Ipamorelin
NNC 26-0161, Aib-His-D-2-Nal-D-Phe-Lys-NH2
Ipamorelin is a five-amino-acid ghrelin receptor agonist developed by Novo Nordisk in the 1990s and named 'the first selective growth hormone secretagogue' because, unlike the earlier GHRPs, it releases growth hormone without also raising cortisol, ACTH or prolactin. That selectivity is real and well documented. What is not documented is any benefit: the only properly randomised trial ever run on ipamorelin was for postoperative ileus, it failed, and development was discontinued.
Mechanism
Ipamorelin is an agonist at the growth hormone secretagogue receptor 1a, the receptor cloned by Howard and colleagues in 1996 and later identified as the target of the endogenous hormone ghrelin. It is structurally derived from the GHRP series but reduced to a pentapeptide, with two non-natural residues (2-aminoisobutyric acid at the N-terminus and D-2-naphthylalanine at position three) that confer protease resistance and receptor affinity. Activation of GHS-R1a on pituitary somatotrophs signals through Gq to phospholipase C, generating inositol trisphosphate and diacylglycerol, mobilising intracellular calcium and triggering growth hormone exocytosis. The receptor is also expressed in the hypothalamic arcuate nucleus, where agonism suppresses somatostatin tone and amplifies endogenous GHRH release. Because this pathway is entirely separate from the GHRH receptor, ghrelin-receptor and GHRH-receptor agonists act synergistically on acute growth hormone release, a genuine pharmacological interaction first demonstrated in humans by Bowers and colleagues in 1990.
Ipamorelin's defining property is selectivity. GHRP-6, GHRP-2 and hexarelin all activate GHS-R1a but also drive the hypothalamic-pituitary-adrenal axis, raising ACTH, cortisol and prolactin, apparently through actions at receptors and pathways beyond the somatotroph. Raun and colleagues showed that ipamorelin releases growth hormone with potency and efficacy comparable to GHRP-6 while leaving ACTH and cortisol at vehicle levels even at doses far above those needed for maximal growth hormone release. That is a real and reproducible pharmacological distinction, and it is the entire basis of ipamorelin's reputation. It is important to be precise about what it means: selectivity describes what a drug does not do. It says nothing about whether the thing it does do produces any benefit.
What the research shows
The founding paper, Raun and colleagues in 1998, characterised ipamorelin in cells, in rats and in swine, establishing its potency, its GHS-R1a dependence and its lack of effect on ACTH, cortisol and prolactin. In humans, Gobburu and colleagues conducted a dose-escalation pharmacokinetic-pharmacodynamic study in healthy male volunteers and found dose-proportional pharmacokinetics with a terminal half-life of about two hours, and a discrete episode of growth hormone release whose magnitude varied considerably between individuals. That study established that ipamorelin does what it says in humans. It measured nothing else.
The only randomised controlled evidence for any clinical outcome comes from a different direction entirely. Ipamorelin was developed for postoperative ileus, on the reasonable ghrelin-based rationale that GHS-R1a agonism promotes gastric emptying and gut motility. Beck and colleagues ran a multicentre, double-blind, placebo-controlled proof-of-concept trial in adults undergoing bowel resection: 117 patients enrolled and 114 were treated, receiving intravenous ipamorelin 0.03 mg/kg or placebo twice daily from postoperative day 1 to day 7 or discharge. Median time to first tolerated meal was 25.3 hours on ipamorelin versus 32.6 hours on placebo, but the difference was not statistically significant (P=0.15), and there were no significant differences in the key or secondary efficacy analyses. The trial was negative, and development was discontinued for lack of efficacy. That is the complete randomised evidence base for ipamorelin in humans: one properly conducted trial, in an indication unrelated to how the peptide is marketed, that did not work.
What this leaves is a striking gap. Ipamorelin is sold almost universally for body composition, recovery, sleep and 'growth hormone optimisation'. There is not a single published human trial measuring any of those outcomes: no randomised trial, no controlled study, not even a published open-label series. The claim most often made for it, that its selectivity makes it safer than other GHRPs, is a statement about acute hormone panels in short studies, not a demonstration of long-term safety, which has never been assessed.
Evidence assessment
Limited evidence
Human data comprise a single phase 1 pharmacokinetic study and one well-conducted phase 2 randomised trial in postoperative ileus that missed its primary endpoint (P=0.15) and all secondary endpoints; the remaining citations are preclinical, and there is no controlled human data of any kind for body composition, recovery or the other uses for which it is marketed.
Tiers are applied consistently across the library and re-checked when new trials read out. Read the grading method.
