GHRP-2 vs GHRP-6: Two Hexapeptides at the Same Receptor

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

Sequence, relative potency and regulatory position — not receptor. Both are C-terminally amidated hexapeptide agonists at the growth hormone secretagogue receptor . GHRP-6 is His-D-Trp-Ala-Trp-D-Phe-Lys-NH₂ and is the founding member of the family, described in 1984 . GHRP-2 is D-Ala-D-2-Nal-Ala-Trp-D-Phe-Lys-NH₂, a later analogue described in the class literature as one of the more potent members .

Q: Have GHRP-2 and GHRP-6 been compared head-to-head?

No randomised head-to-head trial has been published. The comparisons that exist are review-level, setting out the family's members together , or incidental — studies that administered a growth hormone-releasing peptide alongside growth hormone-releasing hormone in the same participants, which compares two receptor routes rather than two peptides .

Q: Do both compounds raise ACTH and cortisol?

Yes. Neither hexapeptide is selective for growth hormone release. That is what separates both of them from ipamorelin, the pentapeptide in the same class described as the first selective secretagogue because it released growth hormone in swine without measurable ACTH or cortisol release . In GHRP-2's case the ACTH response has itself been examined diagnostically in patients with hypopituitarism .

Q: Is either compound approved?

GHRP-2 is approved in Japan as a diagnostic agent for intravenous provocative testing of growth hormone secretion. That approval attaches to a specific manufactured product in one country for one diagnostic purpose. GHRP-6 has no approval anywhere. Neither is approved by the U.S. Food and Drug Administration for any indication, diagnostic or therapeutic.

Q: Why was the ghrelin receptor named after a growth hormone secretagogue?

Because the drugs came first. GHRP-6 was described in 1984 as a synthetic hexapeptide acting on the pituitary to release growth hormone by a route the GHRH receptor could not explain . The search for the receptor responsible ended in 1996 with the identification of a receptor in pituitary and hypothalamus , and its endogenous ligand, ghrelin, was identified three years later .

Q: What does the GHRP-2 diagnostic literature actually establish?

That a provocative test discriminates, in the populations enrolled, against a reference test. In 77 healthy participants and 58 patients, peak growth hormone occurred within 60 minutes in every participant and was significantly lower in patients than in the healthy group, with a sensitivity–specificity crossing point between 15 and 20 µg/l . In 56 children, peak concentrations correlated with the insulin tolerance test and the crossing point was again 15 µg/l . These are threshold-validation studies, not studies of any clinical outcome.

Q: What is the most informative finding in the GHRP-6 human literature?

A study in 21 patients with type 2 diabetes split by body-mass index, plus 8 controls, each given GHRP-6, GHRH, and both together on separate occasions. Growth hormone responses to GHRP-6 did not differ across the three groups, while the response to GHRH was reduced in the overweight diabetic group . It distinguishes the behaviour of two receptor systems in the same people — it is not a comparison with GHRP-2.

Q: Does the receptor have activity without a ligand?

Yes, and unusually so. The growth hormone secretagogue receptor was shown to have high constitutive signalling in the absence of any ligand, with a potent inverse agonist identified . Later work characterised agonism, antagonism and inverse agonism bias in its signalling , and a structure of the antagonist-bound receptor was published in 2020 . None of that work distinguishes GHRP-2 from GHRP-6 specifically.

GHRP-2 and GHRP-6 are hexapeptide agonists at the ghrelin receptor with different sequences, different potencies and very different regulatory positions.

By Elegance Peptides Editorial TeamUpdated September 22, 2026

Unlike most pairs in this library, these two compounds share a receptor, a peptide length and a family name. They are both hexapeptide agonists at the growth hormone secretagogue receptor, and one of them is the reason that receptor was looked for in the first place.

The differences that remain are real but narrow: sequence, relative potency, the strength of the non-growth-hormone responses each produces, and — the widest gap of the four — where each stands with regulators.

