GHRP-6 vs Ipamorelin and GHRP-2: History and Mechanism

GHRP-6 is a short synthetic peptide made of six amino acids. It was created in the laboratory before scientists knew the receptor it acts on, or the natural hormone that activates that receptor in the body. The story of GHRP-6 is therefore also the story of how ghrelin was discovered.

History: From Enkephalin Analogues to Ghrelin

It started with the American endocrinologist Cyril Bowers at Tulane University in New Orleans. In the second half of the 1970s, his laboratory studied synthetic analogues of enkephalins, which are short opioid peptides. Some of them unexpectedly released growth hormone (GH) from rat pituitary cells. The first compounds, however, were weak and worked only in vitro.

Step-by-step changes to the structure led to the hexapeptide His-D-Trp-Ala-Trp-D-Phe-Lys-NH₂. Bowers and colleagues described it in 1984 in the journal Endocrinology. It later became known as GHRP-6, short for growth hormone–releasing peptide. It was the first GHRP with a marked effect in living animals. In experiments it released GH in rats, monkeys, lambs and calves, and the response increased with the dose. Levels of LH, FSH, TSH and prolactin did not change in these experiments. Somatostatin, the natural brake on GH secretion, suppressed the response to GHRP-6.

GHRP-6 does not bind to the receptor for GHRH, the hypothalamic hormone that normally controls GH release. In the 1980s, Bowers therefore proposed that GHRP mimics a natural hormone that had not yet been found. Confirming this took more than ten years:

• 1990: a study in healthy men showed that GHRP-6 and GHRH given together release more GH than the sum of their separate effects (synergy).
• 1996: a Merck team (Howard and colleagues) cloned the receptor through which these compounds act. It is found in the pituitary and the hypothalamus and was named the growth hormone secretagogue receptor (GHS-R). Today it is referred to as GHS-R1a.
• 1999: the Japanese team of Kojima and Kangawa isolated the natural ligand of this receptor from rat stomach. It is a 28-amino-acid peptide, and the authors named it ghrelin.

In the meantime, GHRP-6 also served Merck as a template for developing non-peptide compounds, including ibutamoren (MK-677). Put simply, a synthetic peptide helped find the receptor, and the receptor then led scientists to the natural hormone.

Mechanism of Action as Described in Research

GHRP-6 is an agonist of the GHS-R1a receptor. It binds to the receptor and activates it in a similar way to ghrelin. The receptor belongs to the G protein–coupled receptors and signals via phospholipase C, a different route from the GHRH receptor. Research describes it at two sites:

• in the pituitary, where GHRP-6 directly releases GH from the cells that produce it (somatotrophs) in in vitro experiments,
• in the hypothalamus, where it is involved in controlling pulsatile GH secretion.

The interplay with GHRH is central. In his historical review, Bowers states that GHRP increases pulsatile GH secretion in humans and that the body's own GHRH is needed for this. The synergy described in 1990 supports the conclusion that the two act on separate, complementary pathways. According to the review by Sigalos and Pastuszak, GH release after secretagogues remains pulsatile and subject to negative feedback.

The selectivity of GHRP-6 for growth hormone is not complete, however. In the 1990 study in healthy men, prolactin and cortisol levels roughly doubled at the highest dose tested. LH and TSH levels did not change. A 1995 sleep study, cited in the review by Sigalos and Pastuszak, reported increases in GH, ACTH and cortisol after GHRP-6.

Appetite and the Ghrelin Pathway

Ghrelin is known for influencing hunger signals as well as GH. Research describes a similar feature for GHRPs. Bowers notes that the first sign of a broader action of GHRP was precisely an increase in food intake. In a 1995 experiment, sated rats received GHRP-6 directly into the brain ventricles. The incidence of eating rose with the dose, while blood GH levels did not change in proportion. The authors concluded that the effect works through a mechanism independent of GH release. For the related GHRP-2, small studies in volunteers reported higher food intake at a subsequent meal.

GHRP-6 vs GHRP-2 vs Ipamorelin: Comparison

All three compounds are synthetic growth hormone secretagogues and act through the same receptor. They differ in structure and in how selectively they release GH according to studies. The most direct comparison comes from a 1998 study by Raun and colleagues at Novo Nordisk. In it, the team introduced ipamorelin and compared it with GHRP-6 and GHRP-2 in rat pituitary cells, in rats and in pigs.

