Ipamorelin, GHRP-6, and hexarelin belong to the same pharmacological family: growth hormone secretagogues (GHS) that mimic the endogenous hormone ghrelin at the GHS-R1a receptor, prompting the pituitary to release growth hormone (GH). All three are sold strictly for research use and are not approved for human consumption by the FDA or any major regulatory body. The mechanism is shared, but the literature describes real differences in selectivity, downstream hormone activity, and reported side effects. Those differences are what matter when researchers choose between them.
Mechanism and Receptor Selectivity
All three bind GHS-R1a. What separates them is how much "off-target" activity they show at related receptors and signaling pathways.
GHRP-6 was among the earliest synthetic GHS peptides studied. Research describes it as a relatively non-selective agonist. Alongside GH release, it is reported to significantly stimulate cortisol and prolactin secretion, and to activate ghrelin receptors in the gut, producing a pronounced appetite-stimulating effect. Studies also note that GHRP-6 can trigger the release of ACTH (adrenocorticotropic hormone), which feeds its cortisol signal.
Hexarelin is generally described as the most potent of the three on a per-microgram basis. Research reports that it also produces notable cortisol and prolactin co-release, similar to GHRP-6. One additional area of scientific interest is hexarelin's reported interaction with CD36, a scavenger receptor expressed in cardiac tissue. Work published in Circulation Research (Bodart et al., 2002) identified CD36 as the molecular target mediating hexarelin's reported cardiovascular effects in the heart, a pathway described as largely distinct from, and independent of, GH secretion. What this means in research settings is still being worked out.
Ipamorelin emerged later. The research literature consistently characterizes it as the most selective of the three. In the foundational study describing it (Raun et al., European Journal of Endocrinology, 1998), ipamorelin was reported as the first GHRP-receptor agonist to release GH without producing significant elevations in ACTH or cortisol, even at doses more than 200-fold higher than the ED50 for GH release. That selectivity profile is frequently cited as the reason ipamorelin is the most commonly referenced GHS peptide in contemporary research designs that aim to isolate GH-axis effects.
Reported Side-Effect Profiles
The selectivity differences above carry straight through to the side-effect patterns described across the literature.
| Peptide | Reported GH Release | Cortisol / ACTH Co-release | Prolactin Co-release | Appetite Stimulation | Relative Potency |
|---|---|---|---|---|---|
| Ipamorelin | Yes | Low / minimal | Low / minimal | Low | Moderate |
| GHRP-6 | Yes | Moderate–High | Moderate | Pronounced | Moderate |
| Hexarelin | Yes | Moderate–High | Moderate | Moderate | High |
Ipamorelin's lower off-target activity means research subjects are less likely to show the cortisol and hunger-signaling responses seen with the other two. Studies report a relatively clean GH-release profile. It is not considered as potent as hexarelin on a molar basis.
GHRP-6's pronounced appetite stimulation is one of the most consistently reported attributes in the literature, an effect attributed to its activity at ghrelin receptors in gastric tissue. It comes up both as a variable to control for in research design and as a practically observable response in animal models.
Hexarelin's higher potency comes with a trade-off. Research describes greater desensitization (tachyphylaxis) with repeated dosing compared to ipamorelin, on top of the cortisol and prolactin co-release it shares with GHRP-6. A human dose-response study indexed on PubMed (Rahim et al., 1998) reported that the GH response to repeated hexarelin administration progressively attenuated over weeks of dosing.
Research Context and Framing
These peptides are studied as tools for understanding the GH/IGF-1 axis, pituitary function, and related physiology, not as approved therapeutics. The comparison above summarizes what peer-reviewed literature and pharmacological reference sources report. It is not a recommendation or endorsement of any use. Researchers selecting among these compounds typically weigh the selectivity-versus-potency trade-off against their specific experimental endpoints.
Regulatory status has also been in flux. According to FDA records, ipamorelin acetate was placed in Category 2 of the agency's interim 503A bulk drug substances list in September 2023, a designation that effectively barred its use in pharmacy compounding pending further review. The FDA subsequently moved to remove ipamorelin (and other peptides) from Category 2 in 2024, and as of early 2026 its compounding status remained unresolved, pending Pharmacy Compounding Advisory Committee review and formal rulemaking (as reported by the FDA Law Blog, April 2026). None of these compounds carries an approved clinical indication. All three remain research-only compounds not approved for human consumption.
Sources
- PubMed — National Library of Medicine (search: ipamorelin, GHRP-6, hexarelin, growth hormone secretagogue)
- Raun et al., "Ipamorelin, the first selective growth hormone secretagogue" — PubMed
- Bodart et al., "CD36 Mediates the Cardiovascular Action of Growth Hormone-Releasing Peptides in the Heart" — Circulation Research
- FDA — Bulk Drug Substances Used in Compounding Under Section 503A
- Finnrick — Peptide Research Index
- Peptigrity — Peptide Information