GHRP-2, GHRP-6 and hexarelin: the first-generation secretagogues and why the field moved on

TL;DR

• GHRP-6, GHRP-2 and hexarelin were the first growth-hormone-releasing peptides. They act on the ghrelin receptor and release GH strongly, and they dominate older forum material for that reason.
• They are not selective. Human studies show GHRP-2 and hexarelin also raise prolactin, ACTH and cortisol, and GHRP-6 carries a marked appetite effect. Ipamorelin was developed specifically to remove those off-target signals.
• Hexarelin is the most potent and the least durable. Its GH response attenuates with sustained use, partially and reversibly, in human studies.
• The human evidence for outcomes people want is thin. It consists mostly of small, short endocrine studies from the 1990s and diagnostic work. The large outcome trials were never funded or run, so benefit for body composition or ageing is unestablished rather than disproven.
• None of the three is approved as a therapy in the US. FDA's own list places GHRP-2 and GHRP-6 in Category 2 (significant safety risks) under its 503B interim policy, added September 2023 and still listed after the April 2026 update. All three are on the WADA prohibited list.
• The useful path today is measured and selective: baseline bloodwork on the GH axis, discussed with your own doctor, rather than a first-generation compound from a research-chemical vendor.

Educational content, not medical advice. No doses or protocols appear.

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Read enough older bodybuilding and anti-ageing forum threads and three names keep appearing: GHRP-6, GHRP-2 and hexarelin. They were discussed as if they were the obvious tools for raising growth hormone. Then read anything written by an endocrinologist in the last decade and they have mostly disappeared, replaced by ipamorelin, GHRH analogues and, where a licensed product is genuinely indicated, recombinant GH.

That gap is the subject of this page. It is not that the older compounds stopped working. They release GH, sometimes more strongly than their successors. The field moved on because of what else they release, how quickly one of them loses its effect, how little outcome evidence was ever generated, and where regulators have since placed them. Each of those is a specific, checkable reason, and together they explain why current practice ignores compounds that once dominated the conversation.

If the GH axis itself is unfamiliar, start with the GH axis, which sets out how GHRH analogues and ghrelin-receptor agonists act on different levers of the same system.

The three generations of GH secretagogue, in order

The history matters because it explains the design choices.

First generation: the GHRPs (1980s to early 1990s). In 1984 Cyril Bowers and colleagues described a series of small synthetic peptides, derived from enkephalin structures, that released GH in animals and then in humans. GHRP-6 was the most potent of that early series. GHRP-2 (also called pralmorelin) and hexarelin (examorelin) followed as more potent analogues. At this point nobody knew which receptor they acted on. The compounds were discovered by their effect, not designed for a target.

The receptor, then the hormone (1996 and 1999). In 1996 a Merck team cloned the receptor these compounds bind, the growth hormone secretagogue receptor (GHS-R1a). It was an orphan receptor: its natural ligand was unknown. In 1999 Kojima and colleagues identified that ligand in the stomach and named it ghrelin. The order is unusual in pharmacology. The synthetic drugs came first; the natural hormone they mimic was found fifteen years later. Ghrelin turned out to be a hunger signal as well as a GH signal, which retrospectively explained a great deal about the first-generation compounds.

Second and third generation: selectivity and oral activity (mid 1990s onward). Once the receptor was known, compounds could be designed against it. Ipamorelin, described in 1998, was built to release GH without the hormonal side traffic of its predecessors. In parallel, non-peptide agonists such as ibutamoren (MK-677) pursued oral activity and longer action, and later molecules such as macimorelin and anamorelin were taken through formal development for narrow, specific indications. The trajectory runs from potency, to selectivity, to defined clinical purpose.

What GHRP-2 and GHRP-6 do that ipamorelin deliberately does not

The single most useful fact for anyone comparing GHRP-6 vs ipamorelin is that they are not interchangeable versions of the same thing. They share a receptor and a primary effect. They differ in what comes along with it.

Cortisol and prolactin cross-stimulation

The clearest human data come from the Ghigo and Arvat group in Turin, who in 1997 compared GHRP-2 and hexarelin directly against reference stimuli in healthy volunteers. Both peptides released more GH than GHRH did. Both also raised prolactin, ACTH and cortisol. The prolactin effect was smaller than that of a classic prolactin stimulus; the ACTH and cortisol effect was comparable to that of corticotropin-releasing hormone itself. In other words, at the exposures studied, these compounds were activating the stress axis to roughly the degree that the body's own stress-axis signal does.

