GHRP-2 and GHRP-6 are the two oldest growth-hormone-releasing peptides still sold, and they are compared mostly on two claims: that GHRP-2 is stronger and that GHRP-6 makes people hungrier. The first has human data behind it. The second has a human study, but of GHRP-2.
The single-compound records are on the GHRP-2 dosage page and the GHRP-6 dosage page; the GHRP developed to avoid their side effects is on the ipamorelin dosage page.
| GHRP-2 (pralmorelin) | GHRP-6 | |
|---|---|---|
| Sequence | D-Ala-D-(2-naphthyl)Ala-Ala-Trp-D-Phe-Lys-NH2 | His-D-Trp-Ala-Trp-D-Phe-Lys-NH2 |
| Molecular weight | 818.0 | 873.0 |
| Receptor | Ghrelin receptor (GHS-R1a) | Same |
| Approved product | Diagnostic in Japan, 100 mcg IV once | None |
| Human test dose | 100 mcg IV | 1 mcg/kg IV |
| GH response in humans | Larger | Smaller (Bowers 1993) |
| Food intake measured in people | +35.9% (7 men) | Not found |
| Urine detection after a nasal dose | Up to 47 h | 23 h (metabolites 12 h) |
One receptor, two generations
GHRP-6 was the first of the series to be given to people, and GHRP-2 was built from it: Arvat and colleagues call GHRP-2 and hexarelin "super-analogs of GHRP-6". All of them bind the receptor later found to belong to ghrelin, the stomach hormone that signals hunger and releases growth hormone. Because they share a receptor, they share its effects in kind, and differ in degree.
The GH response
Bowers, who developed the series, reported in 1993 that GHRP-6, GHRP-1 and GHRP-2 were "increasingly more effective in releasing GH in humans", in that order, that each released GH more effectively than GHRH 1-44, and that all three worked by mouth. That ordering is why GHRP-2 became the diagnostic. In Japan it is given once as 100 mcg intravenously, and in its validation study peak GH averaged 84.6 µg/L in healthy adults against 1.36 µg/L in people with GH deficiency. GHRP-6's literature uses it the same way, as a single 1 mcg/kg intravenous test, not as a treatment.
The response fades with repeated use. Nine healthy men given 100 mcg of GHRP-2 under the skin daily had mean GH peaks of 83, 59 and 51 µg/L on days 1, 3 and 5, with no rise in IGF-I, as set out on the GHRP-2 page.
Prolactin, ACTH and cortisol
Neither peptide is specific to growth hormone. In six young adults given 1 and 2 mcg/kg intravenously, GHRP-2 and hexarelin raised prolactin, ACTH and cortisol as well as GH, and the authors concluded the peptides are "not fully specific". Hexarelin's own dose-response studies put numbers on that, with prolactin plateauing at a 180% rise and cortisol stepping up from 0.5 mcg/kg, as set out on the hexarelin dosage page. The same side effects are listed for GHRP-6 in its testing literature, where it is also used to probe the adrenal axis. Ipamorelin was later described as "the first selective growth hormone secretagogue" for that reason (Raun et al., Eur J Endocrinol 1998, PMID 9849822).
Appetite: the measured study is GHRP-2's
The usual line on forum and vendor pages is that GHRP-6 causes strong hunger and GHRP-2 much less. The one human food-intake trial this site found tested GHRP-2. Laferrère and colleagues infused seven lean, healthy men under the skin with GHRP-2 at 1 mcg/kg per hour for 270 minutes, or saline, and then offered a buffet. On GHRP-2 they ate 35.9% more, every man ate more, and growth hormone rose about thirteenfold by area under the curve. At 70 kg that infusion totals about 315 mcg over four and a half hours. A follow-up in obese subjects found the same stimulation (Obesity 2006, PMID 16861611).
For GHRP-6, the first 15 of 27 PubMed records combining it with appetite terms, read on 2026-10-09, were reviews and animal or cell studies; none measured food intake in people. The claim that GHRP-6 is the hungrier of the two may be true, since both act on the hunger hormone's receptor, but it is not a measured human comparison.
Detection in urine
Both are on the World Anti-Doping Agency's prohibited list. In a Russian anti-doping laboratory study with the Cologne sports university, one volunteer per compound took a nasal dose; GHRP-2 and its metabolites were found in urine for up to 47 hours, and GHRP-6, excreted mostly unchanged, for 23 hours, with its metabolites for 12. One person per compound is a method study, not a population figure.
The figures in a vial, and as units
| Amount | Molecules | 2.5 mg/mL | 5 mg/mL |
|---|---|---|---|
| GHRP-2 100 mcg | ≈122 nmol | 4 units | 2 units |
| GHRP-2 300 mcg | ≈367 nmol | 12 units | 6 units |
| GHRP-6 70 mcg (1 mcg/kg at 70 kg) | ≈80 nmol | 2.8 units | 1.4 units |
| GHRP-6 100 mcg | ≈115 nmol | 4 units | 2 units |
| GHRP-6 300 mcg | ≈344 nmol | 12 units | 6 units |
2.5 mg/mL is a 5 mg vial with 2 mL of water; 5 mg/mL is a 5 mg vial with 1 mL. Vendor and protocol pages read for the single-compound pages cite 100 to 300 mcg under the skin up to three times a day; no controlled human trial validates that schedule for either peptide. Small draws such as 2.8 units fall between the marks of a 2-unit-marked barrel; the barrels are set out on insulin syringe units, and any vial and volume are worked on the GHRP-2 calculator and the GHRP-6 calculator.
What is not established
No trial has compared the two head to head at matched doses, given them together, or measured GHRP-6's effect on food intake in people. The human data are mostly single intravenous tests in small groups; repeated injections under the skin, the way vials are used, have been studied for GHRP-2 only, in a handful of men and children. The compounds are covered in Medibact's GHRP-2 guide and GHRP-6 guide. Nothing on this page is an instruction; product and dose belong with a prescriber.
Sources and dates
Opened 2026-10-09: the abstracts of Bowers 1993 (PMID 8374685), Arvat et al. 1997 (PMID 9285939), Laferrère et al. 2005 (PMID 15699539) and Semenistaya et al. 2015 (PMID 25869809), and the title record of Raun et al. 1998 (PMID 9849822); PubMed searches for GHRP-6 with appetite terms (27 records) and with prolactin or cortisol (56); PubChem records for pralmorelin (CID 6918245) and GHRP-6 (CID 4345065). Carried from this site's pages, each with its read date: the Japanese diagnostic, its validation study, the 5-day and paediatric GHRP-2 studies (GHRP-2 page, 2026-09-28), GHRP-6's test dose, registry count and protocol figures (GHRP-6 page, 2026-09-27). Units and molar arithmetic by this site. Corrections go to the contact page.