GHRP-2 vs GHRP-6 compared across sequence, receptor pharmacology, half-life, GH response and appetite — including where the common claim that GHRP-2 avoids hunger breaks down against the published data.
GHRP-2 and GHRP-6 are the two most-referenced members of the growth-hormone-releasing peptide family: synthetic hexapeptides that stimulate pituitary GH release by agonising the growth hormone secretagogue receptor (GHS-R1a) — the same receptor the gut hormone ghrelin binds. They are frequently discussed as though one is simply a cleaner version of the other. The published human literature supports a more nuanced picture, and undercuts at least one claim that circulates widely.
Both compounds trace to the secretagogue work of Cyril Bowers and colleagues, which predated the 1999 identification of ghrelin as the endogenous ligand for GHS-R1a. GHRP-6 carries the sequence His-D-Trp-Ala-Trp-D-Phe-Lys-NH₂. GHRP-2 substitutes D-alanine at position 1 and D-2-naphthylalanine at position 2, giving D-Ala-D-βNal-Ala-Trp-D-Phe-Lys-NH₂. Four of six residues are shared. The two substitutions are what the entire comparison rests on.
Neither is a GHRH analogue. Unlike tesamorelin or CJC-1295, which act at the GHRH receptor, GHRPs work through a parallel pathway — which is why the two classes are so often studied and combined together. The growth-hormone secretagogue category guide sets out that split in full.
| GHRP-2 (pralmorelin) | GHRP-6 | |
|---|---|---|
| Sequence | D-Ala-D-βNal-Ala-Trp-D-Phe-Lys-NH₂ | His-D-Trp-Ala-Trp-D-Phe-Lys-NH₂ |
| Primary target | GHS-R1a (ghrelin receptor) | GHS-R1a (ghrelin receptor); also described as binding CD36 |
| GH release | Strong; exceeds maximal-dose GHRH in healthy volunteers | Strong; the original GHRP benchmark |
| Human half-life | Short — tens of minutes; published PK is thin | Distribution t½ ≈ 7.6 min, elimination t½ ≈ 2.5 h (IV, n=9) |
| Appetite | Increases food intake in controlled human study | Markedly orexigenic; the class archetype |
| Regulatory status | Approved in Japan (2004) as a single-dose GH-deficiency diagnostic | No approval; investigational |
| Depth of human data | Better characterised endocrine dataset | Deeper preclinical breadth, thinner endocrine dataset |
| Non-GH research lines | Diagnostics, appetite, GH axis | Cytoprotection, cardiac ischaemia, wound healing |
The cleanest human endocrine data on GHRP-2 comes from Arvat and colleagues (Peptides, 1997), who dosed GHRP-2 and hexarelin at 1 and 2 µg/kg intravenously in young and elderly subjects alongside GHRH, TRH, and hCRH comparators. GHRP-2 at 1 µg/kg produced a GH response greater than maximal-dose GHRH, and the response was dose-dependent.
What does not exist in comparable quality is a head-to-head human trial of GHRP-2 against GHRP-6 at matched doses. The widely repeated statement that GHRP-2 is "more potent" derives from cross-study comparison and receptor-affinity work, not from a direct clinical comparison. It is a reasonable inference. It is not a measured result, and the size of any difference should be treated as unresolved.
The most common framing is that GHRP-6 causes hunger and GHRP-2 does not. That framing is wrong as stated.
GHRP-6's orexigenic effect is real and pronounced — it is the closest of the two to a functional ghrelin mimetic in this respect. But GHRP-2 also increases food intake in humans. Laferrère and colleagues (JCEM, 2005) infused seven lean healthy men with GHRP-2 at 1 µg/kg/h subcutaneously versus saline and measured intake at a subsequent ad libitum buffet meal; subjects ate roughly 36% more on GHRP-2. A follow-up in subjects with obesity reproduced the stimulatory effect.
The defensible statement is one of degree, not of kind: both compounds engage the appetite arm of GHS-R1a signalling, and GHRP-6 does so more forcefully. Any protocol design that assumes GHRP-2 is appetite-neutral is designing against the literature.
The Arvat data also showed that GHRP-2 raised prolactin (less than TRH did) and raised ACTH and cortisol to a degree comparable with hCRH. That is the central caveat for the whole GHRP class: these are not GH-selective agents. GHRP-6 is generally described as producing the larger cortisol and prolactin signal, though again this rests on cross-study comparison rather than a matched-dose head-to-head. Ipamorelin is the member of the class usually cited for a cleaner selectivity profile.
GHRP-6 has a research line GHRP-2 does not. Work centred at Cuba's Center for Genetic Engineering and Biotechnology has examined GHRP-6 for GH-independent cytoprotection — myocardial ischaemia-reperfusion models, post-infarct remodelling, and wound healing — with proposed mechanisms including CD36 binding and PI3K/AKT survival signalling. This body of work is largely preclinical, with limited early-phase human work, and should not be read across to the GH-release context.
If the question is characterised pituitary GH response with the better-documented human endocrine dataset, GHRP-2 is the more studied compound and the only one of the two with a regulatory approval anywhere — as a diagnostic agent, not a therapeutic. See GHRP-2 and the GHRP-2 research overview.
If the question involves ghrelin-pathway appetite signalling, or the non-GH cytoprotective mechanisms, GHRP-6 is the compound the relevant literature actually used. See GHRP-6 and the GHRP-6 research overview.
Both are short-acting, both are non-selective at the pituitary, and both are outrun in data maturity by the GHRH-analogue side of the secretagogue field.
Is GHRP-2 stronger than GHRP-6? Probably, on the basis of receptor-affinity data and cross-study GH responses — but there is no matched-dose head-to-head human trial of the two, so the magnitude of any difference is not established.
Does GHRP-2 avoid the hunger effect of GHRP-6? No. It reduces it relative to GHRP-6, but a controlled human study found GHRP-2 infusion increased ad libitum food intake by roughly a third. Both engage the appetite arm of ghrelin-receptor signalling.
Why is GHRP-2 approved in Japan if it is not a drug? It was approved there in 2004 as pralmorelin, a single-dose diagnostic agent used to provoke GH release when assessing growth hormone deficiency. That is a diagnostic indication, not approval for ongoing administration.
Do they raise cortisol? GHRP-2 raised ACTH and cortisol comparably to hCRH in healthy volunteers. GHRP-6 is generally reported to produce a larger cortisol and prolactin signal. Neither is GH-selective.
Can the published half-lives be compared directly? Not cleanly. GHRP-6 has a formal intravenous PK study in nine volunteers; GHRP-2's circulating half-life is far less well characterised in the peer-reviewed record, and figures quoted for it usually trace to secondary sources.