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Hexarelin, GHRP-2, GHRP-6: the first-generation secretagogues compared

Last updated: October 3, 2026 · 7 min read · By the Grey Peptides Editorial Board

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Grey Peptides
Grey Peptides Editorial Board
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Key takeaways
  • GHRP-6, GHRP-2 and hexarelin release growth hormone through the ghrelin receptor, and all three also raise the stress hormones ACTH and cortisol. None is a 'clean' growth hormone releaser.
  • GHRP-2 is pralmorelin, approved only in Japan as a diagnostic test. In lean men, an infusion made them eat 35.9% more. Hexarelin is the most potent, but its effect faded by almost half over 16 weeks of daily use.
  • None is approved in the US, GHRP-6 cannot be compounded here, and all three are banned in sport. Their evidence is short studies of hormone levels, not long-term benefits.

Where the GHRPs came from

In the late 1970s and 1980s, chemists led by Cyril Bowers set out to make small synthetic peptides that would make the pituitary release growth hormone. GHRP-6, a six-amino-acid peptide, was the first that worked well, and it worked through a receptor nobody yet understood. That receptor was later identified as the target of ghrelin, the stomach hormone discovered in 1999. GHRP-2 followed as a more potent analogue, and hexarelin was made by swapping GHRP-6's tryptophan for a more stable 2-methyl-tryptophan, giving a peptide that released growth hormone strongly in rats 1.

Comparing them with ghrelin itself is revealing. In seven young men, ghrelin released more growth hormone than hexarelin, and both released more than GHRH 2. These peptides imitate a natural hormone, and like that hormone they act on more than growth hormone.

Side by side

From our encyclopedia entries and the studies cited below, as of October 3, 2026:

GHRP-6GHRP-2 (pralmorelin)Hexarelin
OriginThe first of the seriesA later, more potent analogueGHRP-6 with a more stable tryptophan
ApprovalNever approved; cannot be compounded in the USApproved in Japan only, as a diagnostic testNever approved
Half-life in people2.5 hours elimination (nine men)Not reported in the abstracts we holdNot measured in humans
Cortisol and ACTHRaisedRaised; used as an adrenal testRaised; the cortisol response shrank with daily use
ProlactinSee entryRaised in some studiesRaised
HungerNot directly measured in the cards we hold35.9% more eaten at a buffetNot reported
DesensitisationNot tested long term in peopleResponse pattern changed with chronic useGH response fell by almost half over 16 weeks, then recovered
Evidence gradeMediumMediumMedium
WADAAll prohibited at all times (S2.2.4, growth hormone secretagogues)

Growth hormone release

All three are potent. In 12 healthy men, single intravenous doses of hexarelin raised growth hormone to mean peaks of 26.9, 52.3 and 55.0 ng/mL about 30 minutes later, against 3.9 on placebo 3. GHRP-2 at 100 micrograms intravenously raised growth hormone within 60 minutes in all of 77 healthy adults, to an average of 84.6 µg/L, while responses were much lower in people with severe deficiency, which is the basis of its use as a diagnostic test 4. GHRP-6 given during the night raised average growth hormone from 5.5 to 15.4 ng/mL between 10 pm and 3 am 5, and taken by mouth in children it produced a response close to an intravenous dose 6. GHRP-6 also kept working where GHRH faltered: in primary hypothyroidism it released far more growth hormone than GHRH did 7, and in people with excess cortisol, from disease or steroid drugs, GHRH barely worked while GHRP-6 still produced a response 8. That robustness is why the GHRPs were explored as diagnostic tests, the one use that reached approval, for GHRP-2 in Japan.

Cortisol and ACTH: the side effect they share

Unlike GHRH, the GHRPs also stimulate the stress-hormone axis. During GHRP-6 infusions overnight, ACTH rose from 16.6 to 21.0 pg/mL and cortisol from 25.2 to 56.0 ng/mL 5. In people with Addison's disease, whose adrenal glands do not respond, GHRP-6 produced an ACTH response about 30 times larger than in controls 9. GHRP-2's effect on cortisol is reliable enough that a peak cortisol below a threshold after GHRP-2 predicted secondary adrenal insufficiency with about 90% accuracy 10. Hexarelin also raises cortisol, and with twice-daily injections the cortisol response to the morning dose shrank over 16 weeks and recovered four weeks after stopping 11.

For someone injecting these peptides repeatedly, that means repeated cortisol spikes, whose long-term effects in healthy people have not been studied. Ghrelin itself raised prolactin, ACTH and cortisol in the same comparison study 2.

Hunger: why GHRP-2 and GHRP-6 have a reputation

Ghrelin is a hunger hormone, so peptides that act on its receptor might be expected to increase appetite. The clearest test was with GHRP-2: lean healthy men given a GHRP-2 infusion for four and a half hours before a buffet ate 35.9% more than on saline, every subject increased intake, and the mix of foods did not change 12. GHRP-6's reputation for hunger is widespread in user forums, but none of the studies on our GHRP-6 entry measured food intake directly. Hexarelin is often described as causing less hunger; we found no study that measured it.

