Ipamorelin: The Selective GH Secretagogue

The short version

Ipamorelin is a small synthetic peptide — five amino acids — with the sequence Aib-His-D-2-Nal-D-Phe-Lys-NH2. It activates the ghrelin receptor (GHS-R1a) on pituitary somatotrophs to trigger a pulse of growth hormone release. What earned it the label "the first selective growth hormone secretagogue" is what it does not do: unlike earlier GH-releasing peptides (GHRP-6, GHRP-2), ipamorelin does not meaningfully raise ACTH, cortisol, or prolactin, even at doses more than 200 times its GH threshold ^[6].

The honest assessment: ipamorelin has never been approved for human use by any regulator. Its only published Phase 2 RCT — 114 adults after bowel resection — missed its primary endpoint ^[14]. Human pharmacokinetic data exist from a single early study of intravenous infusions in eight volunteers per dose level ^[15]. The rest of the evidence is animal and cell work. It is sold as a research chemical and is prohibited in sport under WADA Section S2. This page reports the literature; it gives no dose and no medical advice.

What it is

Ipamorelin is a synthetic pentapeptide: alpha-aminoisobutyric acid (Aib) at position 1, histidine, D-2-naphthylalanine (D-2-Nal) at position 3, D-phenylalanine (D-Phe) at position 4, and lysine-NH2 at the C-terminus (amidated). The two D-configured residues at positions 3 and 4 confer protease resistance, increasing metabolic stability relative to endogenous peptides. Also catalogued as NNC 26-0161 and NNC-26-0161 (Novo Nordisk development codes).

Ipamorelin was derived from GHRP-1 by removing the central Ala-Trp dipeptide. It is a selective agonist of the ghrelin/GHS-R1a receptor — a GPCR found on pituitary somatotrophs, hypothalamic neurons, and peripheral tissues including enteric neurons and pancreatic islets. It is not related structurally to CJC-1295 and works through a completely different receptor and intracellular pathway.

How it works

Ipamorelin binds GHS-R1a (the ghrelin receptor), a Gq-coupled GPCR. Receptor activation raises intracellular calcium in pituitary somatotrophs, triggering GH exocytosis — a mechanism that is mechanistically distinct from and complementary to the GHRH-receptor/cAMP pathway that CJC-1295 uses. Because the two pathways converge on GH release through independent intracellular switches, co-stimulating both produces a GH pulse larger than either agent alone ^[5][7].

Ipamorelin's defining pharmacological feature is its selectivity. In the original characterization in anesthetized rats and conscious swine, it did not raise ACTH or cortisol above GHRH-stimulated levels even at doses more than 200 times its GH ED50, while matching GHRP-6's GH efficacy ^[6]. This stands in contrast to GHRP-6 and GHRP-2, which activate the hypothalamic-pituitary-adrenal axis at high doses.

Beyond its pituitary action, GHS-R1a is expressed on enteric and vagal neurons (relevant to its trial in postoperative ileus) and on pancreatic islet cells. Ipamorelin also showed a GH-independent insulinotropic effect on pancreatic beta cells ex vivo and an adipogenic effect in GH-intact mice, indicating that its ghrelin-receptor activity extends beyond simple pituitary GH release ^[2].

Human pharmacokinetics from a single IV study: dose-proportional kinetics with a terminal half-life of approximately 2 hours, clearance 0.078 L/h/kg, steady-state volume of distribution 0.22 L/kg, and a single GH pulse peaking about 40 minutes post-dose ^[15].

What the research shows

Founding selectivity characterization. Ipamorelin at doses exceeding 200 times its GH ED50 did not elevate ACTH or cortisol above GHRH-stimulated levels in rats and swine, while producing GH release comparable to GHRP-6 — the definitive pharmacological profile establishing it as the first selective GH secretagogue ^[6].

Human pharmacokinetics. IV infusions of 4.21–140.45 nmol/kg in healthy male volunteers (n=8 per dose) showed linear, dose-proportional kinetics; terminal half-life approximately 2 hours; single discrete GH pulse peaking around 40 minutes after dosing ^[15].

Bone growth in rats. Subcutaneous ipamorelin at 18, 90, and 450 micrograms per day (divided three times daily for 15 days) dose-dependently increased longitudinal bone growth rate in adult female rats from 42 to 44, 50, and 52 micrometers per day, respectively, with no change in total IGF-1 or bone-turnover markers — suggesting a partly local or GH-pulse-driven skeletal effect ^[16].

