Ipamorelin: What “Selective” Actually Means

For laboratory research use only. Not for human or veterinary use.

Ipamorelin is almost always described with one word attached: selective. It is a fair word, but it comes from one specific finding in one 1998 paper, and it means something narrower than most product listings imply. This guide sets out what ipamorelin is, where it came from, what “selective” actually refers to, what has been measured in people, and what to check on a certificate.

Identity#

Three of the five residues are not ordinary amino acids#

Read the sequence carefully, because most of it is unusual.

• Aib — α-aminoisobutyric acid. Not one of the twenty amino acids biology uses. It carries two methyl groups on the α-carbon, which makes the backbone stiff at that position and awkward for proteases to cut.
• D-2-Nal — D-2-naphthylalanine, the same synthetic residue that sets GHRP-2 apart from GHRP-6. A naphthalene ring in place of a natural aromatic side chain.
• D-Phe — the D form of phenylalanine, the mirror image of the natural residue.

Only histidine and lysine are standard, and the C-terminus is an amide rather than a free acid. None of this is decoration. Non-natural and D-residues are how a five-residue peptide survives long enough in a biological system to be studied at all.

Where it came from#

Ipamorelin was designed at Novo Nordisk in the 1990s and described in 1998. It came out of a chemistry programme working on the earlier growth hormone-releasing peptides: it belongs to a series of compounds that lack the central Ala-Trp dipeptide of GHRP-1. Compare it with GHRP-2 (D-Ala-D-2-Nal-Ala-Trp-D-Phe-Lys-NH₂) and the family resemblance is plain: the D-2-Nal, the D-Phe and the Lys-NH₂ end are all there, and the Ala-Trp in the middle is gone.

Novo Nordisk and collaborators published a run of follow-up work — analogues built on ipamorelin, rat pharmacokinetics, and animal studies of the growth hormone axis — between 1998 and 2002. Our GHRP-2 and GHRP-6 guide covers the older compounds and the receptor they share.

What “selective” actually means#

The 1998 paper compared ipamorelin with GHRP-2 and GHRP-6 in rat pituitary cells, anaesthetised rats and conscious pigs. Two findings matter.

On growth hormone release itself, it was not special. In rat pituitary cells and in pigs, ipamorelin released growth hormone with a potency and maximum response very close to GHRP-6. GHRP-2 was more potent than either, but reached a lower maximum.

On everything else, it was. In pigs, GHRP-2 and GHRP-6 both raised ACTH and cortisol alongside growth hormone. Ipamorelin did not raise either above the level seen with growth hormone-releasing hormone itself — even at amounts more than 200 times those needed for a half-maximal growth hormone response. None of the three compounds moved FSH, LH, prolactin or TSH.

That is the whole of the claim. “Selective” means: it releases growth hormone without the ACTH and cortisol rise the older GHRPs produce, as measured in pigs in 1998. It does not mean it is more potent, and it does not describe anything beyond which hormones moved. A listing that uses the word to imply more than that is extrapolating.

How it behaves: the pharmacokinetic data#

Two studies from the late 1990s are the core of what is known.

• In rats (1998), ipamorelin was cleared from plasma about five times more slowly than GHRP-6 and was excreted mainly in the urine, where GHRP-6 went mainly into bile. Most of it was recovered intact. Absorption through the nasal lining was estimated at around 20%, lower than GHRP-2.
• In healthy men (1999), a study gave short intravenous infusions to eight volunteers at each of five levels. Ipamorelin had a terminal half-life of about two hours. Each infusion produced a single pulse of growth hormone, peaking at about 40 minutes and falling back to baseline. Exposure rose in proportion to the amount given, and responses varied more between people than drug levels did.

Those two papers are why ipamorelin is often described as short-acting and as producing a single growth hormone pulse. Both descriptions are accurate, and both come from small studies.

The clinical trial#

Ipamorelin went further than most research peptides: it reached a phase 2 trial. In a multicentre, double-blind, placebo-controlled study published in 2014, 117 hospital patients recovering from bowel surgery received ipamorelin or placebo. It was well tolerated, with adverse events no more frequent than on placebo. On its main measure, the time to tolerating a solid meal, the difference between the groups (about 25 hours against 33) was not statistically significant.

That is the most recent controlled human trial we have found. Ipamorelin is not a licensed medicine anywhere.