Desensitisation: the effect that fades

Hexarelin has the best human data on what happens with repeated use. Adults injecting it twice daily for 16 weeks saw the growth hormone response to a test dose fall from 19.1 µg/L·h at baseline to 12.3 at week 4 and 10.5 at week 16, recovering to 19.4 four weeks after stopping 13. In beagles given hexarelin twice daily for six weeks, the response rose at first, then fell below its starting level by week 6 14. GHRP-2 given chronically for one to four weeks changed the way it combined with GHRH, from additive to synergistic 15. The practical meaning is that the effect users chase diminishes with continued use, and the cycles recommended online are attempts to work around a tolerance the body builds.

Hexarelin and the heart

Hexarelin has an unusual cardiac story. In seven men given intravenous growth hormone or hexarelin, both raised circulating growth hormone to the same extent, but only hexarelin raised the heart's ejection fraction, from 64.0% to 70.7%, within minutes 16. In men with growth hormone deficiency who barely released growth hormone in response, ejection fraction still rose 17. A separate receptor, CD36, has been identified as a cardiac receptor for hexarelin, and heart research in animals continues. These are acute, small studies, not evidence that hexarelin helps heart disease.

Sleep and other effects

The GHRPs are sometimes promoted for deeper sleep, but the evidence points the other way for hexarelin: four night-time doses in seven young volunteers reduced stage 4 sleep in the first half of the night and EEG delta power across the night, while raising growth hormone and prolactin 18. GHRP-2 infusion in men with prolonged critical illness reactivated growth hormone secretion and normalised IGF-1 and related proteins 19, and intranasal GHRP-2 increased height velocity in children with short stature in an early study 20; neither became an approved treatment outside Japan's diagnostic use.

The pralmorelin naming problem

GHRP-2's international nonproprietary name is pralmorelin. In Japan, Kaken Pharmaceutical sells it as a single-dose intravenous diagnostic for growth hormone deficiency, validated at 100 micrograms; treatment development for short stature in Japan and growth hormone deficiency in the US did not reach approval 21. Research vials labelled 'GHRP-2' are not Kaken's diagnostic and are not approved anywhere. Our entry lists both names so that searches for either lead to the same evidence; the pralmorelin approval should not be read as an approval of the research product.

Regulatory and sport status

None of the three is approved in the US. GHRP-6 is in FDA's 503A Category 3 and has been in 503B Category 2, for significant safety risks, since September 29, 2023, so it cannot be compounded 21. All three are prohibited at all times under WADA's S2.2.4 growth hormone secretagogues section 21, and GHRP-2 has a validated doping-control test that detects the peptide and its metabolite in urine 21. GHRP-6's elimination half-life in nine healthy men was 2.5 hours 22.

How they compare (as of October 3, 2026)

GHRP-2 has the strongest human record, including a diagnostic approval in Japan and a clear appetite effect. Hexarelin is the most potent and the best studied for desensitisation, which halved its effect over months, and it has curious cardiac effects. GHRP-6, the original, has a measured half-life and many short physiological studies. All three raise cortisol, none has evidence for the long-term muscle, fat-loss or anti-ageing uses they are sold for, and all are banned in sport. Our GHRP-2 vs GHRP-6 and hexarelin vs ipamorelin pages cover the pairs in more detail.

Frequently asked questions

What is the difference between GHRP-2 and GHRP-6?

Both release growth hormone through the ghrelin receptor and raise cortisol. GHRP-2 is more potent, made men eat 35.9% more in a buffet study, and is approved in Japan as pralmorelin for diagnosis; GHRP-6 was the original and was never approved.

Does GHRP-6 make you hungry?

It is widely reported to, and it acts on the ghrelin (hunger) receptor, but none of the studies on our entry measured food intake directly. GHRP-2, a close relative, increased food intake by 35.9% in a controlled study.

Is hexarelin stronger than GHRP-6?

Hexarelin is a more stable version of GHRP-6 and a potent growth hormone releaser, but its effect fell by almost half over 16 weeks of twice-daily use and recovered after stopping.

Are GHRPs legal?

None is approved in the US, GHRP-6 cannot be compounded, and all three are prohibited at all times by WADA.

