GHRP-6 is a synthetic hexapeptide (a chain of six amino acids) described in 1984 by Cyril Bowers's team — the first compound that selectively released growth hormone (GH) through an entirely new mechanism, unknown at the time. Today that mechanism is known to be the receptor for ghrelin, the hunger hormone — GHRP-6 mimicked ghrelin before anyone knew ghrelin existed. It has an unusually rich history of human research: from doses given to healthy men around the turn of the 1990s, through sleep studies, to a multicentre diagnostic test combined with GHRH. In recent years a Cuban centre has also studied it as a tissue-protective agent. In this review we explain in plain terms what those studies show — including the fact that GHRP-6 raises ACTH and cortisol, and that its famous effect on hunger has been documented in rats, not in humans.
What GHRP-6 is
The name is short for “growth hormone-releasing peptide”. Bowers et al. (1984) showed that the new hexapeptide released GH from cultured pituitary cells and in living animals — rats, monkeys, lambs, calves — without releasing the other pituitary hormones at the same time. GH rose within 2 minutes of the injection, peaked at 10–20 minutes and had usually returned to normal by 2 hours. In young rats, dosing for 9 or 25 days increased body-weight gain, and the pituitary did not become “deaf” to the peptide. The authors concluded that the small molecule had the attributes of a pituitary-regulating hormone and could serve to study the GH-secreting cells.
How it works — the ghrelin switch
GHRP-6 acts on a receptor (a switch on cells) in the pituitary and the hypothalamus that is naturally flipped by ghrelin. Activating this switch in the pituitary produces a short GH pulse; in the hypothalamus it stimulates appetite and engages the stress axis. Bowers et al. (1990) demonstrated something important in humans: GHRP-6 and the natural GH-releasing hormone (GHRH) given together act synergistically — more strongly than the sum of the two — proof that they work through two independent mechanisms. That synergy later became the basis of a diagnostic test.
Structure and origin
The sequence of GHRP-6 is His-D-Trp-Ala-Trp-D-Phe-Lys-NH2 (molecular mass 872 Da): six amino acids, two of them in the unnatural, “mirror-image” D form. Those two protect the peptide from being digested quickly by enzymes. In the literature GHRP-6 is sometimes described as an “opioid-related” hexapeptide — its structure descends from such peptides, although it acts on GH through a completely different mechanism, independent of GHRH. The later peptides of this family — GHRP-2, hexarelin, ipamorelin — are its derivatives and analogues.
What was studied — cells and animals
Beyond the original growth-hormone work, two animal threads are particularly interesting. First, appetite: Locke et al. (1995) injected GHRP-6 directly into the brain ventricles of sated rats. The higher the dose, the more often the animals started eating — the relationship was almost linear — while blood GH did not depend on the dose. Conclusion: GHRP-6 stimulates eating through the brain, independently of its action on growth hormone. Second, tissue protection: the genetic engineering centre in Havana (Cibrián et al., 2006) showed that GHRP-6 triples the migration rate of intestinal epithelial cells in culture (without stimulating their proliferation), and that in rats given the peptide before liver ischaemia it reduced liver and gut damage by 50–85% and also limited lung and kidney injury. Bear in mind that these are cell and rodent studies, conducted mostly by a single centre.
Human data — growth hormone
One of the first human studies (Bowers et al., 1990) enrolled 18 healthy men. After intravenous doses of increasing size the GH peak was 7.6, 16.5 and 68.7 µg/l versus 1.2 µg/l after placebo — the pulse was therefore strongly dose-dependent. No adverse effects or laboratory abnormalities were observed. Ten years later Popovic et al. (2000) published in The Lancet a multicentre study of 125 adults with pituitary disease and 125 healthy people: GHRH combined with GHRP-6 produced a mean GH peak of 59.2 µg/l in healthy subjects and 4.1 µg/l in patients, separating the two groups more clearly than the classic insulin test. The result did not depend on age, sex or the amount of body fat, and the test caused no side effects. The authors called it “convenient, safe and reliable”. This is the best-documented use of GHRP-6 in humans — single-dose and diagnostic.
The pharmacokinetics were described by the Cuban team (Cabrales et al., 2013) in nine healthy men after a single intravenous dose: the peptide disappeared from the blood in two phases, with a distribution half-life of about 7.6 minutes and an elimination half-life of about 2.5 hours. The authors note that they were studying it as a compound that “enhances tissue viability” — a direction that, beyond this pharmacokinetic study, has no clinical results yet.
Human data — cortisol, prolactin and sleep
GHRP-6 is not selective. Already in the 1990 study prolactin and cortisol roughly doubled — though only after the highest dose. This was shown most precisely by a sleep study from the Max Planck Institute (Frieboes et al., 1995): healthy men received four intravenous doses of GHRP-6 or placebo during the night. Between 22:00 and 03:00 the GH concentration was almost three times higher (15.4 versus 5.5 ng/ml), ACTH was higher (21.0 versus 16.6 pg/ml) and cortisol was more than twice as high (56.0 versus 25.2 ng/ml). Interestingly, GHRH does the opposite — at night it lowers cortisol. Sleep, however, did not get worse: stage-2 sleep lengthened (270 versus 245 minutes) and deep sleep was unchanged. The authors state plainly that GHRP-6 “promotes sleep”, though differently from GHRH. It was a single-night study — it says nothing about what happens to sleep and cortisol after weeks of use.