What they are

Property	GHRP-2	GHRP-6
Development code	KP-102	Not established
Compound type	Synthetic hexapeptide	Synthetic hexapeptide
Peptide family	Growth hormone secretagogues (ghrelin receptor agonists)	Growth hormone secretagogues (ghrelin receptor agonists)
Primary target	Growth hormone secretagogue receptor 1a (GHS-R1a, the ghrelin receptor)	Growth hormone secretagogue receptor 1a (GHS-R1a, the ghrelin receptor)
Secondary targets	Not established	Not established
Receptor family	Class A (rhodopsin-like) G protein-coupled receptors	Class A (rhodopsin-like) G protein-coupled receptors
Agonist / antagonist	Agonist at the growth hormone secretagogue receptor	Agonist at the growth hormone secretagogue receptor
Highest research phase	Approved diagnostic product in Japan; investigator-initiated clinical validation studies continuing	Investigator-initiated clinical pharmacology studies; no registrational programme and no approval
Regulatory status (United States)	Not approved in the United States. GHRP-2 has not been approved by the U.S. Food and Drug Administration for any indication, diagnostic or therapeutic.	Not approved. GHRP-6 has not been approved by the U.S. Food and Drug Administration for any indication, diagnostic or therapeutic.
Human trials published	Yes	Yes

Every value in this table is read from the two compounds’ own library entries when the site is built, so it cannot disagree with them. Nothing here ranks one compound against the other.

GHRP-6 is a synthetic hexapeptide, His-D-Trp-Ala-Trp-D-Phe-Lys-NH₂, CAS registry number 87616-84-0, molecular weight 873.01 g/mol. First described in 1984, it is the founding member of the growth hormone-releasing peptide family ^[1]. It is not approved by the U.S. Food and Drug Administration for any indication.

GHRP-2, whose international non-proprietary name is pralmorelin, is a synthetic hexapeptide, D-Ala-D-2-Nal-Ala-Trp-D-Phe-Lys-NH₂, CAS registry number 158861-67-7, molecular weight 817.99 g/mol. It is approved in Japan as a diagnostic agent for provocative testing of growth hormone secretion, and is not approved in the United States for any use, diagnostic or therapeutic.

Both are entirely synthetic. Neither has a sequence relationship to ghrelin, the endogenous ligand of the receptor they act at ^[12].

Receptor and mechanistic differences

The receptor is the same, and both compounds were part of finding it. GHRP-6 was described in 1984 as a synthetic hexapeptide acting on the pituitary to release growth hormone specifically ^[1]. Because that action could not be explained through the growth hormone-releasing hormone receptor, a search followed for the receptor responsible, and in 1996 a receptor in pituitary and hypothalamus was identified that functions in growth hormone release ^[4]. Its endogenous ligand, ghrelin — an acylated peptide from the stomach — was identified in 1999 ^[12]. Both GHRP-2 and GHRP-6 act at that receptor, GHS-R1a.

The sequence difference is at four of six positions. GHRP-6 carries histidine at position 1 and D-tryptophan at 2; GHRP-2 carries D-alanine at 1 and D-2-naphthylalanine at 2, and alanine where GHRP-6 has alanine at 3. Both end in the same D-Phe-Lys-NH₂. The structure-activity work behind those substitutions, and the development of the class more broadly, is set out in the secretagogue review literature ^{[14, 5]}. GHRP-2 is described in that literature as one of the most potent members of the family.

Neither is selective for growth hormone release. This is the property that separates both of them from ipamorelin, the pentapeptide in the same class that was described as the first selective secretagogue because it released growth hormone in swine without measurable release of ACTH or cortisol ^[9]. GHRP-2 and GHRP-6 both raise ACTH and cortisol. In GHRP-2's case that property has itself been put to diagnostic use in the assessment of the adrenal axis ^[16].

The reported non-endocrine effects are not identical. Central administration of ghrelin and of growth hormone secretagogues activates brain regions associated with food intake in rodents ^[13], and rodent work on GHRP-6 specifically reported that its effects on body mass and fat accrual depend on the pubertal stage and the sex of the animal ^[17]. These are animal findings, reported in the tiered section below.

