Ipamorelin: A Precise GH Pulse With a Thin Human Record

The short version

Ipamorelin is a synthetic peptide five amino acids long. It was designed to activate one specific receptor — GHS-R1a, the ghrelin receptor on pituitary cells — and trigger a pulse of growth hormone release without the cortisol, prolactin, or ACTH spikes that plagued earlier growth-hormone-releasing peptides. On paper, that selectivity is its most interesting quality.

In practice, the human evidence is thin and sobering. The one published Phase 2 randomized controlled trial — in 114 adults after bowel resection surgery — missed its primary endpoint ^[3]. The pharmacokinetics are characterized in a small group of healthy male volunteers ^[4]. No Phase 3 trial exists, no approval exists anywhere, and in 2024 the FDA removed ipamorelin from its 503A compounding drug substances list. Ipamorelin is classified as a research chemical. It is also prohibited in sport at all times under WADA category S2.

This page summarizes what was actually studied. It lists no human dose and gives no medical advice.

What it is

Ipamorelin (also catalogued as NNC 26-0161) is a synthetic pentapeptide with the sequence Aib-His-D-2-Nal-D-Phe-Lys-NH2 (alpha-aminoisobutyric acid at position 1; D-2-naphthylalanine and D-phenylalanine confer protease resistance). It was derived from GHRP-1 by removing the central Ala-Trp dipeptide.

It is a selective agonist of the ghrelin / growth hormone secretagogue receptor, GHS-R1a. This is also the receptor that the hunger hormone ghrelin acts on — a mechanistic connection with appetite that is relevant to reported side effects. Ipamorelin is not the same compound as the CJC-1295 + Ipamorelin blend sold by many commercial suppliers; it is a distinct, single-compound research peptide. It has never been approved as a drug by the FDA, EMA, or any major regulatory authority.

How it works

Ipamorelin activates GHS-R1a on pituitary somatotrophs, triggering a discrete pulse of growth hormone release. Its defining pharmacological feature — established in rats and swine at doses more than 200-fold above its GH half-maximal effective concentration — is that it does not meaningfully raise ACTH, cortisol, or prolactin at doses that produce robust GH stimulation ^[1][5]. Earlier peptides in this class (GHRP-6, GHRP-2) produced those off-target effects; ipamorelin's selectivity removed them.

Growth hormone release driven by ipamorelin operates via a mechanism complementary to that of GHRH analogs like CJC-1295: GHS-R1a and the GHRH receptor use different intracellular signaling pathways (Gq/IP3 and Gs/cAMP respectively), so the two can potentiate each other — the rationale behind combined protocols studied in pharmacology research.

Secondary targets include GHS-R1a-expressing enteric and vagal neurons (gastric motility — the mechanistic basis for the postoperative ileus trial), pancreatic islet cells (direct insulinotropic action at pharmacological concentrations), and hypothalamic appetite circuitry via the ghrelin-receptor class-level orexigenic effect.

What the research shows

Pharmacokinetics in humans. Population PK/PD modeling in healthy male volunteers (n=8 per dose level, five 15-minute IV infusions of 4.21 to 140.45 nmol/kg) showed dose-proportional kinetics with a terminal half-life of approximately 2 hours, clearance of 0.078 L/h/kg, and steady-state volume of distribution of 0.22 L/kg. The GH response peaked at approximately 40 minutes after dosing as a single discrete pulse ^[4]. This is one of only two published human datasets on ipamorelin.

The only human efficacy trial. A prospective, randomized, placebo-controlled Phase 2 trial (NCT00672074, n=114 adults undergoing open or laparoscopic bowel resection) gave ipamorelin 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 — a difference that did not reach statistical significance (p=0.15). Treatment-emergent adverse events occurred in 87.5% of the ipamorelin arm versus 94.8% of placebo. Primary endpoint was not met. This is the definitive human efficacy anchor for ipamorelin, and it is a null result ^[3].