Anti-doping status#

Growth hormone secretagogues, ipamorelin among them, are on the World Anti-Doping Agency Prohibited List, and anti-doping laboratories have published methods for detecting ipamorelin and its breakdown products in urine. Anyone subject to anti-doping testing should not handle it. As with AICAR, this is a plain fact about the compound and belongs on the page.

What to check on the certificate#

• Mass: 711.86, not 768.9. Anti-doping laboratories analysing seized black-market products in 2018 found a glycine-extended ipamorelin — the same sequence with an extra glycine on the front. It is a different compound, 57 mass units heavier. A certificate that reports mass near 768–769 is not describing ipamorelin.
• Not GHRP-2 either. GHRP-2 sits at 817.99, close enough in the catalogue and far enough on a spectrum to tell apart instantly.
• What the spectrum shows. A mass spectrometer reports the monoisotopic mass, 711.39, usually as the singly charged ion near 712.4 or the doubly charged ion near 356.7. Either is fine; the certificate should say which.
• Purity, with the method named, measured on that batch.
• Net peptide content and counter-ion. A short peptide carries a meaningful share of counter-ion — usually acetate — and water. Net peptide content is how much of the vial is peptide, and it is not the same number as purity.

Ask about the documentation for the batch we would ship before you order. See also how to read a COA, how to verify one and what a purity figure means.

Handling and storage#

Standard peptide handling applies. Store the sealed lyophilised vial cold, dry and out of light, and let it reach room temperature before opening. Reconstitute by running bacteriostatic or sterile water down the vial wall rather than onto the powder, and swirl rather than shake. Once in solution, keep it refrigerated and use it within its working window. The reconstitution guide, solvent guide and storage guide cover the detail.

What Pioneer supplies#

Ipamorelin is stocked in a 10mg vial, with 2mg and 5mg vials made to order in one to two weeks. It is also supplied pre-blended with CJC-1295 (without DAC).

Quality comes first#

A five-residue peptide with three non-natural building blocks has several places for synthesis to go wrong, and a black-market variant one glycine longer has already been found in the wild. The mass on the certificate is what separates the real compound from both. Ask about the documentation for the batch we would ship before you order.

Sources#

Each entry links to its PubMed record, where the title and abstract can be read. The descriptions are ours: they say what each paper examined, not what it is claimed to show. Records checked on PubMed on 28 October 2026.

1. Raun K, Hansen BS, Johansen NL, et al. (1998). Eur J Endocrinol 139(5):552–61. The Novo Nordisk paper that first described ipamorelin, comparing it with GHRP-2 and GHRP-6 in rat pituitary cells, rats and pigs, including ACTH and cortisol. PubMed 9849822
2. Johansen PB, Hansen KT, Andersen JV, et al. (1998). Xenobiotica 28(11):1083–92. Rat pharmacokinetics of ipamorelin compared with GHRP-2 and GHRP-6: clearance, routes of excretion, and absorption through the nasal lining. PubMed 9879640
3. Gobburu JV, Agersø H, Jusko WJ, et al. (1999). Pharm Res 16(9):1412–6. Study in healthy men at five infusion levels, modelling ipamorelin’s half-life and the growth hormone pulse that followed. PubMed 10496658
4. Beck DE, Sweeney WB, McCarter MD (2014). Int J Colorectal Dis 29(12):1527–34. Phase 2 double-blind placebo-controlled trial in 117 hospital patients after bowel surgery. Well tolerated; the difference on its main measure was not statistically significant. PubMed 25331030
5. Semenistaya E, Zvereva I, Thomas A, et al. (2015). Drug Test Anal 7(10):919–25. Anti-doping study of how ipamorelin and four related GHRPs are broken down and excreted, noting that they are on the WADA Prohibited List. PubMed 25869809
6. Krug O, Thomas A, Malerød-Fjeld H, et al. (2018). Growth Horm IGF Res 41:1–6. Analysis of seized black-market products by anti-doping laboratories, identifying glycine-extended versions of ipamorelin, GHRP-2 and GHRP-6. PubMed 29864719

In our catalogue#

• Ipamorelin — 10mg in stock; 2mg and 5mg made to order
• CJC-1295 (without DAC) + Ipamorelin — pre-blended with CJC-1295 (without DAC)
• GHRP-2 — the older hexapeptide ipamorelin was compared against
• GHRP-6 — GH-releasing peptide-6

Pioneer Research Peptides — UK supplier of research peptides for laboratory use. Ask about batch documentation before you order.

For laboratory research use only — not for human consumption. 18+.