The comparison shows three main differences:

• Structure. GHRP-2 differs from GHRP-6 in its first two amino acids: instead of His-D-Trp it carries D-Ala and D-2-naphthylalanine (D-2-Nal). Ipamorelin is a shorter pentapeptide. It came from a series of compounds lacking the central Ala-Trp pair of GHRP-1, and it starts with the non-standard amino acid Aib (2-aminoisobutyric acid).
• Selectivity. In pigs, both GHRP-6 and GHRP-2 raised ACTH and cortisol in addition to GH. Ipamorelin did not raise these hormones more than GHRH alone. The authors therefore called it the first selective GH secretagogue. This is a result from an animal study.
• Use. In Japan, GHRP-2 is used in clinical diagnostics as a stimulation test of GH secretion. GHRP-6 remains a research compound.

Other Research Directions

Beyond the growth hormone axis, GHRP-6 is studied mainly in cell and animal models. Part of this work concerns a second receptor, CD36. In 2002, a Canadian team found that hexarelin, another peptide of the GHRP family, binds to it in heart tissue. A 2017 review by a group from the Center for Genetic Engineering and Biotechnology in Havana states that GHRPs bind to both receptors and lists these research areas:

• Heart: models of myocardial infarction, for example in pigs, in which the authors describe a smaller extent of damage to the heart muscle.
• Liver and other organs: in a model of liver ischaemia and reperfusion, the authors reported less damage to the liver and to distant organs, and fewer signs of inflammation and oxidative stress, after prior administration of GHRP-6.
• Fibrosis: models of liver cirrhosis and hypertrophic scars, in which GHRP-6 reduced collagen deposition.

These are results from experimental models. The authors of the review themselves state that GHRPs are still waiting for a clinical role. Results in animals cannot be used to draw conclusions for humans.

Regulatory and Sport Status

GHRP-6 is not approved as a medicinal product in the European Union or the United States. A 2018 review states that GHRPs are not approved by the US Food and Drug Administration (FDA). In September 2023, the FDA also placed GHRP-6 in category 2 of bulk drug substances for pharmacy compounding, meaning substances that may present significant safety risks. As reasons it cites possible immunogenicity due to aggregation and peptide-related impurities, and a possible effect on cortisol and blood glucose.

The 2026 Prohibited List of the World Anti-Doping Agency (WADA) names GHRP-6 explicitly in section S2.2.4, among growth hormone releasing factors. It appears in the group of GH-releasing peptides together with GHRP-2 (pralmorelin), hexarelin and other GHRPs. Ipamorelin is listed in the same section among growth hormone secretagogues. Substances in class S2 are prohibited at all times, in and out of competition, and the ban also covers substances with a similar chemical structure or similar biological effect. The summary of modifications for the 2027 List does not mention any change to section S2.

GHRP-6 Identity and Why It Matters

Two features of GHRP-6 place particular demands on quality control:

• D-amino acids. Tryptophan at position 2 and phenylalanine at position 5 are in the D form, the mirror image of the usual amino acids in proteins. A peptide with a D-amino acid has the same mass as its all-L version. During synthesis, racemisation can also occur and create so-called diastereomeric impurities.
• Amidated C-terminus. The terminal lysine carries an –NH₂ group instead of a free carboxylic acid. The amide and the free acid differ by about 1 dalton.

This is why identity and purity are assessed separately. Chromatography (HPLC) shows purity, meaning the proportion of the main compound in the sample. Mass spectrometry (MS) confirms identity: it measures the molecular mass and reveals, for example, a peptide with a missing or extra amino acid, or the non-amidated form. MS alone cannot tell a D-amino acid from an L-amino acid, however, because the mass does not change. Configuration is checked with specialised methods, such as chiral amino acid analysis after hydrolysis of the peptide.

Certificates of analysis for individual batches are available on the Certificates page. You can match a batch by the production date printed on the packaging. For laboratory and research purposes, GHRP-6 is available as a lyophilised powder in a vial: GHRP-6 10 mg.

Frequently Asked Questions

What is the difference between GHRP-6 and ipamorelin?

Both act through the GHS-R1a receptor. GHRP-6 is a hexapeptide described in 1984, while ipamorelin is a shorter pentapeptide introduced by Novo Nordisk in 1998. In a comparative study in pigs, GHRP-6 and GHRP-2 raised ACTH and cortisol in addition to growth hormone, whereas ipamorelin did not. This is why research describes ipamorelin as more selective.

Is GHRP-6 on the WADA list?

Yes. The 2026 WADA List names it explicitly in section S2.2.4 among GH-releasing peptides. It is prohibited at all times, in and out of competition. GHRP-2 and ipamorelin are in the same section.

How is GHRP-6 related to ghrelin?

GHRP-6 was created as a synthetic compound before ghrelin was discovered, and it acts on the same receptor, GHS-R1a. Research on GHRPs led to the discovery of this receptor in 1996 and to the isolation of ghrelin as its natural ligand in 1999.

Is GHRP-6 an approved medicine?

No. GHRP-6 is not authorised as a medicinal product in the EU or the US. It is available only as a research compound for laboratory purposes.

Sources

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