Receptor: Both — growth hormone secretagogue receptor 1a, the ghrelin receptor ^{[4, 12]}.
Sequence: GHRP-6, His-D-Trp-Ala-Trp-D-Phe-Lys-NH₂. GHRP-2, D-Ala-D-2-Nal-Ala-Trp-D-Phe-Lys-NH₂. Both C-terminally amidated hexapeptides.
Position in the family: GHRP-6 is the founding member, described 1984 ^[1]. GHRP-2 is a later, more potent analogue ^[14].
Selectivity: Neither is selective; both raise ACTH and cortisol, unlike ipamorelin ^{[9, 16]}.
Regulatory position: GHRP-2 is an approved diagnostic product in Japan only. GHRP-6 has no approval anywhere. Neither is approved in the United States.
Clinical use of the literature: GHRP-2's human literature is largely diagnostic validation ^{[15, 18]}. GHRP-6's is largely investigator-initiated pharmacology in defined patient groups ^{[10, 3]}.

What the research compares

No randomised head-to-head trial of GHRP-2 against GHRP-6 has been published. The two have not been randomised against each other in humans, and the comparisons that exist in the literature are of two kinds, neither of which is a trial.

The first is review-level. The growth hormone-releasing peptide family has been reviewed as a family, with the clinical and basic aspects of its members set out together ^[5], and the development of the secretagogue class — including the structure-activity relationships that distinguish its members — has been reviewed since ^[14]. Statements about relative potency within the family come from that literature rather than from a randomised comparison.

The second is incidental. Several studies administered a growth hormone-releasing peptide alongside growth hormone-releasing hormone in the same participants, which compares two receptor routes rather than two peptides ^{[6, 10, 8]}. Those designs are informative about the GHRH axis against the secretagogue axis, and say nothing about GHRP-2 against GHRP-6.

What can be compared directly is the regulatory and documentary position of each, which is a matter of record rather than of inference, and the shape of each compound's own literature. Those comparisons are made below.

What the human research shows for each

Human clinical research

Results from pharmaceutical clinical trials describe the investigational material and populations used in those studies and should not be interpreted as establishing the effects of research-grade materials offered for laboratory use.

GHRP-2 — a diagnostic literature

Validation of the provocative test in adults. 77 healthy participants and 58 patients with a peak growth hormone below 3 µg/l on the insulin tolerance test received a single 100 µg intravenous administration after an overnight fast, with sampling over two hours. The serum growth hormone peak occurred within 60 minutes in every participant. Peak concentrations were significantly lower in patients (1.36 ± 2.60 µg/l) than in the healthy group (84.6 ± 60.9 µg/l), p < 0.001, with no difference between hypothalamic and pituitary disease. Responses were unaffected by sex, slightly lower in older participants and those with adiposity, and reproducible on repeat testing; the sensitivity–specificity crossing point fell between 15 and 20 µg/l ^[15].

In children. In 56 children with growth disorders, a 2 µg/kg intravenous administration correlated favourably with the insulin tolerance test (p < 0.0001), with peak concentrations significantly lower in children with growth hormone deficiency (median 3.39 µg/l) than without (25.10 µg/l); the crossing point was again 15 µg/l ^[18]. An earlier study in 24 children compared responses to GHRP-2 and to GHRH in the same participants and found them similar, with a synergistic response when both were administered together ^[6].

As an alternative reference test. A comparison of the arginine test and the GHRP-2 test against the insulin tolerance test addressed which alternative best reproduces the reference classification when the reference test is contraindicated ^[19]. Separately, the ACTH response to GHRP-2 has been examined in patients with hypopituitarism ^[16], and continuous 24-hour infusion in women has been used to characterise activation of the growth hormone axis ^[11]. Diagnostic use of intravenous and intranasal administration in children of short stature was reported in the mid-1990s ^[7].

Limitations. These are validation studies of a diagnostic threshold, not studies of any clinical outcome. They establish that a test discriminates in the population enrolled, against a reference test whose own thresholds are debated, in a single country.

