Ipamorelin: The First Selective GH Secretagogue

Most growth hormone secretagogues do more than release growth hormone. They pull cortisol and ACTH up with it, which makes the resulting data hard to interpret and the compound less useful as a research tool. Ipamorelin was the first one that did not.

What Ipamorelin is

Ipamorelin is a synthetic pentapeptide: Aib-His-D-2-Nal-D-Phe-Lys-NH₂. Five residues, roughly 711 daltons. It was arrived at by taking GHRP-1 and deleting the central Ala-Trp dipeptide — a subtractive design rather than an additive one.

It binds the growth hormone secretagogue receptor GHS-R1a, the receptor ghrelin acts on. That makes it a releaser rather than a replacement: it acts on the pituitary to discharge stored growth hormone, so the pattern in study models is pulsatile, unlike the sustained elevation you get from administering growth hormone directly.

The claim in the title

The foundational paper is Raun and colleagues, European Journal of Endocrinology, 1998, and it is titled “Ipamorelin, the first selective growth hormone secretagogue.” That is a strong title for a scientific paper. The data behind it are worth walking through, because the claim is narrower and more interesting than it first appears.

How the selectivity was demonstrated

In conscious swine, the potency numbers were unremarkable. Ipamorelin came in at an ED₅₀ of 2.3 ± 0.03 nmol/kg with a maximum response of 65 ng GH per ml of plasma. GHRP-6, the established comparator, managed 3.9 ± 1.4 nmol/kg and 74 ng/ml. Broadly the same compound class doing broadly the same thing.

The separation appeared in the hormones nobody wanted to move:

• ACTH and cortisol. GHRP-6 and GHRP-2 both raised them. Ipamorelin did not — and crucially, still did not at doses more than 200-fold above its ED₅₀ for growth hormone release. That is not a marginal effect hiding below detection; it is an absence across two orders of magnitude.
• FSH, LH, prolactin and TSH. None of the secretagogues tested altered these, ipamorelin included.

The conclusion the authors drew was that this was the first GHRP-receptor agonist whose selectivity for growth hormone release resembled that of GHRH itself — acting through the ghrelin receptor, but behaving like something acting through the GHRH receptor.

What Ipamorelin did in humans

A phase I study in healthy adults reported it as generally well tolerated, with a clear growth hormone response peaking around 40 to 60 minutes after administration and no significant effects on other pituitary or adrenal hormones. Pharmacokinetic-pharmacodynamic modelling in human volunteers has been published separately.

So the selectivity finding held up in people, at least in the setting it was measured in. That is more than can be said for a lot of compounds in this category.

Where Ipamorelin stopped

Development did not go where you might expect. Ipamorelin showed prokinetic effects in preclinical models — the ghrelin receptor has a role in gut motility — and it was taken into a randomised, controlled proof-of-concept study in patients undergoing bowel resection, testing whether it would speed recovery from postoperative ileus.

It did not produce a statistically significant improvement in postoperative gastrointestinal recovery. That programme ended, and Ipamorelin holds no regulatory approval in any jurisdiction for any indication.

Why Ipamorelin and CJC-1295 appear together

These two turn up side by side constantly, and the reason is mechanistic rather than marketing. They act at different receptors: ipamorelin at GHS-R1a, CJC-1295 at the GHRH receptor. Because growth hormone release is regulated by both pathways plus somatostatin inhibition, the research question is whether engaging two arms at once produces a different release pattern than engaging either alone.

That is a legitimate question and there is literature on combined secretagogue administration. It is not, however, the same as evidence that any particular combination produces a particular outcome in people.

What the evidence does not support

Being straight about this matters more than another paragraph of mechanism. The published record on Ipamorelin establishes that it releases growth hormone selectively. It does not establish outcomes.

• There are no controlled human trials showing body composition change.
• There are no controlled human trials showing recovery, sleep or injury-healing outcomes.
• The one randomised human efficacy study that was run — in postoperative gut recovery — did not meet its endpoint.
• It is unapproved everywhere, and material sold under this name is a laboratory reference standard.

A compound can have an elegant, well-characterised mechanism and no demonstrated clinical benefit. Those are separate questions, and the literature has answered only the first one.

Handling Ipamorelin in the lab

Supplied lyophilized and vacuum sealed. Store the sealed vial at minus 20 degrees Celsius, away from light. The dry cake tolerates ambient transit comfortably, so shipping without cold chain says nothing either way about quality.

Reconstitute with bacteriostatic water, running the stream down the glass wall rather than onto the cake, and swirl instead of shaking — peptides denature at the air-liquid interface, and a foamy vial is a damaged one. Our reconstitution calculator will do the volume arithmetic. To check what your certificate is telling you, see our guide to reading a peptide COA, and the 10mg vial we stock ships with a lot-matched certificate.

Frequently asked questions

What is the Ipamorelin sequence?
Aib-His-D-2-Nal-D-Phe-Lys-NH2. A pentapeptide of roughly 711 daltons, derived from GHRP-1.

Does Ipamorelin raise cortisol?
Not in the published comparison. It produced no significant ACTH or cortisol elevation even at over 200 times the ED50 for growth hormone release, which is what distinguished it from GHRP-6 and GHRP-2.

Does it affect prolactin?
No secretagogue in the Raun comparison altered FSH, LH, prolactin or TSH.

How does it differ from growth hormone itself?
It releases stored endogenous hormone through the pituitary rather than supplying exogenous hormone, so the profile is pulsatile rather than sustained.

Is Ipamorelin approved anywhere?
No. Its one randomised human efficacy trial, in postoperative gastrointestinal recovery, did not reach significance.

References

Raun K, Hansen BS, Johansen NL, Thogersen H, Madsen K, Ankersen M, Andersen PH. Ipamorelin, the first selective growth hormone secretagogue. Eur J Endocrinol. 1998;139(5):552-561. PMID 9849822

Pharmacokinetic-pharmacodynamic modeling of ipamorelin, a growth hormone releasing peptide, in human volunteers. PMID 10496658

Ipamorelin, a new growth-hormone-releasing peptide, induces longitudinal bone growth in rats. PMID 10373343

The emerging landscape of performance-enhancing peptides modulating the GH-IGF1 axis. PMC13322892