Two compounds on the growth-hormone shelf act at the same receptor, come from the same era of medicinal chemistry, and get sorted in vendor copy along a single axis: hexarelin is "the potent one," ipamorelin is "the clean one." That framing survives because it is half right. The two really do differ — but not on one axis, on three, and only one of them is about the ghrelin receptor at all.
Ipamorelin and Hexarelin are both growth hormone-releasing peptides (GHRPs) acting at GHS-R1a, the ghrelin receptor. Neither is subtype-selective in the receptor-pharmacology sense, because there is no meaningful family of GHS-R subtypes to be selective between. What separates them is what else happens downstream, what else each molecule binds, and what kind of human literature each one accumulated.
Built from the same parent by opposite operations
Both molecules are edits of earlier GHRPs, and the edits go in opposite directions.
Hexarelin is GHRP-6 plus one methyl group. GHRP-6 is His-D-Trp-Ala-Trp-D-Phe-Lys-NH₂. Hexarelin is the same hexapeptide with a methyl on the indole ring of the position-2 D-tryptophan — His-D-2-methyl-Trp-Ala-Trp-D-Phe-Lys-NH₂. The formulas make the relationship literal: C₄₆H₅₆N₁₂O₆ at ~873 Da for GHRP-6, C₄₇H₅₈N₁₂O₆ at ~887 Da for hexarelin. One CH₂, 14 daltons.
Ipamorelin is GHRP-1 minus a dipeptide. Raun and colleagues at Novo Nordisk (European Journal of Endocrinology 1998;139:552–561) describe it as emerging from a series of compounds that lacked the central Ala-Trp dipeptide of GHRP-1. The result is a pentapeptide, Aib-His-D-2-Nal-D-Phe-Lys-NH₂, C₃₈H₄₉N₉O₅, ~712 Da — shorter than the hexapeptides, and carrying two residues no ribosome can install: Aib (α-aminoisobutyric acid) at the N-terminus and D-2-naphthylalanine at position 3.
Addition of a methyl versus deletion of a dipeptide. Both are chemical routes by necessity — non-proteinogenic residues and D-amino acids rule out expression, as covered in our piece on synthesis routes and impurity profiles.
"Selective" here means hormonal spillover, not receptor subtype
The Raun paper is the source of ipamorelin's reputation, and it is worth reading precisely, because two of its findings get quoted and one gets dropped.
The dropped one is potency. In primary rat pituitary cells, ipamorelin released GH with potency and efficacy similar to GHRP-6 (EC₅₀ 1.3 ± 0.4 nmol/L, Emax 85 ± 5% versus 2.2 ± 0.3 nmol/L and 100%). In conscious swine the two were again comparable, while GHRP-2 showed higher potency but lower efficacy. So the common claim that ipamorelin is both more selective and more potent than the older GHRPs is not what the founding paper reports. On the GH-release axis it is roughly a peer.
The quoted finding is the endocrine profile. In swine, none of the secretagogues tested moved FSH, LH, prolactin or TSH. GHRP-6 and GHRP-2 raised ACTH and cortisol; ipamorelin did not — and did not even at doses the authors describe as more than 200-fold above its ED₅₀ for GH release. That is the basis for calling it "the first GHRP-receptor agonist with a selectivity for GH release similar to that displayed by GHRH."
Two caveats belong with it. First, this is downstream hormonal specificity, not receptor selectivity — the distinction we drew in the receptor pharmacology primer. Both compounds engage the same receptor; they differ in what the axis does afterward. Second, it is a swine result. Ghigo and colleagues' human review (Endocrine 2001;14:87–93) describes GHSs in people as having slight prolactin-releasing activity and centrally-dependent ACTH-releasing activity — the class is "not fully specific for GH" in humans. The species and the endpoint both matter.
Hexarelin's second target is not a GH receptor at all
This is the genuine dividing line, and it has nothing to do with selectivity as usually discussed.
Bodart and colleagues (Circulation Research 2002;90:844–849) labelled rat cardiac membranes with a radioactive photoactivatable hexarelin derivative and purified what it stuck to. The result was an 84 kDa binding protein whose N-terminal sequence was identical to rat CD36 — a scavenger receptor (SR-B2), fatty acid translocase, and Toll-like receptor co-receptor, expressed on cardiomyocytes and microvascular endothelial cells. It is not a GPCR and it is not a GHS-R subtype.
The functional half is worth stating carefully, because it is often reported backwards. In perfused hearts, hexarelin activation of CD36 produced a dose-dependent increase in coronary perfusion pressure — a vasoconstrictive response. It was absent in CD36-null mice and in spontaneously hypertensive rats genetically deficient in CD36. The paper's own framing is that CD36 may mediate coronary vasospasm in hypercholesterolaemia and atherosclerosis. This is a mechanism paper identifying a molecular target, not a cardioprotection result.
That the two activities are separable is the strongest evidence they are genuinely distinct. The Université de Montréal group has spent two decades developing EP 80317, a GHRP-6-family peptide used as a CD36 modulator without the secretagogue activity — most recently in a hind-limb ischaemia-reperfusion model (Journal of Biochemical and Molecular Toxicology 2024;38:e70057). Medicinal chemistry pulled the two apart, which means hexarelin was carrying both.
So: ipamorelin is a compound with one established target. Hexarelin is a compound with two, and the second one sits in lipid handling and innate immune signalling.