Preclinical: bone growth. Subcutaneous ipamorelin at 18, 90, and 450 micrograms/day (divided three times daily for 15 days) dose-dependently increased longitudinal bone growth rate in adult female Sprague-Dawley rats, without changing total IGF-1 or bone turnover markers — suggesting a partly local or GH-pulse-driven skeletal effect ^[5].

Preclinical: weight loss and emesis model. In a 2024 ferret study, ipamorelin (1-3 mg/kg intraperitoneally) inhibited cisplatin-induced body-weight loss by approximately 24% on the last day of the delayed phase, but had no anti-emetic effect. The related compound anamorelin reduced acute emesis by 60% via a central mechanism; ipamorelin appears to act peripherally for this indication ^[1].

Class-level cardiovascular caution. A 28-day integrated preclinical safety study of a different GHS-R1a agonist (GSK894281) found dose-dependent myocardial degeneration and necrosis in rats, detectable by histopathology and elevated heart-type fatty-acid-binding protein. Ipamorelin was not the tested compound, but this is a receptor-class-level signal that makes chronic GHS-R1a agonism in subjects with cardiac vulnerability a genuine concern ^[2].

Reported effects, cautions & safety

Research-use communities report a range of anecdotal effects from ipamorelin. These are anecdotal, not clinical evidence — effects reported by individuals using research-grade material of unverified purity, without medical supervision, at uncharacterized doses and by routes with no published human PK data.

Frequently reported anecdotal benefits: deeper and more restorative sleep (the most consistently cited effect; often appearing within the first one to two weeks of a pre-bedtime protocol), more vivid dreams in the early weeks, faster physical recovery and reduced post-training soreness.

Occasionally reported anecdotal benefits: gradual leaner body composition over weeks to months (subtle and slow; heavily confounded by concurrent diet and training).

Frequently reported anecdotal adverse effects: a transient facial flush or head-rush approximately 5-15 minutes post-injection, lasting up to an hour — widely described in community threads as similar to a niacin flush. Tingling or numbness in hands and feet. Mild water retention and puffiness, particularly in the first weeks.

Occasionally reported anecdotal adverse effects: increased hunger in hours after injection (mechanistically expected from GHS-R1a agonism), transient lightheadedness or a spacey feeling shortly after injecting, injection-site redness or itching, and diminishing response over months of continuous use.

Evidence-based cautions from the published literature:

• Active or recent malignancy: GH stimulates hepatic IGF-1 production, and IGF-1 is a mitogen. Chronically raising GH pulse amplitude carries a class-level theoretical concern for proliferative conditions — not a documented ipamorelin-specific oncologic event, but a mechanism-based caution.
• Diabetes and insulin dysregulation: ipamorelin has a GH-independent direct insulinotropic action on pancreatic islet cells (demonstrated ex vivo), while GH itself reduces peripheral insulin sensitivity. The combined effect in subjects with pre-existing insulin dysregulation is unpredictable.
• Cardiovascular disease: class-level GHS-R1a agonism raises fluid retention concerns via GH-axis sodium handling, plus the rat myocardial degeneration signal from a related compound ^[2].
• Long-term human safety data do not exist. The only human safety window is a 7-day perioperative IV trial and an acute IV PK study. Subcutaneous research use, which is the predominant route in the community, has no published human PK characterization ^[3][4].

Where it fits in the growth-hormone axis research

Among the GH axis compounds on this desk, ipamorelin is the pharmacologically cleaner secretagogue — but cleaner selectivity did not translate into efficacy in the only human indication tested. It occupies an instructive position: mechanistically elegant, pharmacokinetically well-characterized in an acute IV context, and with a null Phase 2 result that explains why it never reached approval ^[3].

Read alongside CJC-1295, which targets the GHRH receptor and drives a sustained multi-day GH elevation rather than a discrete pulse, ipamorelin illustrates the mechanistic complementarity of the two secretagogue classes. NAD+ and MOTS-c work at a cellular rather than hormonal level — a different layer of the same aging biology. Compare all four on the compare page.