Hexarelin and ipamorelin are two of the growth hormone secretagogues the research market sells side by side, and they are usually compared by the claims attached to them. The useful comparison is narrower: what each did in people when someone measured it. They share a receptor — the ghrelin receptor, GHS-R1a — and both make the pituitary release growth hormone. After that the records diverge. This page sets them out side by side and says what has never been tested. Neither is an approved medicine; doses are not the subject here.
The two at a glance
| Hexarelin | Ipamorelin | |
|---|---|---|
| What it is | Synthetic hexapeptide derived from GHRP-6 (1990s, Europe) | Synthetic pentapeptide, "the first selective GH secretagogue" (Novo Nordisk, 1998) |
| Receptor | Ghrelin receptor (GHS-R1a); also binds the cardiac receptor CD36 in animal work | Ghrelin receptor (GHS-R1a) |
| Raises cortisol / prolactin in people | Yes — cortisol ~40% from 0.5 µg/kg IV; prolactin dose-dependent | Not raised in the developers' animal studies; no dedicated human test |
| Acute effect on the heart in people | Ejection fraction 64.0% → 70.7% (7 men); rises in bypass patients | Not studied |
| Longest human exposure | 16 weeks, twice daily | 7 days (post-surgical trials) |
| Human half-life | Not cleanly reported | About 2 hours (IV, healthy men) |
| Phase 2 trials | None | Two, for postoperative ileus |
| ClinicalTrials.gov studies (2026-10-09) | 0 | 3 |
| PubMed records (2026-10-09) | 169 tagged as human studies | 50 in total, most of them in animals |
| FAERS reports (openFDA, 2026-10-09) | 1 | 11 |
| Approved anywhere | No | No |
The pattern is unusual. Hexarelin has the larger human literature — a run of Italian and British endocrinology studies in the 1990s — but never reached a registered trial. Ipamorelin has the thinner literature but was taken into phase 2 by a drug developer. The research market sells both as if the evidence were similar.
Selectivity: the cortisol and prolactin question
Every early GH-releasing peptide also released ACTH, cortisol and prolactin. That was the problem ipamorelin was made to solve. In the 1998 paper that introduced it, its developers reported that ipamorelin did not release ACTH or cortisol above the level seen with GHRH, even at doses more than 200 times its ED50 for GH — where GHRP-6 and GHRP-2 both raised them (PMID 9849822). That finding is from animal experiments; no published human study was designed to repeat it.
Hexarelin's selectivity was measured in people, and it is not selective. In healthy adult men given intravenous doses up to 1.0 µg/kg, GH release plateaued at 1.0 µg/kg, cortisol rose by about 40% from 0.5 µg/kg, and prolactin rose with the dose; a low dose combined with GHRH gave a large GH release with no cortisol rise (Massoud 1996, PMID 8954038).
What happens with repeated use is the more useful finding. After 16 weeks of twice-daily subcutaneous hexarelin at 1.5 µg/kg, the cortisol response to a test injection was smaller than at baseline (area under the curve 1,506 → 1,222 nmol/L·h) and recovered after stopping; 24-hour urinary cortisol, ACTH and prolactin responses did not change significantly (Rahim 1999, PMID 10341859). The authors' conclusion: at that regimen, over-stimulation of the adrenal axis and prolactin did not occur.
The heart: a hexarelin-only finding
Hexarelin has a cardiac effect that does not come from growth hormone. In seven male volunteers, intravenous hexarelin and recombinant GH were given so that both produced the same GH exposure. Only hexarelin changed the heart: left-ventricular ejection fraction rose from 64.0% to 70.7%, with no change in blood pressure or heart rate; GH left it at 62.4% versus 62.1% (Bisi 1999, PMID 10342360). In patients with coronary artery disease undergoing bypass surgery, hexarelin at 2.0 µg/kg raised ejection fraction and cardiac index within ten minutes, while GH, GHRH and placebo had no haemodynamic effect (Broglio 2002, PMID 12144941).
Animal and cell work, reviewed by Mao and colleagues in 2014 (PMID 25278975), attributes this to hexarelin binding a second receptor in heart tissue, the scavenger receptor CD36, as well as the ghrelin receptor. Whether that is a benefit, a risk or neither has not been studied in people beyond single doses under monitoring. Ipamorelin's effect on the heart has never been measured.