Phase 2 bowel-resection trial (missed endpoint). The only published Phase 2 RCT of ipamorelin enrolled 114 adults undergoing open or laparoscopic bowel resection, given 0.03 mg/kg IV twice daily for up to 7 days. Median time to first tolerated meal was 25.3 hours with ipamorelin versus 32.6 hours with placebo — not statistically significant (p=0.15). Treatment-emergent adverse events occurred in 87.5% of the ipamorelin arm versus 94.8% of placebo; no ipamorelin-specific safety signal was observed, but the primary efficacy endpoint was not met ^[14].

Chemotherapy-related weight loss. The most recent published ipamorelin study (2024) found that intraperitoneal ipamorelin (1–3 mg/kg) in ferrets inhibited cisplatin-induced body-weight loss by approximately 24% on the last day of the delayed phase, with no anti-emetic effect — a peripheral mechanism distinct from the central action of the related compound anamorelin ^[12].

Class-level cardiovascular signal. A 28-day preclinical safety study of a structurally distinct GHS-R1a agonist (GSK894281) found dose-dependent myocardial degeneration and necrosis in rats, detectable by histopathology, electron microscopy, and elevated cardiac FABP3 — a class-level cardiovascular signal that makes chronic systemic GHS-R1a agonism a concern in subjects with underlying cardiac vulnerability. Ipamorelin itself was not the compound tested in that study ^[13].

Reported effects, cautions & safety

Community-reported effects (anecdotal, not clinical evidence). Research-use communities most consistently describe: deeper, more restorative sleep and faster physical recovery with reduced post-training soreness (both frequently reported); vivid dreams especially in early weeks; gradual lean body composition improvement; facial flushing and head-rush shortly after injection (all frequently reported). Adverse effects include tingling or numbness in hands and feet, mild water retention and puffiness, increased hunger hours after injection, early fatigue or a spacey feeling, injection-site irritation, and diminishing response over months of continuous use (all occasionally reported). These are anecdotal, unverified, dose and source unknown — not clinical findings.

Cited safety cautions. The following arise directly from the literature:

• Active or recent malignancy. GH drives hepatic IGF-1 production; IGF-1 is a well-characterized mitogen. Ipamorelin potently releases GH ^[6], and the theoretical concern — IGF-1-mediated promotion of proliferative activity — is mechanistic reasoning, not an observed oncologic event in any ipamorelin study.
• Diabetes or impaired glucose tolerance. GH is a counter-regulatory hormone reducing insulin sensitivity; ipamorelin additionally has a GH-independent insulinotropic action on pancreatic beta cells ^[2]. The dual metabolic influence creates unpredictable net glycemic impact in subjects with pre-existing insulin dysregulation.
• Active cardiovascular disease or heart failure. A 28-day preclinical study of a related GHS-R1a agonist found dose-dependent myocardial degeneration in rats ^[13]. Ipamorelin was not the compound tested; this is a class-level signal, not an observed ipamorelin cardiovascular event.
• Appetite dysregulation. GHS-R1a agonism activates hypothalamic appetite centers; ipamorelin also showed GH-independent adipogenic and leptin-elevating effects in mice ^[2]. Subjects for whom increased appetite or fat deposition would be harmful should be aware of this class-level orexigenic and adipogenic signal.
• Unknown long-term safety. The only controlled human dataset is a 7-day perioperative IV study in 114 patients ^[14] and an acute PK study in 8 volunteers ^[15]. No Phase 3 trial exists; no long-term human safety database. Subcutaneous self-administration — the dominant off-label route — has no published human PK characterization. Research-grade purity from unregulated suppliers is unverified.
• Selective cortisol sparing (relative advantage). Unlike GHRP-6 and GHRP-2, ipamorelin does not meaningfully raise ACTH or cortisol even at very high doses ^[6]. This removes the adrenocortical concern that attaches to less selective GHRPs, though it does not eliminate other risks.

Where it fits in GH-axis research

Ipamorelin occupies a precise niche on this desk: the selective ghrelin-receptor component that gives the CJC-1295 / Ipamorelin combination its second receptor pathway — and which, studied alone, illustrates exactly what that half of the combination contributes and what the evidence limits are. Its story is in some ways the starkest on the desk: highly selective pharmacology established in animal work ^[6], a single brief human pharmacokinetic characterization ^[15], and a Phase 2 trial in humans that missed its endpoint ^[14]. Marketing for ipamorelin has traveled well beyond that evidence base. Read alongside CJC-1295 for the complementary GHRH-analogue half, and see the comparison page for how the three relate.