Growth hormone secretagogues (GHS) are a class of compounds that stimulate endogenous growth hormone (GH) release from the anterior pituitary. Unlike exogenous recombinant GH, secretagogues preserve the pulsatile, physiological pattern of GH secretion by amplifying the body's own signalling. Three of the most studied compounds — GHRP-2, Ipamorelin, and Sermorelin — differ in their receptor targets, selectivity, and pulse characteristics.
The GH Axis: A Brief Primer
GH secretion is regulated by two primary hypothalamic hormones: growth hormone-releasing hormone (GHRH), which stimulates release, and somatostatin, which inhibits it. Pulsatile GH secretion follows a circadian pattern, with peaks typically occurring during deep sleep. Downstream, GH stimulates IGF-1 production in the liver, which mediates most of GH's anabolic, lipolytic, and recovery-promoting effects.
Sermorelin
Sermorelin is a synthetic analogue of GHRH comprising the first 29 amino acids of the native 44-amino-acid peptide. It binds directly to the GHRH receptor (GHRHR) on pituitary somatotrophs, stimulating GH release. Because it acts through the natural GHRH pathway, sermorelin's effects are subject to normal negative feedback regulation — somatostatin can still inhibit the response, maintaining physiological GH pulsatility.
Sermorelin has the most established human safety data of the three compounds, having been studied in children with GH deficiency and adults with age-related GH decline. Its FDA approval history (it was approved, then voluntarily withdrawn for commercial reasons) gives it a relatively well-characterised pharmacological profile.
GHRP-2
GHRP-2 (growth hormone-releasing peptide 2) is a synthetic hexapeptide that activates the ghrelin receptor (GHS-R1a). This is a distinct receptor from the GHRHR targeted by sermorelin. Ghrelin receptor activation stimulates GH release through a different intracellular pathway and also inhibits somatostatin, meaning GHRP-2 effectively works by both stimulating release and reducing inhibition simultaneously.
GHRP-2 produces a pronounced GH pulse that is larger in amplitude than GHRH alone but shorter in duration. It also stimulates prolactin and cortisol — not significantly at low doses but meaningfully at higher doses or with chronic use. This selectivity profile is important for research protocol design.
Ipamorelin
Ipamorelin is a third-generation GHRP — also a ghrelin receptor agonist — engineered for improved selectivity. Compared to GHRP-2, ipamorelin produces GH pulses of similar or slightly lower amplitude with minimal concurrent stimulation of cortisol, prolactin, or ACTH. This clean selectivity profile has made ipamorelin one of the most studied secretagogues for research into sustained GH axis stimulation without off-target hormonal effects.
Preclinical studies have demonstrated ipamorelin's ability to increase bone mineral density and lean mass in aged rat models. Its selectivity makes it well-suited for longer-duration studies where cortisol and prolactin confounding are undesirable.
Combination Protocols in Research
A well-documented synergy exists between GHRH-pathway agonists (sermorelin) and ghrelin receptor agonists (ipamorelin, GHRP-2). Administered together, the two pathway types produce a GH pulse substantially larger than either compound alone — often described as a 3–5x amplification compared to additive effects. This synergy is the basis for the common sermorelin + ipamorelin combination approach in research protocols.
Comparison Summary
- Sermorelin: GHRH receptor; physiological pulsatility maintained; most human data; mild GH pulse
- GHRP-2: Ghrelin receptor; larger GH pulse; cortisol and prolactin co-stimulation at higher doses
- Ipamorelin: Ghrelin receptor; selective GH release only; minimal off-target hormonal effects; preferred for longer studies
- Combinations: GHRH + GHRP/ghrelin agonist produces synergistic GH release
For research and educational purposes only. These compounds are not approved for use outside of licensed clinical trials in most jurisdictions. Protocol design should involve qualified researchers.