GHRP-6 — investigator-initiated pharmacology

In type 2 diabetes, with and without adiposity. 21 patients with non-insulin-dependent diabetes, split by body-mass index, plus 8 normal-weight controls, each received GHRP-6 90 µg intravenously, GHRH 100 µg intravenously, and both together on three separate occasions. Growth hormone responses to GHRP-6 did not differ across the three groups — peaks of 50.95 ± 11.55, 51.96 ± 7.71 and 70.07 ± 15.59 mU/L, p > 0.05 — while the response to GHRH was reduced in the overweight diabetic group relative to the normal-weight diabetic group (peak 8.25 ± 1.90 against 22.19 ± 8.81, p < 0.05) ^[10].

That result is the most directly informative thing in the GHRP-6 human literature, and what it informs is not a comparison with GHRP-2: it distinguishes the behaviour of two receptor systems in the same people.

In other endocrine conditions. Growth hormone responses to GHRP-6 and to GHRH were examined in hyperthyroidism ^[3], and in patients with microprolactinoma and macroprolactinoma before and after bromocriptine therapy, with both agents administered separately and together ^[8].

Limitations. Small, single-centre, condition-specific pharmacological studies with hormone concentrations as endpoints. None measured a clinical outcome and none assessed repeated administration over time.

What the two human records have in common

Both literatures measure hormone concentrations rather than clinical outcomes; both consist of small studies, mostly single-centre; and neither compound has a registrational programme outside GHRP-2's Japanese diagnostic approval. On that much the two are genuinely alike, and it is worth stating because the depth of GHRP-2's diagnostic validation can give the impression of a broader evidence base than either compound has.

What the animal research shows

Animal research

The rodent literature is where the non-endocrine properties of this class were characterised, and it is also where the two compounds diverge most in what has actually been studied.

GHRP-6's original characterisation reported that growth hormone rose within two minutes of intravenous administration in rats, peaked at 10 to 20 minutes and had usually returned to baseline by two hours, with somatostatin-14 and somatostatin-28 both inhibiting the response and somatostatin-28 the more active ^[1].

Central administration of ghrelin and of growth hormone secretagogues was later shown to activate brain regions associated with food intake in rodents ^[13] — the finding that connects this receptor to functions beyond the pituitary. Rodent work on GHRP-6 specifically reported that its effects on body mass and fat accrual depend on the pubertal stage and the sex of the animal, which is an unusually explicit statement of how conditional such findings are ^[17].

Ipamorelin's discovery paper provides the within-class contrast: in swine, that pentapeptide released growth hormone without measurable ACTH or cortisol release, a selectivity neither hexapeptide has ^[9].

Findings described in this section were observed in animals. Nothing in them establishes anything about humans.

What the in vitro and receptor research shows

In vitro research

The receptor work is common to both compounds and is the reason they belong in the same section rather than separate ones.

A receptor in pituitary and hypothalamus functioning in growth hormone release was identified in 1996 through the synthetic secretagogues, defining the target both hexapeptides act at ^[4]. Ghrelin was identified as its endogenous acylated ligand in 1999 ^[12]. The receptor was subsequently shown to have unusually high constitutive signalling — activity in the absence of any ligand — and a potent inverse agonist was identified, which means a full account of secretagogue pharmacology at this receptor has to address basal tone and not only agonist occupancy ^[2]. Later work characterised agonism, antagonism and inverse agonism bias in its signalling ^[20], and a structure of the antagonist-bound receptor was published in 2020 ^[21].

None of that structural or signalling work distinguishes GHRP-2 from GHRP-6 specifically. Statements about their relative potency come from the structure-activity and review literature of the class ^{[14, 5]}, not from a comparative binding study reproduced here.

Research status of each

GHRP-2 is an approved diagnostic product in Japan, used for intravenous provocative testing of growth hormone secretion. That approval attaches to a specific manufactured pharmaceutical product in one country for one diagnostic purpose. It has not been approved by the U.S. Food and Drug Administration for any indication, diagnostic or therapeutic, and investigator-initiated validation work continues.

