Ipamorelin and selectivity: what the founding study actually measured

Published August 28, 2026 · Artemis Labs

Ipamorelin’s reputation rests on one 1998 paper in the European Journal of Endocrinology, whose title calls it “the first selective growth hormone secretagogue.” The claim behind that title is narrow and checkable. In conscious swine, ipamorelin released growth hormone about as strongly as the older peptide GHRP-6 — and unlike GHRP-6 and GHRP-2, it did not raise ACTH or cortisol above the levels seen with the body’s own releasing hormone. The authors reported that this held “even at doses more than 200-fold higher than the ED50 for GH release.” That is a hormone-measurement finding in pigs, rats and cultured cells. It is not a finding about safety, and the study did not test one.

Key findings

• In primary rat pituitary cells, ipamorelin released growth hormone with “EC50 = 1.3+/-0.4nmol/l and Emax = 85+/-5%” against GHRP-6’s “2.2+/-0.3nmol/l and 100%” (PMID 9849822).
• In conscious swine the two were again close: ipamorelin “ED50 = 2.3+/-0.03 nmol/kg”, GHRP-6 “ED50 = 3.9+/-1.4 nmol/kg”. GHRP-2 was more potent but had a lower maximum effect.
• The selectivity sentence, verbatim: “Very surprisingly, ipamorelin did not release ACTH or cortisol in levels significantly different from those observed following GHRH stimulation.”
• The authors’ own conclusion was a comparison, not a claim of benefit: ipamorelin is “the first GHRP-receptor agonist with a selectivity for GH release similar to that displayed by GHRH.”

What does “selective” mean here?

The pituitary gland makes several hormones from several different cell types. Growth hormone comes from cells called somatotrophs. ACTH — which drives cortisol release from the adrenal glands — comes from a different cell type, and prolactin from a third.

Earlier peptides in this family were not tidy. When the 1998 team gave GHRP-6 and GHRP-2 to swine, both raised ACTH and cortisol alongside growth hormone. Ipamorelin, at the doses tested, did not. Nor did any of the compounds move FSH, LH, prolactin or TSH.

So “selective” in this literature means one specific thing: the compound moved the hormone it was designed to move, and left the others where they were, in that experiment, in that species. It does not mean the compound has no other effects — the same class of compounds turned out to raise body fat in mice through a route that had nothing to do with growth hormone at all.

How strong is the finding?

Stronger than most single results, for three reasons that are visible in the paper itself.

First, it was tested across three systems rather than one — isolated rat pituitary cells, anesthetized rats, and conscious swine — and the growth hormone comparison held in each.

Second, the authors used pharmacological tools to check the mechanism rather than inferring it. They used GHRP and GHRH antagonists to show that ipamorelin “stimulates GH release via a GHRP-like receptor,” so the effect was traced to a receptor instead of merely observed.

Third, and most usefully, the negative result was pushed hard. The absence of ACTH and cortisol release was checked at doses more than two hundred times the dose that produced half-maximal growth hormone release. A null result at a dose that high is a much better null result than one measured at a single dose.

What limits the finding is equally plain. It is one research group, at the company that made the compound, in 1998. The affiliation lines read Novo Nordisk. That is not a reason to discount it — it is a reason to say so, which most writing about this compound does not.

What the research does not show

The selectivity result is not a human finding. Nothing in the 1998 paper involved people. The only human study that measured hormone response was a 1999 pharmacokinetic study, and it measured growth hormone, not the hormones the selectivity claim is about.

It is also not a safety comparison. A study showing that a compound did not raise two hormones is not a study of harm, and the paper makes no such claim. Describing ipamorelin as “safer” than GHRP-6 on the strength of this result goes past what was measured.

And selectivity at the pituitary says nothing about the rest of the body. The ghrelin receptor is also expressed in the gut, which is why ipamorelin was later tested for bowel motility rather than for growth. A compound can be selective among pituitary hormones and still act in several organs.

Finally, the counter-evidence that belongs in the same breath: in mice, ipamorelin increased fat pad weight relative to body weight even in animals that were growth-hormone deficient, and the authors concluded these compounds “increase body fat by GH-independent mechanisms that may include increased feeding” (PMID 11162489). Selectivity for one hormone did not mean a single, narrow effect on the animal.

Frequently asked questions

Did ipamorelin release more growth hormone than older peptides?

No. In the comparisons reported, it was roughly equivalent to GHRP-6, and GHRP-2 was more potent though with a lower maximum. The difference the paper reports is in what the compounds did not do.

Was the selectivity confirmed by anyone else?

Later reviews repeat it, but the primary comparison traces to the 1998 work and the group that developed the compound. No independent replication of the hormone-selectivity comparison appears in the record we retrieved.

Does selectivity mean fewer side effects?

That is not what was measured. The study recorded hormone concentrations in animals. Adverse effects in people were assessed only in the 2014 trial described on our human trials page, which reported the compound was well tolerated but did not outperform placebo on effectiveness.

How does ipamorelin compare with the other peptides in its class?

Side by side on our comparison of ipamorelin, GHRP-2, GHRP-6 and hexarelin.

References

1. Raun K, Hansen BS, Johansen NL, Thøgersen H, Madsen K, Ankersen M, et al. Eur J Endocrinol. 1998. PMID 9849822. DOI 10.1530/eje.0.1390552.
2. Lall S, Tung LY, Ohlsson C, Jansson JO, Dickson SL. Biochem Biophys Res Commun. 2001. PMID 11162489.
3. Beck DE, Sweeney WB, McCarter MD. Int J Colorectal Dis. 2014. PMID 25331030.
4. Ahnfelt-Rønne I, Nowak J, Olsen UB. Endocrine. 2001. PMID 11322495.

Methodology: quotations are taken verbatim from the PubMed abstracts of the cited studies, retrieved through NCBI E-utilities on August 28, 2026. Where a study reports a value with its uncertainty, the value is reproduced as published.

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