Desensitisation: hexarelin loses about 45%
The 16-week hexarelin study also measured the GH response to a test dose over time (Rahim 1998, PMID 9589671):
| Week | GH response, area under curve (µg/L·h) |
|---|---|
| 0 | 19.1 |
| 1 | 13.1 |
| 4 | 12.3 |
| 16 | 10.5 |
| 20 (4 weeks after stopping) | 19.4 |
From week 0 to week 16 the response fell by 45% (our arithmetic from the published means), and recovered fully after a four-week break. Over the same 16 weeks IGF-1, IGF binding protein-3, body fat, lean body mass and bone mineral density did not change significantly; of the bone markers, only one collagen marker rose. The authors described the biological impact on the GH–IGF-1 axis as "minimal".
No equivalent long-term study exists for ipamorelin, so the research market's claim that ipamorelin does not desensitise is not something the human record tests. How the two families of GH secretagogues compare on the GHRP side is on our GHRP-2 vs GHRP-6 page.
Ipamorelin's human record
Ipamorelin's human pharmacology comes from one study: eight healthy men at each of five infusion rates (4.21 to 140.45 nmol/kg over 15 minutes), showing dose-proportional exposure and a terminal half-life of about 2 hours (Gobburu 1999, PMID 10496658).
Its clinical development went to the gut, not to growth. In a phase 2 trial in 117 patients after bowel resection (NCT00672074), ipamorelin 0.03 mg/kg was infused twice daily for up to seven days. Treatment-emergent adverse events occurred in 87.5% on ipamorelin and 94.8% on placebo, and the authors described it as well tolerated; the time to a first tolerated meal did not differ significantly (Beck 2014, PMID 25331030). A second phase 2 study of 320 patients (NCT01280344) is marked completed on the registry with no posted results, and development stopped. Its side-effect record in full is on our ipamorelin side-effects page.
What neither record contains
- No head-to-head study. Nobody has given hexarelin and ipamorelin to the same people, so "stronger" and "cleaner" are inferences from separate studies.
- No long-term ipamorelin study, and no cardiac measurement of it.
- No long-term cardiac study of hexarelin, despite the one effect that sets it apart.
- No subcutaneous self-administration study of either of the kind research buyers practise, and nothing about the contents of research vials — see what research-use-only labelling means.
- Anti-doping status is not the question here, but both appear in published urine-testing methods for GH-releasing peptides (Semenistaya 2015, PMID 25869809).
How ipamorelin compares with the other secretagogues buyers ask about is on our tesamorelin vs ipamorelin and ipamorelin vs sermorelin pages. Peptide Lexicon's hexarelin entry covers what the peptide is. A decision about either compound — or about a symptom, especially a cardiac or hormonal one — belongs with a clinician.
Sources and dates
- Raun K et al. Ipamorelin, the first selective growth hormone secretagogue. Eur J Endocrinol. 1998;139(5):552-61. PMID 9849822.
- Gobburu JV et al. Pharmacokinetic-pharmacodynamic modeling of ipamorelin, a growth hormone releasing peptide, in human volunteers. Pharm Res. 1999. PMID 10496658.
- Beck DE, Sweeney WB, McCarter MD; Ipamorelin 201 Study Group. Prospective, randomized, controlled, proof-of-concept study of the ghrelin mimetic ipamorelin for the management of postoperative ileus in bowel resection patients. Int J Colorectal Dis. 2014;29(12):1527-34. PMID 25331030.
- Massoud AF et al. Hexarelin-induced growth hormone, cortisol, and prolactin release: a dose-response study. J Clin Endocrinol Metab. 1996. PMID 8954038.
- Rahim A, O'Neill PA, Shalet SM. Growth hormone status during long-term hexarelin therapy. J Clin Endocrinol Metab. 1998;83(5):1644-9. PMID 9589671.
- Rahim A et al. The effect of chronic hexarelin administration on the pituitary-adrenal axis and prolactin. Clin Endocrinol (Oxf). 1999. PMID 10341859.
- Mao Y et al. The cardiovascular action of hexarelin. J Geriatr Cardiol. 2014. PMID 25278975.
- Bisi G et al. Acute cardiovascular and hormonal effects of GH and hexarelin, a synthetic GH-releasing peptide, in humans. J Endocrinol Invest. 1999;22(4):266-72. PMID 10342360.
- Broglio F et al. Effects of acute hexarelin administration on cardiac performance in patients with coronary artery disease during by-pass surgery. Eur J Pharmacol. 2002;448(2-3):193-200. PMID 12144941.
- Semenistaya E et al. Determination of growth hormone releasing peptides metabolites in human urine after nasal administration of GHRP-1, GHRP-2, GHRP-6, hexarelin, and ipamorelin. Drug Test Anal. 2015. PMID 25869809.
- ClinicalTrials.gov API v2 (interventions "hexarelin", "ipamorelin"), PubMed (hexarelin[tiab] AND humans[mh]; ipamorelin[tiab]) and openFDA adverse-event endpoint, all queried 2026-10-09.