Ipamorelin: a swine GH ED50 of 2.3 nmol/kg, a ~2-hour human half-life, and a single GH pulse peaking near 40 minutes.

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Ipamorelin is a small lab-made peptide (a short chain of five amino acids) that nudges the pituitary gland to release a pulse of growth hormone (GH) — the hormone the body uses to build and repair tissue. What makes it stand out is selectivity: it raises GH without much of the cortisol (a stress hormone) or prolactin spillover that older peptides cause ^[1]. People are mostly interested in it for body composition — leaner mass, less fat — and for sleep. The honest state of the evidence: the animal data is real but the human data is thin and, where it has been tested, disappointing. The one published human trial measured no clear benefit ^[3]. It has never been approved as a drug. Despite the "rx" in this domain name, ipamorelin is not a prescription medicine. What people report — including the downsides — is on the effects page.

What the body-composition literature actually measured

The body-composition case for ipamorelin starts with a counterintuitive finding: a chunk of the effect is not about growth hormone at all. Twice-daily subcutaneous ipamorelin for two weeks raised body weight by roughly 15% in both GH-deficient and GH-intact mice, with fat-pad weight and serum leptin (the hormone fat tissue uses to signal fullness) elevated in both groups ^[6]. In other words, part of what ipamorelin does to fat and appetite runs through the ghrelin receptor directly, sidestepping the GH axis entirely.

The weight signal shows up again in the most recent in-vivo study. In a 2024 ferret model, intraperitoneal ipamorelin (1–3 mg/kg) cut cisplatin-induced body-weight loss by about 24% on the last day of the delayed phase — a peripheral anti-wasting effect, with no anti-nausea benefit ^[5].

Growth hormone is the louder mechanism. A single intravenous infusion in healthy men produced one discrete GH pulse peaking near 40 minutes, with a terminal half-life around 2 hours ^[2]. But that GH pulse does not always translate into elevated IGF-1: in a 15-day rat bone-growth study, longitudinal growth rate climbed dose-dependently from 42 to 52 microm/day with no measurable change in total IGF-1 ^[4]. The body-composition story here is more nuanced than "more GH, more IGF-1, leaner body" — and this digest keeps that nuance in view. The full breakdown of reported leanness, fat, and appetite signals is on the Ipamorelin benefits page.

Selectivity: the one trait that defines the molecule

Ipamorelin's defining feature is what it does not do. Its founding 1998 characterization (sequence Aib-His-D-2-Nal-D-Phe-Lys-NH2) released GH potently in rat pituitary cells, anaesthetised rats, and conscious swine — a swine ED50 (the dose producing half the maximal effect) of 2.3 nmol/kg, versus 3.9 nmol/kg for the older peptide GHRP-6 — yet it did not raise ACTH or cortisol above the level seen with GHRH, even at doses more than 200-fold above its GH ED50 ^[1].

That selectivity is the reason ipamorelin gets singled out from the broader growth-hormone-releasing-peptide (GHRP) family. Older GHRPs reliably bumped cortisol and prolactin; ipamorelin, in its characterization, did not. Whether that cleaner profile produces better real-world body-composition outcomes in humans has never been tested in a controlled trial — and this digest does not pretend otherwise.

The human data is thin — and where it exists, it underwhelmed

Two human datasets carry the entire clinical record. The first is the 1999 pharmacokinetic study (n=8 per dose) that established the ~2-hour half-life and the single 40-minute GH pulse ^[2]. The second is the only published Phase 2 randomized controlled trial: 114 adults undergoing bowel resection received 0.03 mg/kg intravenously twice daily for up to 7 days, and the trial missed its primary endpoint — median time to first tolerated meal was 25.3 hours with ipamorelin versus 32.6 hours with placebo (p=0.15) ^[3].

No Phase 3 trial has been run. There is no approved indication, and no long-term human body-composition outcome study. The detailed reading of the studies sits on the Ipamorelin research page; the reported effects and the cited safety reasoning are on the Ipamorelin effects page; the full citation list is on the Ipamorelin references page.