Ipamorelin is a five-residue growth hormone secretagogue, and the molecule that retired an assumption the field had carried for a decade: that you could not stimulate growth hormone release through the ghrelin receptor without also driving cortisol. Raun and colleagues at Novo Nordisk characterized it in 1998 as "the first selective growth hormone secretagogue." This profile covers what the molecule is, where the roughly two-hour half-life figure comes from (and why the popular "30 minutes" is wrong), why it is almost always paired with a GHRH analog, and the handling factors that matter in a research setting. It contains no dosing information.
Structural identity
- Sequence:
Aib-His-D-2-Nal-D-Phe-Lys-NH₂(a C-terminally amidated pentapeptide) - Molecular formula: C₃₈H₄₉N₉O₅ (free base)
- Average mass: ≈711.9 g/mol
- CAS: 170851-70-4
- Origin: Engineered at Novo Nordisk from the GHRP-1 scaffold and first described by Raun et al. in the European Journal of Endocrinology (1998). The peptide is fully synthetic.
Three of the five positions are not ordinary L-amino acids. Position 1 is α-aminoisobutyric acid (Aib), a methyl-doubled, non-proteinogenic residue. Positions 3 and 4 are D-amino acids: D-2-naphthylalanine and D-phenylalanine. Those three substitutions are the entire reason ipamorelin behaves the way it does, both in protease resistance and in receptor selectivity. The sequence carries no cysteine, no methionine, and no tryptophan, so disulfide scrambling, methionine-sulfoxide oxidation, and tryptophan photolysis are all off the table.
What makes ipamorelin "selective"?
It releases growth hormone without meaningfully raising cortisol or ACTH. The older growth-hormone-releasing peptides GHRP-6, GHRP-2, and hexarelin all stimulate GH but also provoke the stress-hormone axis. Raun's group found ipamorelin released GH at a potency comparable to GHRP-6 while leaving ACTH and cortisol near GHRH-level baselines.
The structural source of that selectivity is the D-2-naphthylalanine and D-phenylalanine pair at positions 3 and 4. Ipamorelin descends from GHRP-1, and the engineering goal at Novo Nordisk was explicitly to keep the GH-releasing activity while dropping the corticotropic spillover. Among the common secretagogues, the cortisol-provocation order runs hexarelin > GHRP-2 > GHRP-6 ≫ ipamorelin. For a researcher who wants to isolate the GH axis without confounding the hypothalamic-pituitary-adrenal axis, that ordering is the whole argument for choosing ipamorelin as the probe.
Receptor target and the ghrelin connection
Ipamorelin is an agonist at the growth hormone secretagogue receptor type 1a (GHSR-1a), the same receptor the stomach hormone ghrelin activates. GHSR-1a is a class-A G-protein-coupled receptor that signals through Gq and phospholipase C. Activating it on pituitary somatotrophs does two things at once: it directly triggers GH release, and it suppresses somatostatin, the endogenous brake on GH secretion. Ipamorelin is a synthetic ghrelin mimetic in the GH-axis sense, without the appetite and gut-motility breadth of ghrelin itself.
Half-life: the number everyone gets wrong
Vendor pages routinely list ipamorelin's half-life as "about 30 minutes." That figure belongs to GHRP-2 and GHRP-6, not ipamorelin. The authoritative number comes from a phase 1 pharmacokinetic study in healthy volunteers (Gobburu et al., Pharmaceutical Research, 1999), which reported a terminal elimination half-life of roughly two hours, dose-proportional kinetics, and a GH peak around 40 minutes after administration.
The distinction that resolves the confusion is this. The roughly two-hour figure is the terminal elimination half-life of the peptide in plasma, while the biological action is a single GH pulse that rises and falls over about an hour. The pulse, not the elimination tail, is what drives the pulsatile administration pattern seen across the research literature.
| Secretagogue | Receptor | Approx. plasma half-life | ACTH / cortisol provocation |
|---|---|---|---|
| GHRP-6 | GHSR-1a | ~15–30 min | Moderate |
| GHRP-2 | GHSR-1a | ~30 min | Moderate–high |
| Hexarelin | GHSR-1a | ~30–70 min | Highest of the group |
| Ipamorelin | GHSR-1a | ~2 h (terminal) | Minimal (near GHRH baseline) |
Why researchers pair it with a GHRH analog
Ipamorelin is rarely studied alone. In published research designs it is co-administered with a growth-hormone-releasing hormone (GHRH) analog, most often the modified GRF(1-29) sequence sold as CJC-1295 without DAC. The rationale is mechanistic rather than additive: GHRH and the ghrelin receptor drive GH release through two independent pituitary pathways, so stimulating both at once produces a supra-additive (synergistic) response that neither generates on its own. Our CJC-1295 with DAC vs without DAC article covers why the short-acting, pulsatile GHRH partner is the one that matches ipamorelin's kinetics.