The human files answer different questions
Here the usual expectation inverts. Ipamorelin is generally described as the untested research compound. It is not.
Ipamorelin was developed into human trials — not for GH or body composition, but for gastrointestinal motility, exploiting the ghrelin receptor's promotility effects in the gut. Helsinn Therapeutics ran two Phase 2 studies. NCT00672074, in postoperative ileus after bowel resection, was published: Beck, Sweeney and McCarter, International Journal of Colorectal Disease 2014;29:1527–1534. Multicentre, double-blind, placebo-controlled, 117 enrolled and 114 in the safety and modified intent-to-treat populations. It was well tolerated. On the key efficacy endpoint — time to tolerance of a standardised solid meal — median times were 25.3 hours versus 32.6 hours, p = 0.15. The authors report no significant differences on the key or secondary analyses.
A second and larger Phase 2, NCT01280344, enrolling 320 participants for recovery of gastrointestinal function, is listed as completed in June 2013 with no results posted. That is the whole registered human record.
Hexarelin's file is a different shape entirely. It has zero ClinicalTrials.gov records — its human literature is 1990s endocrine physiology from Ghigo, Deghenghi and colleagues in Turin and collaborating groups, published before trial registration was expected. That work characterised GH responses across age and disease states, interaction with GHRH, and desensitisation behaviour (European Journal of Endocrinology 1997;136:445–460). It is real human data, and it is provocation-test physiology, not efficacy for any indication.
Asking which compound "has better human data" is therefore malformed, in the way we described in the evidence hierarchy piece. One has a small completed negative trial in a gut indication. The other has descriptive endocrinology and no trials at all.
Desensitisation makes timing a variable
The 1997 Ghigo review states that the GHRP effect undergoes partial desensitisation — more during continuous infusion, less during intermittent administration — while prolonged administration still raised IGF-1. Popovic and colleagues (Clinical Endocrinology 1997;46:539–543) reported that in normal-weight women, prior hexarelin blocked the GH-releasing capability of GHRH given two hours later.
This is the principle that dominates the hormonal class: on an axis with feedback, the pattern of exposure is a pharmacological variable in its own right. It applies to both compounds as GHS-R1a agonists, and it is one reason single-timepoint GH responses do not extrapolate to repeated exposure.
What the documentation has to answer
The two need different things from a COA, and both have a specific trap.
Hexarelin's most likely near-mass impurity is another catalog compound. Because it is GHRP-6 plus a methyl, an incomplete or des-methyl species is GHRP-6 — a 14 Da difference, trivially resolvable by mass spectrometry, but only if the theoretical mass on the paperwork is hexarelin's and not a generic figure. On the useful side, hexarelin carries two indole rings, so UV quantitation at 280 nm actually works — unusual on this shelf, where most sequences have no Trp or Tyr at all.
Ipamorelin is the opposite case. It has no Trp and no Tyr; its aromatic content is His, D-Phe and 2-naphthylalanine. Standard tabulated Trp/Tyr extinction coefficients do not describe that chromophore, so A280 quantitation is not a drop-in method — which pushes weight determination back to net peptide content by amino acid analysis. And AAA has its own limit here: Aib and 2-naphthylalanine are not standard analytes. Both molecules are C-terminally amidated, so a free-acid species roughly 1 Da heavier is a plausible impurity in each. Both carry D-residues, and mass spectrometry is blind to chirality — a matching mass confirms composition, not stereochemistry.
Where the literature stands in 2026
Neither compound gained human data this year. Hexarelin's recent output is animal work — MPTP-induced lung injury in mice (Journal of Basic and Clinical Physiology and Pharmacology, online August 2026) and retinal ganglion cell survival after optic nerve transection in hamsters (Indian Journal of Pharmacology 2026;58:126–130). Both sit on rung two.
The more useful development is the review wave. A UCLA scoping review in the American Journal of Sports Medicine (11 August 2026) covered ipamorelin among six commonly-marketed peptides and found 67% of the identified literature was preclinical animal work, with human studies limited, heterogeneous and modest at best. The Poznań narrative review in Frontiers in Endocrinology (2026;17:1822475) stratifies both compounds by evidence tier. Neutral, citable, freely available maps of the same shelf — better sources than vendor copy.
FAQ
Is hexarelin more potent than ipamorelin? The founding comparison in rat pituitary cells and swine put ipamorelin close to GHRP-6 on GH release, with GHRP-2 more potent but less efficacious. Cross-paper potency figures are not comparable anyway — receptor reserve and assay system shift EC₅₀ by orders of magnitude. Potency is also not the axis on which these two most clearly differ.
Does the CD36 finding mean hexarelin has cardiac effects? It means hexarelin has a second identified binding target expressed in the heart and on macrophages. The characterised functional consequence in that study was coronary vasoconstriction in perfused hearts, and the target has since been pursued through a separate non-secretagogue analog. Read it as target identification, not as an outcome.
Why does ipamorelin have completed Phase 2 trials nobody mentions? Because they were in the wrong indication for the marketing story. The trials tested gut motility after abdominal surgery, not growth hormone or body composition. One was published and did not separate from placebo on its key endpoint; the larger one has no posted results. A completed trial that answered a different question is still more information than silence — see our note on trial registry literacy.
More on documentation standards at /quality/, and compound-by-compound entries in the /library/.
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