Sources

  1. Deghenghi, R., et al. (1994). GH-releasing activity of Hexarelin, a new growth hormone releasing peptide, in infant and adult rats. Life Sci, 54(18), 1321-8. PMID: 7910650
  2. Arvat, E., et al. (2001). Endocrine activities of ghrelin, a natural growth hormone secretagogue (GHS), in humans: comparison and interactions with hexarelin, a nonnatural peptidyl GHS, and GH-releasing hormone. J Clin Endocrinol Metab, 86(3), 1169-74. PMID: 11238504
  3. Imbimbo, B. P., et al. (1994). Growth hormone-releasing activity of hexarelin in humans. A dose-response study. Eur J Clin Pharmacol, 46(5), 421-5. PMID: 7957536
  4. Chihara, K., et al. (2007). A simple diagnostic test using GH-releasing peptide-2 in adult GH deficiency. Eur J Endocrinol, 157(1), 19-27. PMID: 17609397
  5. Frieboes, R. M., et al. (1995). Growth hormone-releasing peptide-6 stimulates sleep, growth hormone, ACTH and cortisol release in normal man. Neuroendocrinology, 61(5), 584-9. PMID: 7617137
  6. Bellone, J., et al. (1995). Growth hormone-releasing effect of oral growth hormone-releasing peptide 6 (GHRP-6) administration in children with short stature. Eur J Endocrinol, 133(4), 425-9. PMID: 7581965
  7. Pimentel-Filho, F. R., et al. (1997). Growth hormone responses to GH-releasing peptide (GHRP-6) in hypothyroidism. Clin Endocrinol (Oxf), 46(3), 295-300. PMID: 9156038
  8. Borges, M. H., et al. (1997). Different effects of growth hormone releasing peptide (GHRP-6) and GH-releasing hormone on GH release in endogenous and exogenous hypercortisolism. Clin Endocrinol (Oxf), 46(6), 713-8. PMID: 9274702
  9. Martins, M. R., et al. (2003). GH-releasing peptide (GHRP-6)-induced ACTH release in patients with addison's disease: effect of glucocorticoid withdrawal. J Endocrinol Invest, 26(2), 143-7. PMID: 12739742
  10. Arimura, H., et al. (2016). Investigation of the clinical significance of the growth hormone-releasing peptide-2 test for the diagnosis of secondary adrenal failure. Endocr J, 63(6), 533-44. PMID: 27020037
  11. Rahim, A., et al. (1999). The effect of chronic hexarelin administration on the pituitary-adrenal axis and prolactin. Clin Endocrinol (Oxf), 50(1), 77-84. PMID: 10341859
  12. Laferrère, B., et al. (2005). Growth hormone releasing peptide-2 (GHRP-2), like ghrelin, increases food intake in healthy men. J Clin Endocrinol Metab, 90(2), 611-4. PMID: 15699539
  13. Rahim, A., et al. (1998). Growth hormone status during long-term hexarelin therapy. J Clin Endocrinol Metab, 83(5), 1644-9. PMID: 9589671
  14. Rigamonti, A. E., et al. (1999). Six-week treatment with hexarelin in young dogs: evaluation of the GH responsiveness to acute hexarelin or GHRH administration, and of the orexigenic effect of hexarelin. Eur J Endocrinol, 141(3), 313-20. PMID: 10474131
  15. Bowers, C. Y., et al. (1996). GHRP-2, GHRH and SRIF interrelationships during chronic administration of GHRP-2 to humans. J Pediatr Endocrinol Metab, 9 Suppl 3, 261-70. PMID: 8887169
  16. Bisi, G., et al. (1999). Acute cardiovascular and hormonal effects of GH and hexarelin, a synthetic GH-releasing peptide, in humans. J Endocrinol Invest, 22(4), 266-72. PMID: 10342360
  17. Bisi, G., et al. (1999). Cardiac effects of hexarelin in hypopituitary adults. Eur J Pharmacol, 381(1), 31-8. PMID: 10528131
  18. Frieboes, R. M., et al. (2004). Hexarelin decreases slow-wave sleep and stimulates the secretion of GH, ACTH, cortisol and prolactin during sleep in healthy volunteers. Psychoneuroendocrinology, 29(7), 851-60. PMID: 15177700
  19. Van den Berghe, G., et al. (2002). The combined administration of GH-releasing peptide-2 (GHRP-2), TRH and GnRH to men with prolonged critical illness evokes superior endocrine and metabolic effects compared to treatment with GHRP-2 alone. Clin Endocrinol (Oxf), 56(5), 655-69. PMID: 12030918
  20. Pihoker, C., et al. (1997). Treatment effects of intranasal growth hormone releasing peptide-2 in children with short stature. J Endocrinol, 155(1), 79-86. PMID: 9390009
  21. Grey Peptides encyclopedia entries for GHRP-2 (pralmorelin approval details; doping-control test), GHRP-6 (FDA 503A Category 3 and 503B Category 2 since September 29, 2023) and hexarelin; WADA 2026 and 2027 S2.2.4 for all three. Read October 3, 2026.
  22. Cabrales, A., et al. (2013). Pharmacokinetic study of Growth Hormone-Releasing Peptide 6 (GHRP-6) in nine male healthy volunteers. Eur J Pharm Sci, 48(1-2), 40-6. PMID: 23099431

Educational information, not medical advice. Each dose in this guide names its source, an approved label or a published study. None is a recommendation for you. An unapproved compound has no established safe or effective human dose, and products sold for “research use only” are not made or tested for people. Talk to a doctor before acting on anything on this site, including before you start, stop or change any medicine or dose.

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