That matters for the reason people use them. Cortisol is catabolic and suppresses immune function. Prolactin elevation has its own endocrine consequences. A compound marketed for recovery or body composition that also nudges both upwards is pulling in two directions at once. FDA's own summary for GHRP-6 notes "potential effect on cortisol" among its safety concerns.

The 1998 paper that introduced ipamorelin (Raun and colleagues, European Journal of Endocrinology) made exactly this comparison its headline, in animal studies: GH release comparable to the older GHRPs, without meaningful increases in ACTH or cortisol, and selectivity comparable to GHRH. That was the point of the molecule. A longer account is in ipamorelin and why selectivity matters.

The appetite effect as a side effect, not a feature

Once ghrelin was identified as a hunger hormone, the intense appetite reported with GHRP-6 made sense: it is a ghrelin mimic, and ghrelin tells the brain to eat. In forum material this is sometimes presented as a benefit for people trying to gain mass. In a clinical frame it is an off-target effect. It is not what a GH-axis intervention is for, it is difficult to control, and it interacts with the blood-glucose and insulin-sensitivity concerns that already sit over the whole class.

There is one setting where appetite stimulation through this receptor is the intended effect, and it shows the difference between a side effect and an indication. Anamorelin, an oral ghrelin-receptor agonist, was approved in Japan (approval announced by the manufacturer in January 2021) for cancer cachexia in specific tumour types, where loss of appetite and lean mass is the disease being treated. That is a defined patient population, a defined endpoint and a formal trial programme. It is not evidence that appetite stimulation is desirable in a healthy adult.

Hexarelin: potency, and the desensitisation problem

What is hexarelin? It is a synthetic hexapeptide, structurally close to GHRP-6, and among the most potent GH releasers of the first generation in acute testing. It shares GHRP-2's prolactin, ACTH and cortisol co-stimulation in the Turin data.

Its defining practical problem is attenuation. In a 1998 study in The Journal of Clinical Endocrinology & Metabolism, Rahim and colleagues gave hexarelin to healthy older adults over sixteen weeks and found the GH response fell significantly by week four and further by week sixteen. The attenuation was partial and reversible: after treatment was stopped, the response returned to roughly its original level. A compound whose main effect fades with continued use is a poor candidate for anything intended to be sustained, and that alone explains much of its decline.

Hexarelin also binds a second receptor, CD36, found on heart and blood-vessel cells. Experimental work, mostly in animals and isolated tissue, has explored cardiovascular effects through that route that appear independent of GH. This is genuinely interesting pharmacology. It is also investigational, it has not been developed into a therapy, and it means hexarelin has biological activity beyond the GH axis that is not well characterised in humans. Neither of those is a reason to use it outside a trial.

Why selectivity became the design goal

Put the three problems together: stress-hormone co-stimulation, an unwanted appetite signal, and, for hexarelin, a response that fades. Each is an effect that is not GH release. The obvious engineering response was to find molecules that kept the GH effect and dropped everything else, and that is what the second generation did. Selectivity, more than raw potency, became what ipamorelin and its contemporaries were designed around.

Two clarifications keep this honest.

Selectivity is a pharmacological distinction, not a regulatory one. Ipamorelin is cleaner on cortisol and prolactin. It is still investigational and not FDA-approved, and FDA's Category 2 page lists ipamorelin acetate under the same 503B interim policy as GHRP-2 and GHRP-6. "More selective" does not mean "approved", and it does not mean outcome benefit has been shown.

Selectivity does not remove the class concerns. Anything that raises GH raises IGF-1, and the theoretical concern about growth signalling wherever cell proliferation matters applies to every secretagogue. Blood glucose and insulin sensitivity warrant monitoring across the class. What a raised IGF-1 does and does not tell you is set out in what an IGF-1 level means.

What the human evidence actually consists of (and how thin it is)

It is worth being precise about what was studied, because the volume of forum discussion creates an impression of a large evidence base that does not exist.

