Ipamorelin and GHRP-6: what differs?

Ipamorelin and GHRP-6 belong to the same class — peptides that cause the pituitary gland to secrete growth hormone. However, there are more than a decade of development and several fundamental differences between them. The editors tell where both molecules came from, how they differ practically, and what has been scientifically confirmed about it.

Two generations of the same class

GHRP-6 is one of the oldest synthetic growth hormone (GH) secretagogues. It was described by Bowers et al in 1984, long before the discovery of the receptor it acts on. During the 1980s and 1990s, GHRP-6 became a reference substance for studying a new way of stimulating GH secretion.

Ipamorelin appeared in the late 1990s in the laboratories of Novo Nordisk. In 1998, Raun et al published an article with the telling title "Ipamorelin—the first selective growth hormone secretagogue." The goal of development was to obtain a compound that would stimulate GH without affecting other hormonal systems.

Both molecules act on the ghrelin receptor GHS-R1a, i.e. they mimic the natural "hunger hormone". But if GHRP-6 reproduces ghrelin quite completely - with all its effects on appetite and stress hormones - then ipamorelin was conceived as a "point" tool aimed specifically at the release of GH.

Therefore, comparing these two peptides is essentially a comparison of “broad” and “narrow” approaches to the same receptor. Next, we will consider how far this idea has been implemented and what practical differences arise from it.

Chemical structure and pharmacokinetics

GHRP-6 is a hexapeptide (six amino acids): His-D-Trp-Ala-Trp-D-Phe-Lys-NH2. Ipamorelin is a pentapeptide (five amino acids): Aib-His-D-2Nal-D-Phe-Lys-NH2. Its composition includes the unusual amino acid aminoisobutyric acid (Aib) and D-2-naphthylalanine, which affect the stability and method of binding to the receptor.

These structural features explain the different behaviour of peptides in the receptor. Small changes in the amino acid sequence can "tune" which receptor signaling pathways are activated more and which are less activated. This is how pharmacologists try to separate the desired effects from the undesirable ones.

Regarding pharmacokinetics: In a study in healthy volunteers, Gobburu et al (1999) estimated the half-life of ipamorelin to be approximately two hours after intravenous administration. GHRP-6 is characterized by rapid elimination, and the release of GH after the introduction of both peptides is short — the peak occurs within a few tens of minutes.

Selectivity and appetite: main differences

The most important result of the work of Raun et al. (1998) was that ipamorelin in experiments in rats and pigs stimulated GH with a potency comparable to GHRP-6, but did not increase the level of ACTH and cortisol even at doses that significantly exceeded the effective. GHRP-6 and GHRP-2 under the same conditions caused such increases.

The second difference concerns appetite. Among secretagogues, GHRP-6 is distinguished by a strong "ghrelin-type" effect: a person feels hungry soon after administration. For ipamorelin, this effect is usually described as weak. However, it is worth emphasizing: direct comparative studies of appetite in humans are limited, and in reality the response is individual.

Why is this important? For a person who considers such substances, "selectivity" sounds like a synonym for "safety". But it is not so. Selectivity only means less effect on cortisol and prolactin; the consequences of increasing GH itself — fluid retention, influence on glucose — remain common to all secretagogues.

In addition, data on the selectivity of ipamorelin were obtained mainly on animals and in short experiments. To what extent it is preserved in people with long-term use is a question on which there are no major studies.

Clinical studies

GHRP-6 has mostly been used as a physiological research tool: to study GH regulation, interaction with GHRH, and the effect of age and obesity on secretion. It did not reach the stage of development as a medicinal product.

Ipamorelin went through a more "classic" development path. It has been studied as a means of accelerating the recovery of intestinal function after operations (post-operative ileus), because the ghrelin system affects the motility of the digestive tract. In a randomised phase II pilot study (Beck et al., 2014) in patients after bowel resection, ipamorelin was well tolerated but did not show a statistically significant advantage over placebo on the primary endpoint.

These results are important for two reasons. First, they provided some data on the short-term tolerability of ipamorelin in humans. Second, they show that even with targeted development, the peptide has not proven clinical benefit, and it has not received registration as a medicinal product.

Neither peptide has been studied in high-quality randomised trials for effects on muscle mass, strength, or recovery in healthy athletes. Therefore, statements such as "Ipamorelin is milder, but effective for weight gain" are not based on clinical data, but on extrapolations and user experience.

Status, control and myths

Both substances are banned by WADA in Section S2 as GH secretagogues, with ipamorelin and GHRP-6 specifically named on the Prohibited List among the examples. The ban is in effect permanently. Modern laboratories determine both peptides and their metabolites in urine.

In the US, the FDA in 2023 included a number of peptides, including ipamorelin, on the list of substances with safety concerns when manufactured in compounding pharmacies. That is, even in countries with a developed practice of pharmaceutical manufacturing, these peptides are not considered standard means of therapy.

Among the common myths, the editors single out the following:

• "Ipamorelin has no side effects." It has fewer off-target hormonal effects in preclinical studies, but the effects of elevated GH and the risks of illegal production do not disappear anywhere.
• "GHRP-6 is outdated and therefore safer." A molecule's age does not make it safer; long-term data are not available for either GHRP-6 or ipamorelin.
• "Peptides are not detected during doping control". Both are on the list of substances that WADA laboratories look for in a targeted manner.

The quality of products on the informal market is a separate issue: peptides are sold as "reagents for research", without a guarantee of content, purity and sterility.

Editorial conclusions

GHRP-6 and ipamorelin are representatives of different generations of GH secretagogues with the same target. GHRP-6 affects more broadly, including appetite, cortisol, and prolactin; ipamorelin was created as a more selective molecule.

The selectivity of ipamorelin is confirmed mainly by preclinical data, and its only significant clinical development has not proven efficacy. No clinical development was conducted for GHRP-6 at all.

For sports, there is no difference: both peptides are prohibited. For health, the difference relates to the side effect profile, but does not make either peptide a safe choice.

We recommend that you also read our materials "Ipamorelin vs GHRP-6: a comparison of the mechanism of action and side effects", "GHRP-2 or GHRP-6: what's the difference" and "Ibutamoren vs growth hormone".

List of used literature

1. Raun K, Hansen BS, Johansen NL, et al. Ipamorelin, the first selective growth hormone secretagogue. Eur J Endocrinol. 1998;139(5):552–561.
2. Bowers CY, Momany FA, Reynolds GA, Hong A. On the in vitro and in vivo activity of a new synthetic hexapeptide that acts on the pituitary to specifically release growth hormone. Endocrinology. 1984;114(5):1537–1545.
3. Gobburu JV, Agersø H, Jusko WJ, Ynddal L. Pharmacokinetic-pharmacodynamic modeling of ipamorelin, a growth hormone releasing peptide, in human volunteers. Pharm Res. 1999;16(9):1412–1416.
4. Beck DE, Sweeney WB, McCarter MD; Ipamorelin 201 Study Group. Prospective, randomized, controlled, proof-of-concept study of the ghrelin mimetic ipamorelin for the management of postoperative ileus in bowel resection patients. Int J Colorectal Dis. 2014;29(12):1527–1534.
5. Ghigo E, Arvat E, Muccioli G, Camanni F. Growth hormone-releasing peptides. Eur J Endocrinol. 1997;136(5):445–460.
6. Sigalos JT, Pastuszak AW. The safety and efficacy of growth hormone secretagogues. Sex Med Rev. 2018;6(1):45–53.
7. World Anti-Doping Agency. International Standard — Prohibited List (розділ S2). Montreal: WADA; актуальна редакція.