Published research scope
The ipamorelin literature is predominantly preclinical. Common experimental contexts in the published record include:
- Pituitary and somatotroph pharmacology. The original Novo Nordisk work characterized GH release in rat pituitary cells and in vivo in swine, establishing the selectivity profile against GHRP-6 and GHRP-2.
- Gastrointestinal motility. As a ghrelin-receptor agonist, ipamorelin has been studied in rodent models of postoperative ileus and gastric emptying, work independent of the GH axis.
- Bone and connective-tissue endpoints. A subset of rodent studies examined bone mineral content and body-composition readouts downstream of GH-axis stimulation.
Controlled human data are limited to early pharmacokinetic and tolerability work; the compound did not advance to a marketed product. Researchers should treat it as a preclinical GH-secretagogue probe and design accordingly. As with every compound in this library, the above is summarized observationally from the peer-reviewed record and is not treatment guidance.
More is not better: the receptor pushes back
The counterintuitive part of working with any ghrelin-receptor agonist is that the constraint is the receptor, not the dose. GHSR-1a is a GPCR, and under sustained agonism it runs the classic desensitization cascade: GRK phosphorylation, β-arrestin recruitment, internalization, and downregulation. Continuous or stacked high exposure erodes the very GH response it is meant to drive. Short, spaced pulses let the receptor dephosphorylate and recycle back to the surface.
This is the deeper reason a short-acting GHRH partner outperforms a long-acting one, and it reframes what ipamorelin is good for. Its value is selectivity, not raw potency. GHRP-2 and hexarelin are more potent GH releasers on paper; their cortisol and, in clinical reports, prolactin baggage is what makes them worse tools when the experimental goal is a clean GH signal.
Stability and handling
Ipamorelin is one of the better-behaved peptides on the bench, for a structural reason. With no cysteine, methionine, or tryptophan, it has no oxidation or photolysis watchpoint, and the Aib residue plus the two D-amino acids give it real resistance to peptidases. It is typically supplied as the acetate salt, the conventional counter-ion for a reference-grade secretagogue, though some research lots ship as the trifluoroacetate salt depending on the final purification.
- Lyophilized, sealed: stable for roughly 18–24 months refrigerated and longer frozen at −20°C, consistent with the standard peptide storage envelope.
- Reconstituted in bacteriostatic water: 2–8°C, protected from light, typically usable for several weeks. Aliquot and freeze for longer holds rather than freeze-thawing one vial repeatedly.
The general workflow applies without exception: stream the diluent down the glass wall rather than onto the cake, swirl rather than shake, and equilibrate a cold vial to room temperature before adding diluent to avoid condensation. The mechanics are in our reconstitution guide, and the degradation-pathway framework is in the storage and stability article.
Common analytical and handling notes
On reverse-phase HPLC ipamorelin elutes as a single sharp peak. The two D-residues and the naphthyl side chain make it more hydrophobic than a same-length all-L peptide, so it retains slightly longer under a standard water/acetonitrile gradient. On ESI-MS the dominant ion is the singly or doubly protonated species against a theoretical mass near 711.9 g/mol. The visual cues for reading a lot's chromatogram and mass spectrum are in our CoA guide, and every lot we release publishes its report in the open CoA library.
Common errors to avoid:
- Quoting the 30-minute half-life. That is a GHRP-2/GHRP-6 figure. Ipamorelin's terminal half-life is roughly two hours.
- Substituting CJC-1295 with DAC for the no-DAC form. The continuous-exposure profile breaks the pulsatile synergy ipamorelin is chosen for.
- Assuming more is better. Sustained high agonism desensitizes GHSR-1a; the receptor, not the dose, sets the ceiling.
- Shaking the vial. Like every peptide, ipamorelin aggregates at the air-water interface. Swirl only.
- Confusing it with GHRP-2. They share a receptor, not a side-effect profile. The cortisol-sparing behavior is the entire point of ipamorelin.
Where to find primary literature
PubMed indexes the ipamorelin record under "ipamorelin" and the original development designation "NNC 26-0161". The foundational characterization is Raun et al., European Journal of Endocrinology, 1998; the human pharmacokinetic parameters are in Gobburu et al., Pharmaceutical Research, 1999. For the GHRH side of the pairing and why partner kinetics matter, the CJC-1295 comparison and the tesamorelin profile are the companion references. HelixCore supplies ipamorelin pre-combined with modified GRF(1-29) in the CJC + Ipamorelin blend.