Acute endocrine studies. The bulk of human GHRP research consists of single-exposure or short-course studies measuring hormone levels over hours: how much GH, prolactin, ACTH and cortisol rise, how responses change with age, and how the peptides interact with GHRH and somatostatin. This work is careful and informative about physiology. It does not measure muscle, fat, function or health outcomes.

Diagnostic work. Because these compounds provoke a reliable GH response in a healthy pituitary, they were natural candidates for testing whether a pituitary can release GH. GHRP-2 (pralmorelin) has been marketed in Japan by Kaken Pharmaceutical as a single-use diagnostic for GH deficiency. A diagnostic approval establishes that a compound produces a measurable, interpretable response in a test setting. It says nothing about therapeutic benefit from repeated use.

Small longer studies. A handful of studies ran for weeks to months in older adults, children with short stature, or specific clinical groups, typically with small numbers and hormonal endpoints. The hexarelin attenuation study is one of the more informative because it was long enough to show the response fading.

Critically ill patients. GHRP-2 was studied in prolonged critical illness in the 1990s as a way to restore GH pulsatility. FDA's safety summary for GHRP-2 cites reports of serious adverse events including increased insulin requirement, death of critically ill study subjects, infection and pancreatitis, while noting that causality has not been established. That caveat matters, and so does the signal: the population was already gravely ill, and the data cannot separate drug from disease.

What is missing is the evidence people are actually looking for: adequately sized, randomised, placebo-controlled trials of sustained use measuring body composition, strength, function, metabolic health or anything describable as ageing. Those trials were never funded. After selective successors appeared, there was little commercial reason to run them for off-patent first-generation peptides. That is a statement about where research money went, not a finding that the compounds fail. It does mean any claim of benefit for those outcomes is unestablished. The general skill of reading such claims is covered in what a study actually measured.

Where these sit on the regulatory map today

Regulatory status in this area changes, so the most useful thing this section can do is tell you exactly what was checked, when, and where to look yourself.

United States: no therapeutic approval. None of GHRP-2, GHRP-6 or hexarelin is approved by FDA as a treatment for any condition.

FDA Category 2. FDA maintains a public page titled Certain Bulk Drug Substances for Use in Compounding that May Present Significant Safety Risks, listing substances placed in Category 2 under its interim compounding policies. As checked for this article (page marked current as of 22 April 2026), it lists GHRP-2 (for injectable and nasal routes) and GHRP-6 under the 503B interim policy, both added on 29 September 2023. Secondary reporting describes both as remaining restricted after FDA's April 2026 changes, in which a number of other peptides were removed from Category 2; the FDA page is consistent with that. Hexarelin does not appear on the page. Absence from a list is not an approval, and the categories are interim policies that FDA revises. Before relying on any of this, check FDA's current list directly, because a summary on any website, including this one, can go out of date.

Japan: a diagnostic, not a therapy. GHRP-2, as pralmorelin, has had a diagnostic use in Japan for assessing GH deficiency.

A related molecule that did reach FDA approval, as a diagnostic. Macimorelin (Macrilen), an oral ghrelin-receptor agonist, was approved by FDA in December 2017 for the diagnosis of adult GH deficiency. It is a test, given once, under supervision. It is not a GH-raising therapy, and its approval does not extend to the older peptides.

Sport. GHRP-2, GHRP-6 and hexarelin are named on the World Anti-Doping Agency prohibited list under S2 (peptide hormones, growth factors, related substances and mimetics), alongside other ghrelin-receptor agonists including ipamorelin.

The pattern across jurisdictions is consistent: where this receptor has been formally approved for anything, it has been for a narrow diagnostic or a defined disease, never for general GH enhancement.

What a clinician would use instead, and why

A clinician approaching the GH axis today does not start with a compound. They start with the question of whether there is anything to treat, and the answer comes from history, examination and bloodwork, including IGF-1 and, where deficiency is genuinely suspected, a formal stimulation test.

From there, the options sort roughly as follows.

Confirmed GH deficiency is a diagnosis with licensed treatment. The comparison between recombinant GH and secretagogues is set out in HGH vs GH peptides.