Key studies
Ipamorelin, the first selective growth hormone secretagogue Preclinical only
Ipamorelin released growth hormone with potency and efficacy comparable to GHRP-6 but, unlike GHRP-6, produced no increase in ACTH or cortisol even at doses well above those giving maximal GH release.
Pharmacokinetic-pharmacodynamic modeling of ipamorelin, a growth hormone releasing peptide, in human volunteers Limited evidence
Dose-proportional pharmacokinetics with a short terminal half-life of about 2 hours; a discrete episode of growth hormone release, with inter-individual variability in the pharmacodynamic response larger than in the pharmacokinetic parameters.
Prospective, randomized, controlled, proof-of-concept study of the Ghrelin mimetic ipamorelin for the management of postoperative ileus in bowel resection patients Mixed evidence
Median time to first tolerated meal was 25.3 hours versus 32.6 hours on placebo, but the difference was not statistically significant (P=0.15) and there were no significant differences in the key or secondary efficacy analyses. The trial was negative.
A receptor in pituitary and hypothalamus that functions in growth hormone release Preclinical only
Identified and cloned GHS-R1a, the G-protein-coupled receptor in pituitary and hypothalamus through which ipamorelin and other growth hormone secretagogues act.
Safety
Ipamorelin's short-term tolerability looks unremarkable. In the postoperative ileus trial, treatment-emergent adverse events occurred in 87.5 per cent of ipamorelin recipients versus 94.8 per cent of placebo recipients, in a bowel resection population where nearly everyone has adverse events regardless. In the phase 1 study no significant safety signals were reported. The selectivity finding is genuinely relevant here: the absence of cortisol and prolactin elevation removes one class of concern that applies to GHRP-2, GHRP-6 and hexarelin. But the safety picture beyond about a week is entirely blank. Nobody has studied what repeated ghrelin-receptor agonism does over months or years in humans, and the theoretical concerns are the same as for the class: sustained IGF-1 elevation and neoplasia risk, impaired glucose tolerance, fluid retention and joint pain. Ghrelin-receptor agonism also stimulates appetite, which is a therapeutic goal in cachexia and an unwanted effect in most of the contexts where ipamorelin is actually used. Finally, ipamorelin sold outside a regulated supply chain has no verified identity, purity, sterility or endotoxin testing; FDA's 2024 review of the compound cited immunogenicity concerns and inadequate human data.
Regulatory status
| Jurisdiction | Status |
|---|---|
| United Kingdom | No UK marketing authorisation. Supply for human use is unlawful under the Human Medicines Regulations 2012. Sold as a laboratory research chemical only, and the MHRA treats accompanying dosing or injection guidance as marketing an unlicensed medicine. |
| United States | Not approved by the FDA for any indication. Development was discontinued after phase 2 failure. Ipamorelin acetate and ipamorelin free base were reviewed at the FDA Pharmacy Compounding Advisory Committee meeting of 29 October 2024, for growth hormone deficiency and postoperative ileus, and the committee voted against including either on the section 503A bulks list; it is therefore not lawfully available for human compounding in the US. The broader FDA position on compounded peptides has been the subject of litigation brought by compounding interests. Material sold online is labelled 'for research use only', a designation that reflects no regulatory review whatever and functions primarily to permit sale of an unapproved drug to consumers. |
| WADA (sport) | Prohibited at all times under section S2.2.4 of the WADA Prohibited List, which names ipamorelin explicitly among prohibited growth hormone secretagogues and mimetics. |
Questions
No. Ipamorelin has never been approved for any indication. Development was discontinued after its phase 2 trial in postoperative ileus failed. In October 2024 the FDA's Pharmacy Compounding Advisory Committee reviewed ipamorelin and voted against allowing it in 503A compounding, so it is not lawfully available for human use in the US through any route.
It means ipamorelin activates the ghrelin receptor to release growth hormone without also raising ACTH, cortisol or prolactin, which the earlier peptides GHRP-2, GHRP-6 and hexarelin all do. This is well documented and reproducible in animal work. But selectivity describes an absence of unwanted hormone effects, not a presence of benefit. It says nothing about whether ipamorelin achieves anything useful.
No human study has ever tested this. There is no randomised trial, no controlled study, and no published open-label series measuring body composition, muscle mass, strength or fat on ipamorelin. The only outcome trial ever run was for postoperative ileus and it missed its endpoints.
It avoids one specific problem those peptides have (cortisol and prolactin elevation), and that difference is real. But 'safer in one respect over a few days' is not the same as 'safe'. No study has examined repeated ipamorelin use in humans beyond about a week, so its long-term safety is genuinely unknown.