Ipamorelin: Selective GH Secretagogue Research Profile

What Is Ipamorelin?

Ipamorelin is a synthetic pentapeptide with the sequence Aib-His-D-2-Nal-D-Phe-Lys-NH₂ — a short chain built partly from non-natural residues (α-aminoisobutyric acid and D-configured aromatic amino acids) and capped with a C-terminal amide. It was developed at Novo Nordisk and carries the development code NNC 26-0161.^[1][4]

Functionally, ipamorelin belongs to the family of growth-hormone secretagogues (GHS), also called growth-hormone-releasing peptides (GHRPs): small synthetic molecules that stimulate the pituitary to release growth hormone. It is not a growth hormone itself and it is not a GHRH analog — it works through a separate receptor described in the next section. The non-natural residues make the molecule more resistant to rapid peptidase breakdown than a fully L-amino-acid peptide of the same length.^[4]

For a research reader, the practical takeaway is that ipamorelin is best understood as a tool compound for probing the growth-hormone axis rather than a drug. The literature gives clear preclinical pharmacology to study; it does not give a finished, approved clinical product. This review deliberately omits any molecular-formula, dosing, or human-use figures that are not directly traceable to the cited primary literature.

Where Ipamorelin Sits in the Ghrelin/GHS Story

Growth-hormone secretagogues act at the GHS-R1a receptor, a G-protein-coupled receptor expressed in the pituitary and hypothalamus that was identified by Howard et al. in 1996 as the target through which synthetic GHRPs trigger growth-hormone release.^[7] That same receptor was later shown to be the endogenous receptor for ghrelin, the stomach-derived hunger hormone — which is why modern literature describes ipamorelin as a ghrelin-receptor agonist or "ghrelin mimetic."

Ipamorelin sits firmly in this GHS/ghrelin lane rather than the GHRH lane. Raun et al. showed that its growth-hormone release is blocked by a GHRP-receptor antagonist but not by a GHRH antagonist, which pharmacologically separates it from growth-hormone-releasing-hormone analogs such as CJC-1295 and tesamorelin.^[1] The two receptor systems are complementary — GHRH analogs and ghrelin-receptor agonists engage the somatotroph through different doors — which is the basic rationale behind the research blends discussed in the GHRH and GH-secretagogue comparison hub.

Mechanism: Selective GHS-R1a Agonism and the GH Pulse

Mechanistically, ipamorelin is an agonist at GHS-R1a. Binding activates the receptor on pituitary somatotrophs and evokes a discrete pulse of growth-hormone secretion rather than a sustained elevation. Raun et al. quantified the pharmacology across three systems:^[1]

• In vitro (rat pituitary cells): a half-maximal effective concentration (EC₅₀) of about 1.3 nmol/L, with an intrinsic efficacy (E_max) of roughly 85% relative to the GHRP-6 maximum.
• Anaesthetised rat: a half-maximal effective dose (ED₅₀) of about 80 nmol/kg.
• Conscious swine: an ED₅₀ of about 2.3 nmol/kg for growth-hormone release.

The defining pharmacological control experiment is the antagonist profile: the growth-hormone response to ipamorelin was abolished by a GHRP-receptor antagonist but was insensitive to a GHRH antagonist.^[1] That is the mechanistic evidence placing ipamorelin in the ghrelin/GHS pathway rather than making it a GHRH-pathway compound. What this gives a research reader is a well-characterized, single-receptor agonist story with measured potency parameters — not a vague "boosts growth hormone" assertion.

Selectivity: GH Release Without ACTH, Cortisol or Prolactin

Selectivity is the reason ipamorelin is discussed at all. Earlier growth-hormone-releasing peptides such as GHRP-2 and GHRP-6 release growth hormone but also drive the hypothalamic-pituitary-adrenal axis, raising ACTH and cortisol. Ipamorelin was the compound that appeared to break that coupling.

In conscious swine, Raun et al. reported that ipamorelin released growth hormone without raising ACTH or cortisol above the levels seen with an equivalent GHRH stimulus, even at doses more than 200-fold its growth-hormone ED₅₀. Over the same range it produced no change in prolactin, FSH, LH or TSH. By direct contrast, GHRP-2 and GHRP-6 tested in the same framework both raised ACTH and cortisol.^[1] The authors summarized the finding by describing ipamorelin as the first GHRP receptor agonist with a selectivity for GH release similar to GHRH.

It is worth stating the limit of that claim precisely. "Selective for GH release over ACTH/cortisol/prolactin in swine" is a specific, measured preclinical result. It is not the same as "side-effect-free," and it does not establish a safety profile in humans — a point the human-trial section below makes concrete.

Preclinical and Human Trials: What Was Actually Shown

1. Potency and the Growth-Hormone Pulse

Across rat and swine models, ipamorelin reproducibly triggered a dose-related growth-hormone pulse with the potency parameters given above, and with the selectivity profile that distinguishes it from other GHRPs.^[1] That is the core, well-replicated pharmacology.

2. Bone Growth in Rats — With No Change in IGF-I

Johansen et al. dosed adult female rats with ipamorelin and reported a dose-dependent increase in longitudinal bone-growth rate and in body-weight gain. Critically for how this compound is often mis-marketed, the same study found no change in total IGF-I, in IGF-binding proteins, or in markers of bone turnover.^[3] In other words, the classic "growth-hormone secretagogue therefore raises IGF-1" story is not supported by this dataset — it is contradicted by it. Any claim that ipamorelin is "anabolic" or "raises IGF-1" goes beyond, and against, the cited evidence.