Where a secretagogue approach is being considered under supervision, selective options are preferred over first-generation ones for the reasons this page describes: less stress-axis and prolactin activity, no prominent appetite effect, and no characteristic loss of response. Those are typically discussed alongside GHRH analogues, which act on a different receptor. The logic of combining the two receptor systems is covered in CJC-1295 and ipamorelin, and the GHRH options are compared in sermorelin vs CJC-1295 vs tesamorelin. Oral non-peptide alternatives carry their own concerns, set out in MK-677.

Monitoring is part of the choice, not an add-on. Baseline and follow-up IGF-1, glucose markers and a cancer-history screen are what separate supervised use from an unmonitored experiment. That is the case regardless of which compound is under discussion.

The constructive reading of the first-generation story is that it worked as science. The GHRPs revealed a receptor, led to the discovery of ghrelin, and showed exactly which off-target effects a better molecule needed to avoid. The useful step for anyone reading old material today is to take that lesson and bring the actual question, whether the GH axis is relevant to you at all, to a physician who can test it.

Frequently asked questions

GHRP-6 vs ipamorelin: what is the difference? Both act on the ghrelin receptor (GHS-R1a) to release growth hormone. GHRP-6 is a first-generation compound with low selectivity: it produces a strong appetite effect, because it mimics the hunger hormone ghrelin, and FDA notes potential effects on cortisol and blood glucose. Ipamorelin was designed later specifically to release GH without meaningful cortisol, ACTH or prolactin elevation, as shown in its foundational 1998 pharmacology study. Neither has large outcome trials for body composition or ageing, both are WADA-prohibited, and neither is FDA-approved. The difference is selectivity, not proven benefit.

What is hexarelin? Hexarelin (examorelin) is a synthetic six-amino-acid peptide from the first generation of GH secretagogues, close in structure to GHRP-6 and among the most potent of that group in acute testing. Human studies show it also raises prolactin, ACTH and cortisol, and that its GH response attenuates over weeks of sustained use, partially and reversibly. It also binds the CD36 receptor on cardiovascular cells, which has been explored experimentally but not developed into a therapy. It is not an approved medicine.

Why is GHRP-2 not used anymore? Mainly because more selective alternatives replaced it. GHRP-2 co-stimulates prolactin, ACTH and cortisol in human studies, and FDA has placed it in Category 2 (significant safety risks) under its 503B compounding policy since September 2023, citing immunogenicity concerns and adverse-event reports in which causality was not established. It retains a narrow diagnostic role in Japan as pralmorelin. Large trials of sustained use for the outcomes people seek were never run, so any such benefit is unestablished rather than disproven.

Do GHRPs raise cortisol and prolactin? GHRP-2 and hexarelin do, in controlled human studies: the ACTH and cortisol response was comparable to that produced by corticotropin-releasing hormone, and the prolactin response was smaller but present. GHRP-6 is less consistently documented on prolactin, but FDA lists a potential cortisol effect among its concerns. Ipamorelin was developed to avoid these effects. This is one of the main reasons the first-generation compounds fell out of clinical interest.

Related reading

• The GH axis - how GHRH analogues and ghrelin-receptor agonists act on different levers of the same system
• Ipamorelin and why selectivity matters - the second-generation secretagogue in detail
• Why CJC-1295 and ipamorelin are paired - the two-receptor logic, without a protocol
• MK-677 - the oral non-peptide agonist and its safety record
• HGH vs GH peptides - supplying the hormone versus prompting the gland
• Part of The Complete Guide to Growth-Hormone Peptides

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Start with whether the GH axis is relevant to you at all. First-generation secretagogues are not something Vivre Labs supplies, and nothing on this page is a reason to seek them out. The useful first step is a baseline biomarker panel including IGF-1 and glucose markers, reviewed with your own doctor alongside your history, to establish whether there is anything to address and, if so, what a selective approach would look like. Join the founding cohort to book a consult.

GH-axis compounds and stacks on Vivre Labs: Sermorelin · Tesamorelin · CJC-1295 (no DAC) · Ipamorelin · VG-1 lean mass and sleep quality protocol (CJC-1295 + ipamorelin). Each page sets out the compound's evidence and regulatory status. Tesamorelin is FDA-approved for one narrow indication; sermorelin's previously approved US product was discontinued in 2008 for reasons other than safety; CJC-1295 and ipamorelin are investigational.

