[Macimorelin] Diagnosis and testing for growth hormone deficiency across the ages: a global view of the accuracy, caveats, and cut-offs for diagnosis — Endocr Connect 2023. PMID 37052176
[Ibutamoren] MK-677, an orally active growth hormone secretagogue, reverses diet-induced catabolism — J Clin Endocrinol Metab 1998. PMID 9467534
[Ibutamoren] Repeat administration of the GH secretagogue MK-0677 increases and maintains elevated IGF-I levels in beagles — J Endocrinol 1997. PMID 9071975
[Ibutamoren] Hepatotoxicity induced by MK-677 — BMJ Case Rep 2025. PMID 40675653

FREQUENTLY ASKED

What is the difference between Macimorelin and Ibutamoren?

What is the regulatory status of Macimorelin and Ibutamoren?

Macimorelin: Approved as a diagnostic agent (adult GHD); also explored in a pilot cancer-cachexia study. Ibutamoren: Studied in clinical trials from the 1990s (catabolism, ageing, frailty); never approved. Widely sold as a "research chemical" and banned in sport.

What safety signals have been reported?

Macimorelin: In diagnostic use it is a single dose; the phase 3 trial compared its accuracy with the insulin tolerance test, which carries hypoglycaemia risk. Ibutamoren: Reported effects include oedema, increased appetite and muscle pain; a 2025 case report described hepatotoxicity, and a 2024 case linked an adulterated performance supplement to gynaecomastia.

Which studies is this comparison based on?

For Macimorelin: “Macimorelin as a Diagnostic Test for Adult GH Deficiency” (PMID 29860473); “Pilot clinical trial of macimorelin to assess safety and efficacy in patients with cancer cachexia” (PMID 36860137); “Diagnosis and testing for growth hormone deficiency across the ages: a global view of the accuracy, caveats, and cut-offs for diagnosis” (PMID 37052176). For Ibutamoren: “MK-677, an orally active growth hormone secretagogue, reverses diet-induced catabolism” (PMID 9467534); “Repeat administration of the GH secretagogue MK-0677 increases and maintains elevated IGF-I levels in beagles” (PMID 9071975); “Hepatotoxicity induced by MK-677” (PMID 40675653).