GHRP-2 vs GHRP-6 vs Hexarelin: the GHRPs compared

Research context (RUO): GHRP-2, GHRP-6 and hexarelin are three synthetic growth hormone-releasing peptides (ghrelin agonists) that are studied in laboratories for their influence on the growth hormone axis. They are supplied exclusively for in vitro laboratory research and are not intended for human or animal use.

GHRP-2 vs GHRP-6 vs hexarelin: the classic GHRPs compared

The comparison GHRP-2 vs GHRP-6 vs hexarelin revolves around three closely related peptides from the same family: the growth hormone-releasing peptides (GHRPs). All three are synthetic ghrelin agonists that act on the same receptor, but in research they differ clearly in potency, in their effect on appetite, and in their secondary influence on hormones such as cortisol and prolactin. This pillar sets those differences side by side, so researchers understand why the three molecular variants show divergent behaviour in preclinical studies.

What are GHRPs? The common basis

GHRP’s are short peptides that the growth hormone secretagogue receptor (GHS-R1a) activate — the same receptor on which the body's own hormone ghrelin acts. When that receptor is activated in models, it stimulates the pituitary to release growth hormone (GH). GHRP-6 was historically the first of this class; GHRP-2 and hexarelin were later developed as more powerful analogues. Because they act via the ghrelin route, they differ fundamentally from GHRH analogues such as sermorelin or CJC-1295, which act on a different pituitary receptor. In research, GHRPs are therefore often studied alongside a GHRH peptide, because the two routes reinforce each other in animal models.

A second similarity: none of the three is fully GH-selective. In a classic human study by Arvat and colleagues (Peptides, 1997), both GHRP-2 and hexarelin, in addition to GH, also increased prolactin, ACTH and cortisol, with the researchers concluding that the peptides are "not fully specific". This is a recurring theme in the literature: the extent to which a GHRP influences these secondary hormones in addition to GH is one of the most important distinguishing features between the three.

GHRP-2: the potent and relatively clean GH stimulator

GHRP-2 (pralmorelin) is regarded in research as the most powerful GH releaser of the three on a weight basis. In the Arvat (1997) study, 1 microgram/kg GHRP-2 intravenously produced a strong GH response significantly higher than that of GHRH. Characteristic of GHRP-2 is that in models it gives a relatively limited appetite stimulus compared with GHRP-6, while still yielding a robust GH peak. The influence on ACTH and cortisol is present, however: in the human comparison, cortisol and ACTH stimulation was comparable to that of corticotropin-releasing hormone (hCRH). Researchers therefore often study GHRP-2 as the peptide that combines a favourable ratio between GH potency and appetite side effect.

GHRP-6: the appetite peptide

GHRP-6 distinguishes itself mainly by its pronounced orexigenic (appetite-stimulating) action. Because GHRP-6 is a strong ghrelin agonist, in models it activates the same hunger signalling as ghrelin itself. That the ghrelin route drives appetite is well documented: in a human study (Neuropsychopharmacology, 2005), a single ghrelin injection increased appetite in eight of nine subjects and evoked vivid images of food. In preclinical research, GHRP-6 is therefore studied par excellence when the appetite axis or ghrelin signalling is the subject of study. The flip side in comparative research: GHRP-6 is less potent on a weight basis as a pure GH releaser than GHRP-2, and the appetite stimulus is, for GH-focused studies, more of a confounding variable than a desired effect.

Hexarelin: the most potent — with a cardiovascular dimension

Hexarelin is the third and, at receptor level, one of the most powerful GHRPs. In the human comparison by Arvat (1997), hexarelin gave a GH response as strong as GHRP-2. Two properties, however, set hexarelin apart in the literature. First, hexarelin causes a relatively strong rise in prolactin, ACTH and cortisol in research — more than a GH-selective peptide such as ipamorelin. Second, hexarelin acts not only via the GHS-R1a receptor but also binds to CD36, a receptor in cardiovascular tissue. Research into the cardiovascular action of hexarelin (2014) describes that this CD36 route may be responsible for GH-independent effects on the heart. Hexarelin is thereby the GHRP most often studied outside the pure GH context. A well-known point of attention in the literature is that hexarelin shows faster desensitisation (a decline in response) with repeated administration in models.