Educational content, not medical advice. GHRP-2, GHRP-6 and hexarelin are not approved as therapies in the United States; ipamorelin is investigational and not FDA-approved; "reviewed" or "studied" is not "approved". Regulatory statements reflect sources checked on 7 October 2026 and can change; confirm against FDA's current list and your local regulator before relying on them. All compounds named here that act on the ghrelin receptor are prohibited in sport under the WADA list. Raising GH and IGF-1 is a cancer-history screening matter, and blood glucose warrants monitoring across the class. No doses, schedules, protocols or sourcing information appear anywhere on this page, and nothing here is a basis for starting, stopping or changing any therapy. Compound pages are linked for reference. Position stated as of 7 October 2026.

References

• 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. (Origin of GHRP-6 and the first-generation series.)
• Howard AD, Feighner SD, Cully DF, et al. A receptor in pituitary and hypothalamus that functions in growth hormone release. Science. 1996;273(5277):974-977. (Cloning of the GH secretagogue receptor, GHS-R1a.)
• Kojima M, Hosoda H, Date Y, et al. Ghrelin is a growth-hormone-releasing acylated peptide from stomach. Nature. 1999;402:656-660. (Identification of the endogenous ligand.)
• Arvat E, Di Vito L, Maccagno B, et al. Effects of GHRP-2 and hexarelin, two synthetic GH-releasing peptides, on GH, prolactin, ACTH and cortisol levels in man. Comparison with the effects of GHRH, TRH and hCRH. Peptides. 1997;18(6):885-891. (GH release exceeding GHRH; prolactin, ACTH and cortisol co-stimulation, the latter comparable to hCRH.)
• Raun K, Hansen BS, Johansen NL, et al. Ipamorelin, the first selective growth hormone secretagogue. European Journal of Endocrinology. 1998;139(5):552-561. (GH release without meaningful ACTH or cortisol elevation.)
• Rahim A, O'Neill PA, Shalet SM. Growth hormone status during long-term hexarelin therapy. Journal of Clinical Endocrinology & Metabolism. 1998;83(5):1644-1649. (Partial, reversible attenuation of GH response over sixteen weeks in older adults.)
• Bodart V, Febbraio M, Demers A, et al. CD36 mediates the cardiovascular action of growth hormone-releasing peptides in the heart. Circulation Research. 2002;90(8):844-849. (Hexarelin binding to CD36; experimental cardiovascular effects.)
• Wakabayashi H, Arai H, Inui A. The regulatory approval of anamorelin for treatment of cachexia in patients with non-small cell lung cancer, gastric cancer, pancreatic cancer, and colorectal cancer in Japan: facts and numbers. Journal of Cachexia, Sarcopenia and Muscle. 2021;12(1):14-16. · Ono Pharmaceutical. Approval of Adlumiz (anamorelin) for cancer cachexia in Japan. Press release, 22 January 2021.
• US Food and Drug Administration. Certain Bulk Drug Substances for Use in Compounding that May Present Significant Safety Risks (Category 2 of the bulk substances nominated under sections 503A or 503B). Content current as of 22 April 2026; accessed 7 October 2026. (GHRP-2 for injectable and nasal routes, and GHRP-6, listed under 503B, added 29 September 2023; ipamorelin acetate listed under 503B, added 29 September 2023; hexarelin not listed.) fda.gov
• Secondary reporting on FDA's April 2026 removal of a group of peptides from Category 2, describing GHRP-2 and GHRP-6 as remaining restricted (e.g. Pharmacy Times; Latham & Watkins client insight, 2026). Used for context only; the FDA page above is the primary source.
• US Food and Drug Administration. Approval of Macrilen (macimorelin) oral solution for the diagnosis of adult growth hormone deficiency, 20 December 2017.
• Pralmorelin (GHRP-2): diagnostic agent for GH deficiency marketed in Japan by Kaken Pharmaceutical.
• World Anti-Doping Agency. Prohibited List, section S2 (peptide hormones, growth factors, related substances and mimetics): growth hormone secretagogues and GHRPs, including GHRP-2 (pralmorelin), GHRP-6, hexarelin (examorelin), ipamorelin, macimorelin and anamorelin.