GHRP-6 has no approval anywhere. Its published clinical work is investigator-initiated pharmacology in defined patient groups, with no registrational programme.

Neither status extends to research-grade material. An approved diagnostic product in one country is a manufactured product with a label and a marketing authorisation, and nothing about it transfers to material supplied for laboratory use.

Frequently Asked Questions

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

Sequence, relative potency and regulatory position — not receptor. Both are C-terminally amidated hexapeptide agonists at the growth hormone secretagogue receptor ^[4]. GHRP-6 is His-D-Trp-Ala-Trp-D-Phe-Lys-NH₂ and is the founding member of the family, described in 1984 ^[1]. GHRP-2 is D-Ala-D-2-Nal-Ala-Trp-D-Phe-Lys-NH₂, a later analogue described in the class literature as one of the more potent members ^[14].

Have GHRP-2 and GHRP-6 been compared head-to-head?

No randomised head-to-head trial has been published. The comparisons that exist are review-level, setting out the family's members together ^{[5, 14]}, or incidental — studies that administered a growth hormone-releasing peptide alongside growth hormone-releasing hormone in the same participants, which compares two receptor routes rather than two peptides ^{[6, 10]}.

Do both compounds raise ACTH and cortisol?

Yes. Neither hexapeptide is selective for growth hormone release. That is what separates both of them from ipamorelin, the pentapeptide in the same class described as the first selective secretagogue because it released growth hormone in swine without measurable ACTH or cortisol release ^[9]. In GHRP-2's case the ACTH response has itself been examined diagnostically in patients with hypopituitarism ^[16].

Is either compound approved?

GHRP-2 is approved in Japan as a diagnostic agent for intravenous provocative testing of growth hormone secretion. That approval attaches to a specific manufactured product in one country for one diagnostic purpose. GHRP-6 has no approval anywhere. Neither is approved by the U.S. Food and Drug Administration for any indication, diagnostic or therapeutic.

Why was the ghrelin receptor named after a growth hormone secretagogue?

Because the drugs came first. GHRP-6 was described in 1984 as a synthetic hexapeptide acting on the pituitary to release growth hormone by a route the GHRH receptor could not explain ^[1]. The search for the receptor responsible ended in 1996 with the identification of a receptor in pituitary and hypothalamus ^[4], and its endogenous ligand, ghrelin, was identified three years later ^[12].

What does the GHRP-2 diagnostic literature actually establish?

That a provocative test discriminates, in the populations enrolled, against a reference test. In 77 healthy participants and 58 patients, peak growth hormone occurred within 60 minutes in every participant and was significantly lower in patients than in the healthy group, with a sensitivity–specificity crossing point between 15 and 20 µg/l ^[15]. In 56 children, peak concentrations correlated with the insulin tolerance test and the crossing point was again 15 µg/l ^[18]. These are threshold-validation studies, not studies of any clinical outcome.

What is the most informative finding in the GHRP-6 human literature?

A study in 21 patients with type 2 diabetes split by body-mass index, plus 8 controls, each given GHRP-6, GHRH, and both together on separate occasions. Growth hormone responses to GHRP-6 did not differ across the three groups, while the response to GHRH was reduced in the overweight diabetic group ^[10]. It distinguishes the behaviour of two receptor systems in the same people — it is not a comparison with GHRP-2.

Does the receptor have activity without a ligand?

Yes, and unusually so. The growth hormone secretagogue receptor was shown to have high constitutive signalling in the absence of any ligand, with a potent inverse agonist identified ^[2]. Later work characterised agonism, antagonism and inverse agonism bias in its signalling ^[20], and a structure of the antagonist-bound receptor was published in 2020 ^[21]. None of that work distinguishes GHRP-2 from GHRP-6 specifically.