Comparison table: the key differences

• GH potency: hexarelin and GHRP-2 are the strongest GH releasers; GHRP-6 is the least potent on a weight basis.
• Appetite (ghrelin effect): GHRP-6 the strongest, GHRP-2 moderate, hexarelin moderate.
• Cortisol & prolactin: hexarelin gives the largest increase; GHRP-2 moderate; all three are not fully GH-selective.
• Special pathway: hexarelin binds, besides GHS-R1a, also to CD36 in cardiac tissue — unique among these three.
• Research focus: GHRP-2 for potent GH release, GHRP-6 for appetite/ghrelin models, hexarelin for cardiovascular and GH research.

The three GHRPs in laboratory research: handling & dissolving

All three peptides are supplied as freeze-dried (lyophilised) powder that is reconstituted before in vitro use, usually with bacteriostatic or sterile water. The choice of solvent and the concentration influence stability during research; our pillar dissolve peptide — which liquid and the reconstitution guide describe this step by step. Because GHRPs contain peptide bonds that are sensitive to repeated freeze-thaw cycles, laboratories work with small aliquots and cold storage.

Quality & purity

In comparative research between three very closely related peptides, molecular purity is decisive: a contaminant can mask the difference between the molecules. Every relevant batch of GHRP-2, GHRP-6 and hexarelin is therefore independently HPLC-tested by an external laboratory; the certificate of analysis (CoA) is publicly verifiable per batch. In our pillar why we rejected a batch you can read how that control works in practice.

Frequently asked questions about GHRP-2 vs GHRP-6 vs hexarelin

What is the main difference between GHRP-2, GHRP-6 and hexarelin?
All three activate the same ghrelin receptor, but GHRP-2 is regarded as the potent and relatively clean GH releaser, GHRP-6 as the strongest appetite peptide, and hexarelin as the most powerful with an additional cardiovascular (CD36) route. The differences therefore lie in potency, appetite effect and side effects on cortisol/prolactin.

Which of the three is most potent for growth hormone release?
In comparative human research (Arvat, 1997), GHRP-2 and hexarelin gave a comparable, strong GH response that was higher than that of GHRH; GHRP-6 is the least potent on a weight basis. This concerns research data only, not usage advice.

Why does GHRP-6 cause hunger and the others less?
GHRP-6 is a particularly strong ghrelin agonist, and ghrelin is the body's own hunger signal. In models, GHRP-6 therefore activates appetite signalling more powerfully than GHRP-2 or hexarelin, which makes it interesting for research into appetite and ghrelin.

Why do cortisol and prolactin rise with these peptides?
None of the three GHRPs is fully GH-selective. The study by Arvat (1997) showed that both GHRP-2 and hexarelin, in addition to GH, also increased prolactin, ACTH and cortisol. In the literature, hexarelin gives the largest rise in these secondary hormones.

What makes hexarelin cardiovascularly interesting in research?
Hexarelin binds not only to the GHS-R1a receptor but also to CD36 in cardiovascular tissue. Research into the cardiovascular action of hexarelin (2014) suggests GH-independent effects on the heart via this route, which distinguishes hexarelin from the other two.

How do these GHRPs relate to GHRH peptides such as CJC-1295?
GHRPs act via the ghrelin receptor, GHRH analogues via a separate pituitary receptor. In animal models, the two routes reinforce each other, which is why they are regularly studied together in research. See also our pillar on growth hormone peptides.

Read more & research at Peplife

• Research GHRP-2 at Peplife
• Research GHRP-6 at Peplife
• Research Hexarelin at Peplife
• View all GH axis peptides
• GHRP-2 research — in-depth pillar
• GHRP-6 research — in-depth pillar
• Hexarelin research — in-depth pillar
• Growth hormone peptides: the complete overview

Sources: Arvat et al., Peptides 1997 — GHRP-2 & hexarelin on GH, prolactin, ACTH, cortisol · Ghrelin stimulates appetite, GH, ACTH and cortisol, Neuropsychopharmacology 2005 · The cardiovascular action of hexarelin, 2014 · Mechanism of action of hexarelin and GHRP-6

Research Use Only. All products are supplied exclusively for in vitro laboratory research. Not intended for diagnostic or therapeutic use in humans or animals, and not approved by the EMA or FDA.