3. Human Intravenous Pharmacokinetics

The single published human study is a pharmacokinetic-pharmacodynamic paper in healthy men given ipamorelin intravenously. It reported dose-proportional kinetics, a short terminal half-life of about two hours, a clearance near 0.078 L/h/kg, and a volume of distribution at steady state of about 0.22 L/kg, with a single growth-hormone pulse peaking at roughly 0.67 hours.^[2] A companion pharmacokinetic characterization of ipamorelin and related peptidyl secretagogues appears in the Johansen/Xenobiotica work.^[4] This is the entirety of the published human pharmacokinetic record — one intravenous study.

4. Gastrointestinal Motility

Because GHS-R1a is a ghrelin receptor and ghrelin is promotile, ipamorelin has been studied as a gut-motility agent. In a rodent model of post-operative ileus, ipamorelin accelerated gastrointestinal transit, consistent with a ghrelin-receptor promotility effect.^[5] That preclinical promotility signal is what motivated the one human efficacy trial.

5. The One Human Efficacy Trial Was Negative

The single registered human efficacy study is Beck et al. 2014 (the Ipamorelin 201 Study Group) — a prospective, randomized, placebo-controlled Phase-2 proof-of-concept trial of intravenous ipamorelin (0.03 mg/kg twice daily for up to seven days) for the management of post-operative ileus. The result was negative: ipamorelin was generally well tolerated, though adverse events including nausea and vomiting were recorded, and it showed no statistically significant benefit over placebo on the trial's endpoints. Clinical development did not advance beyond this study.^[6]

The honest summary across these models is consistent. Ipamorelin has a reproducible, selective growth-hormone-releasing pharmacology in animals, a single short human PK study, and one failed Phase-2 efficacy trial. It is a well-characterized research compound with an unproven clinical profile.

Ipamorelin vs CJC-1295 vs GHRP-2/6 — Where the Lanes Differ

Researchers frequently ask how ipamorelin positions next to the other growth-hormone peptides it is stacked or compared with. The three most common comparators engage the somatotroph through different mechanisms, and the honest framing keeps them in separate lanes. The table below summarizes that, without claiming clinical interchangeability. A fuller head-to-head sits in the CJC-1295 vs ipamorelin comparison and the CJC-1295 + ipamorelin blend guide.

The single sourced distinction that matters most in this table is the adrenal column: in the Raun framework, ipamorelin released growth hormone without the ACTH/cortisol rise that accompanied GHRP-2 and GHRP-6.^[1] CJC-1295 belongs to a different receptor family entirely — it is a long-acting GHRH analog, not a ghrelin-receptor agonist — which is why the two are often studied together rather than as substitutes. None of these molecules is an approved therapeutic, and none has a peptide-specific late-stage human-trial program of the kind seen with incretin agents.

What the Ipamorelin Literature Does Not Yet Give You

Cautious reading of ipamorelin requires acknowledging several real gaps:

• The human record is extremely sparse. It consists of one intravenous pharmacokinetic study in healthy volunteers and a single Phase-2 efficacy trial that was negative.^[2][6] There is no positive registered efficacy trial for any human indication.
• No IGF-1 or anabolic validation. The rat data specifically showed no change in total IGF-I, IGF-binding proteins, or bone-turnover markers, so "ipamorelin raises IGF-1," "ipamorelin is anabolic," or "ipamorelin is an anti-aging peptide" are not supported by the cited literature — they are contradicted by it.^[3]
• Not approved anywhere; dose translation is unsupported. Ipamorelin is not an approved drug in any market. The published animal and single-study human parameters do not map to a validated human protocol, and this article makes no dosing recommendation. Third-person context on how doses appear in the research literature is discussed separately in the ipamorelin dosage in research reference.
• Formulation, stability and long-term data are thin. Beyond the short intravenous PK picture, route-dependent bioavailability, long-term exposure, and formulation stability are not resolved in the public literature the way they are for established protein medicines.

These gaps are not arguments against studying ipamorelin. They are arguments against overselling it.

Where Ipamorelin Fits in a Modern RUO Research Catalog

For research use, ipamorelin belongs in the category of growth-hormone-secretagogue research tools. In the current catalog it is most commonly supplied as part of a research blend — the CJC-1295 + Ipamorelin research vial — reflecting the complementary GHRH-analog and ghrelin-receptor-agonist pairing described above. Material is typically supplied lyophilized with cold-chain handling and reconstituted with bacteriostatic water for in-vitro work, supplied to a >99% HPLC line standard for in-vitro research.

It is not framed for human use, not framed for veterinary use, and not framed as a treatment. The /products/ catalog documents available formats, each supplied to a >99% HPLC line standard; published Janoshik batch reports for the retatrutide range sit in the COA library. For reconstitution and handling context, see the bacteriostatic water guide and the peptide stability and storage guide.

For researchers reading this page as a starting point, the most useful adjacent references on this site are the GHRH and GH-secretagogue comparison hub, the CJC-1295 vs ipamorelin comparison, the CJC-1295 + ipamorelin blend guide, the tesamorelin GHRH-analog review, and the ipamorelin dosage in research reference.