References

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. PMID 6714155 doi:10.1210/endo-114-5-1537
High constitutive signaling of the ghrelin receptor--identification of a potent inverse agonist Molecular Endocrinology; 2003. PMID 12907757 doi:10.1210/me.2003-0069
Different growth hormone (GH) response to GH-releasing peptide and GH-releasing hormone in hyperthyroidism The Journal of Clinical Endocrinology and Metabolism; 1996. PMID 8636330 doi:10.1210/jcem.81.4.8636330
A receptor in pituitary and hypothalamus that functions in growth hormone release Science; 1996. PMID 8688086 doi:10.1126/science.273.5277.974
Growth hormone-releasing peptides: clinical and basic aspects Hormone Research; 1996. PMID 8950613 doi:10.1159/000185015
Diagnostic studies with intravenous and intranasal growth hormone-releasing peptide-2 in children of short stature The Journal of Clinical Endocrinology and Metabolism; 1995. PMID 7559885 doi:10.1210/jcem.80.10.7559885
Growth hormone (GH)-releasing effects of synthetic peptide GH-releasing peptide-2 and GH-releasing hormone (1-29NH2) in children with GH insufficiency and idiopathic short stature Metabolism: Clinical and Experimental; 1995. PMID 7666796 doi:10.1016/0026-0495(95)90016-0
Growth hormone secretion elicited by GHRH, GHRP-6 or GHRH plus GHRP-6 in patients with microprolactinoma and macroprolactinoma before and after bromocriptine therapy Clinical Endocrinology; 1998. PMID 9509075 doi:10.1046/j.1365-2265.1998.00360.x
Ipamorelin, the first selective growth hormone secretagogue European Journal of Endocrinology; 1998. PMID 9849822 doi:10.1530/eje.0.1390552
Growth hormone (GH) response to GH-releasing peptide-6 and GH-releasing hormone in normal-weight and overweight patients with non-insulin-dependent diabetes mellitus Metabolism: Clinical and Experimental; 1999. PMID 10206449 doi:10.1016/s0026-0495(99)90115-4
Tripartite neuroendocrine activation of the human growth hormone (GH) axis in women by continuous 24-hour GH-releasing peptide infusion: pulsatile, entropic, and nyctohemeral mechanisms The Journal of Clinical Endocrinology and Metabolism; 1999. PMID 10372723 doi:10.1210/jcem.84.6.5687
Ghrelin is a growth-hormone-releasing acylated peptide from stomach Nature; 1999. PMID 10604470 doi:10.1038/45230
Acute central ghrelin and GH secretagogues induce feeding and activate brain appetite centers Endocrinology; 2002. PMID 11751604 doi:10.1210/endo.143.1.8561
Development of growth hormone secretagogues Endocrine Reviews; 2005. PMID 15814848 doi:10.1210/er.2004-0019
A simple diagnostic test using GH-releasing peptide-2 in adult GH deficiency European Journal of Endocrinology; 2007. PMID 17609397 doi:10.1530/EJE-07-0066
Diagnostic usefulness of the growth hormone-releasing peptide-2 test as a substitute for the insulin tolerance test in hypopituitarism Endocrine Journal; 2008. PMID 18493103 doi:10.1507/endocrj.k07e-168
The positive effects of growth hormone-releasing peptide-6 on weight gain and fat mass accrual depend on the insulin/glucose status Endocrinology; 2010. PMID 20219977 doi:10.1210/en.2009-1394
Growth hormone response to GH-releasing peptide-2 in children Journal of Pediatric Endocrinology & Metabolism; 2010. PMID 20662346 doi:10.1515/jpem.2010.078
The arginine and GHRP-2 tests as alternatives to the insulin tolerance test for the diagnosis of adult GH deficiency in Japanese patients: a comparison Endocrine Journal; 2013. PMID 23079545 doi:10.1507/endocrj.ej12-0230
Agonism, Antagonism, and Inverse Agonism Bias at the Ghrelin Receptor Signaling The Journal of Biological Chemistry; 2015. PMID 26363071 doi:10.1074/jbc.M115.659250
Structure of an antagonist-bound ghrelin receptor reveals possible ghrelin recognition mode Nature Communications; 2020. PMID 32814772 doi:10.1038/s41467-020-17554-1

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