Ipamorelin vs GHRP-6: the same receptor, one designed not to raise cortisol, and no human comparison

Same receptor, different manners. Both act on the ghrelin receptor to make the pituitary release growth hormone. GHRP-6 came first, in the 1980s; ipamorelin was designed afterwards to do the same job with fewer side effects.

The difference is selectivity. GHRP-6 raises ACTH and cortisol as well as growth hormone, and produces strong hunger. Ipamorelin was selected for releasing growth hormone without those.

Neither has shown a clinical benefit in a randomised human trial. Ipamorelin's largest, in 114 people after bowel surgery, found no significant difference from placebo. GHRP-6's most recent, in 95 people after stroke, found no difference in disability, function or survival.

The direct comparisons are in rats. Several studies gave both compounds to the same animals, for bowel motility and for growth hormone release.

GHRP-6 has a real clinical use that has nothing to do with why it is sold: combined with growth hormone-releasing hormone, it is a diagnostic test of pituitary reserve.

The ghrelin receptor sits on pituitary cells that release growth hormone, and also on pathways that release ACTH, which drives cortisol, and on the circuits that drive appetite. A compound that activates the receptor without discrimination does all of it.

GHRP-6 does all of it. Its own record shows repeated intravenous dosing raising ACTH and cortisol alongside growth hormone, and in Cushing's disease that ACTH response is exaggerated. The hunger it produces is not a side effect in the incidental sense: it is the ghrelin pathway doing what ghrelin does.

Ipamorelin was developed from that starting point with the explicit aim of separating the growth hormone effect from the rest, and its selectivity is the reason it displaced the older compounds in research use.

What follows from that is narrower than the marketing suggests. Selectivity is a statement about which hormones move, demonstrated in pharmacology studies. It is not evidence that either compound produces a result a person would notice, and on that question the two are level, because neither has.

The pattern is the same for both and it is worth stating plainly: each reliably does something measurable to hormones, and neither has been shown to change an outcome that matters to a patient. Ipamorelin's failure was in a condition, slow return of bowel function, chosen because a ghrelin agonist should plausibly help it. GHRP-6's 2026 stroke trial measured disability and survival and found nothing.

That is a stronger statement than either vendor page makes, and it rests on the trials themselves rather than on mechanism.

Unusually for this site, direct comparisons exist. They are animal studies, and several of them gave both compounds to the same animals under the same conditions.

In a 2009 rat model of postoperative ileus, compared with the vehicle, a single dose of ipamorelin (1 mg/kg) or GHRP-6 (20 microg/kg) decreased the time to the first bowel movement. Note the doses: ipamorelin was given at fifty times the amount, which is a potency difference rather than an effect difference, and it is the kind of detail that disappears when a comparison is made from prose.

A 2000 study set out to investigate whether the GHSs, ipamorelin (IPA) and GH-releasing peptide-6 (GHRP-6), increase the measure it was testing, in seven rats. A 1998 study compared the pharmacokinetics of ipamorelin with two related compounds in rats. And a 2019 forensic analysis identified seized powders as analogues of these same secretagogues, which is a comparison of a different kind.

No human study has given both.

These two sit in a family of ghrelin-receptor agonists that includes GHRP-2, more potent than GHRP-6 and also cortisol-raising; hexarelin, the most potent and the most prolactin-raising; and MK-677, an orally active non-peptide with a much longer duration, which is a different proposition because it raises growth hormone around the clock rather than in a pulse.

The ordering people want, from weakest to strongest, is not the useful axis. The useful axis is how much else moves with the growth hormone, and on that axis ipamorelin is at one end and hexarelin at the other.

Related pages: ipamorelin, GHRP-6, and the pairing with a releasing-hormone analogue at CJC-1295 with ipamorelin.

The rat comparison used 1 mg/kg of ipamorelin against 20 micrograms per kilogram of GHRP-6, which is the clearest published statement of their relative potency in a matched setting. Human doses for each come from their own trials and sit on their own records, with the studies that produced them.

Neither has an approved label, so there is no dose from a regulator for either.

Both are unapproved and sold as research chemicals. GHRP-6 has a clinical footprint as a diagnostic agent in endocrine testing, used with growth hormone-releasing hormone to assess pituitary reserve, and that use is a laboratory procedure rather than a treatment. Both are prohibited in sport as growth hormone secretagogues, which is why they appear together in doping-control assay papers.

Supported: both release growth hormone through the ghrelin receptor; GHRP-6 also raises ACTH, cortisol and appetite and ipamorelin largely does not; ipamorelin is less potent per milligram; both improved bowel transit in rats; neither changed a clinical outcome in its randomised human trial.

Not supported: that either produces a benefit a person would notice; that selectivity translates into better results rather than fewer hormonal side effects; that the rat potency difference tells you anything about human dosing; that either is safer, since neither has a safety database worth the name.

1. Treating selectivity as efficacy. It means fewer hormones move, not that more happens.
2. Omitting the failed trials. Both compounds have a randomised human trial that found nothing.
3. Comparing potency without the doses. The rat study used fifty times more ipamorelin than GHRP-6.
4. Reading GHRP-6's hunger as a bonus or a flaw. It is the ghrelin pathway working as designed.
5. Assuming a human head-to-head exists. The direct comparisons are in rats.
6. Ignoring that GHRP-6 has a legitimate diagnostic use. It is a pituitary test agent, which is not what it is sold for.