And hunger? We found no study in the literature that measured appetite or food intake in humans after GHRP-6. Strong stimulation of eating has been documented in rats, and in humans for the related GHRP-2, which increased a meal by about a third. The widespread belief in “ravenous hunger” after GHRP-6 is therefore mechanistically plausible, but there are no human figures behind it.
Safety and the limits of the evidence
GHRP-6 is not an approved medicine. Its only well-documented human use is a single-dose diagnostic test, in which it was safe. All human studies concern single doses or a single night in a dozen to a few dozen people; there are no studies of multi-week use in adults, so nothing is known about the consequences of chronically raising cortisol and prolactin. A 2026 review by Polish endocrinologists (Dominikowski and colleagues), devoted to peptides sold as “research compounds”, lists for this group rises in prolactin and cortisol, appetite changes, blood-glucose disturbances, fluid retention, muscle and joint pain and injection-site reactions — and stresses that none of these peptides is approved for physique or performance purposes, and that the composition of products from unregulated sources is uncertain. We deliberately give no methods of use and no doses.
The wider context — the GHRP family
GHRP-6 is the ancestor of the whole family of ghrelin-receptor agonists. GHRP-2 and hexarelin are its “reinforced” analogues with a stronger GH pulse, while ipamorelin was created to keep the action on GH without affecting cortisol and prolactin. A separate group are the GHRH analogues such as CJC-1295, which act on a different switch — which is exactly why GHRH and GHRP-6 given together produce the synergy exploited by the diagnostic test.
Summary
GHRP-6 is the first synthetic ghrelin mimic — a historic compound through which the whole mechanism was discovered. In humans it produces a dose-dependent growth-hormone pulse, and together with GHRH it forms a well-validated diagnostic test for GH deficiency. At the same time it raises ACTH, cortisol and prolactin, and in the only sleep study it lengthened stage-2 sleep without changing deep sleep. Appetite stimulation has been documented in rats; nobody has measured it in humans. Evidence of any benefit from long-term use does not exist, and the safety data end at single doses.
Sources
- Popovic V, Leal A, Micic D, et al. GH-releasing hormone and GH-releasing peptide-6 for diagnostic testing in GH-deficient adults. Lancet. 2000;356(9236):1137–1142. PMID: 11030292. DOI: 10.1016/S0140-6736(00)02755-0. pubmed.ncbi.nlm.nih.gov/11030292
- Frieboes RM, Murck H, Maier P, et al. Growth hormone-releasing peptide-6 stimulates sleep, growth hormone, ACTH and cortisol release in normal man. Neuroendocrinology. 1995;61(5):584–589. PMID: 7617137. DOI: 10.1159/000126883. pubmed.ncbi.nlm.nih.gov/7617137
- Bowers CY, Reynolds GA, Durham D, et al. Growth hormone (GH)-releasing peptide stimulates GH release in normal men and acts synergistically with GH-releasing hormone. Journal of Clinical Endocrinology and Metabolism. 1990;70(4):975–982. PMID: 2108187. DOI: 10.1210/jcem-70-4-975. pubmed.ncbi.nlm.nih.gov/2108187
- Cabrales A, Gil J, Fernández E, et al. Pharmacokinetic study of Growth Hormone-Releasing Peptide 6 (GHRP-6) in nine male healthy volunteers. European Journal of Pharmaceutical Sciences. 2013;48(1–2):40–46. PMID: 23099431. DOI: 10.1016/j.ejps.2012.10.006. pubmed.ncbi.nlm.nih.gov/23099431
- Bowers CY, Momany FA, Reynolds GA, Hong A. On the in vitro and in vivo activity of a new synthetic hexapeptide that acts on the pituitary to specifically release growth hormone. Endocrinology. 1984;114(5):1537–1545. PMID: 6714155. DOI: 10.1210/endo-114-5-1537. pubmed.ncbi.nlm.nih.gov/6714155
- Locke W, Kirgis HD, Bowers CY, Abdoh AA. Intracerebroventricular growth-hormone-releasing peptide-6 stimulates eating without affecting plasma growth hormone responses in rats. Life Sciences. 1995;56(16):1347–1352. PMID: 8614257. DOI: 10.1016/0024-3205(95)00087-9. pubmed.ncbi.nlm.nih.gov/8614257
- Cibrián D, Ajamieh H, Berlanga J, et al. Use of growth-hormone-releasing peptide-6 (GHRP-6) for the prevention of multiple organ failure. Clinical Science (London). 2006;110(5):563–573. PMID: 16417467. DOI: 10.1042/CS20050374. pubmed.ncbi.nlm.nih.gov/16417467
- Dominikowski A, Rękoś Z, Olejarz M, et al. The emerging landscape of performance-enhancing peptides modulating GH-IGF1 axis: bridging the gap between clinical evidence and patient self-administration. Frontiers in Endocrinology. 2026;17:1822475. PMID: 42395176. DOI: 10.3389/fendo.2026.1822475. pubmed.ncbi.nlm.nih.gov/42395176
For in-vitro laboratory research only. It is not a human medicine and is not for treatment.