Ipamorelin: a selective GH secretagogue that doesn't spike cortisol or prolactin.
Ipamorelin is a synthetic pentapeptide growth-hormone-releasing peptide (GHRP) developed in the late 1990s at Novo Nordisk. It is a selective agonist at the ghrelin receptor (GHS-R1a), and its distinguishing feature in the GHRP class is that it induces a GH pulse without the parallel cortisol and prolactin spikes that characterize GHRP-6 and hexarelin.
- Pentapeptide. Five residues, most of them non-canonical amino acids (Aib, D-2-Nal, D-Phe) for proteolytic stability.
- Selective at GHS-R1a — does not significantly activate other receptors involved in cortisol or prolactin release.
- Original Novo Nordisk development program (NNC 26-0161) advanced to Phase II for ICU-acquired weakness; did not meet endpoints, program discontinued.
- Used research-side in combination with GHRH analogs (e.g., sermorelin, CJC-1295) to exploit the synergistic GH pulse.
- Not FDA-approved. Sold on the research-chemical market under RUO framing.
- Plasma half-life ~2 hours SC; pulsed dosing (2–3x daily) is typical in the research literature.
Mechanism — the ghrelin system
The growth hormone secretagogue receptor (GHS-R1a) is a GPCR expressed on pituitary somatotrophs and in hypothalamic arcuate nucleus neurons. Its endogenous ligand is ghrelin, an acylated 28-residue peptide from the stomach. Binding triggers phospholipase C activation, IP3-mediated Ca²⁺ release, and downstream GH exocytosis.
Ipamorelin mimics this activation selectively. Importantly, ghrelin itself promotes appetite, and many GHRP-class peptides have non-trivial effects on cortisol (via direct hypothalamic-pituitary-adrenal axis input) and prolactin. Ipamorelin was specifically selected during its discovery to minimize these cross-activations while retaining GH-releasing potency.
Pharmacokinetics
| Parameter | Value | Notes |
|---|---|---|
| Route | Subcutaneous | IV PK exists in early-stage lit but not typical |
| Half-life | ~2 hr | Longer than GHRP-6 (~15–30 min) |
| Time to peak GH | ~60–90 min | After SC dose |
| Typical research dose | 100–300 mcg per pulse | From published PK work; not a recommended human dose |
| Tachyphylaxis | Partial, at continuous high exposure | Pulsed dosing preserves receptor sensitivity |
Clinical development: what happened
Novo Nordisk advanced ipamorelin (NNC 26-0161) into Phase II for postoperative ileus and ICU-acquired muscle weakness in the 2000s. The compound failed to meet primary endpoints in the ileus program and development was discontinued. Published trial data from this era (Beck et al., 2014; Gonzalez et al., 2013) provide the majority of the human PK information that's publicly available. Ipamorelin was thus a well-characterized clinical candidate that did not reach market.
The "synergy stack": GHRH + GHRP
A frequently discussed research pattern combines a GHRH analog (sermorelin, CJC-1295-no-DAC, or tesamorelin) with a GHRP (ipamorelin, GHRP-2, or GHRP-6). The rationale is mechanistic: GHRH drives somatotroph GH synthesis, while ghrelin-receptor agonism amplifies the somatotroph's response. Additively, these produce a larger GH pulse than either alone.
The "GHRH + ipamorelin" pairing is preferred in the research community specifically because ipamorelin adds GH release without the cortisol/prolactin noise. Important honest caveat: most evidence for the synergy is from small-N pharmacology work, not from outcome-driven RCTs. Whether the larger GH pulse translates to better physiologic outcomes is not settled.
Safety signals
- Cortisol / prolactin. The distinguishing positive feature — no meaningful rises, unlike GHRP-6 and hexarelin.
- Glucose / HbA1c. GH-axis activation produces modest insulin-resistance effects. Generally small at research-scale doses; worth monitoring.
- Appetite. Less ghrelin-like appetite stimulation than GHRP-6, but not zero.
- Injection-site reactions. Transient erythema/itching in a minority of subjects.
- Tachyphylaxis. GHS-R1a downregulates under continuous stimulation. Pulsed dosing is both mechanistically and pharmacodynamically appropriate.
Regulatory status
Not FDA-approved. Not a compounding-pharmacy-available substance in most US states following the 2023 FDA actions on certain peptides. Sold under research-use-only framing by research-chemical vendors. WADA prohibits growth-hormone secretagogues in sport (class S2.5).
Reconstitution math (5 mg vial)
| BAC water added | Concentration | Per IU (U-100) | For 200 mcg dose |
|---|---|---|---|
| 2 mL | 2,500 mcg/mL | 25 mcg | 8 IU |
| 2.5 mL | 2,000 mcg/mL | 20 mcg | 10 IU |
| 5 mL | 1,000 mcg/mL | 10 mcg | 20 IU |
A 2.5 mL reconstitution gives the cleanest math: 10 IU on a U-100 insulin syringe = 200 mcg. For research literature doses of 100–300 mcg per pulse, this is usable calibration.
Where to read further
- Raun K, et al. "Ipamorelin, the first selective growth hormone secretagogue." Eur. J. Endocrinol. 1998;139:552–561.
- Gobburu JV, et al. "Pharmacokinetic-pharmacodynamic modeling of ipamorelin, a growth hormone releasing peptide." J. Pharmacol. Exp. Ther. 1999;286(1).
- Beck DE, et al. "Safety and efficacy of ipamorelin in postoperative ileus." J. Am. Coll. Surg. 2014;219(3).
- Smith RG, et al. "The growth hormone secretagogue receptor." Endocr. Rev. 2005;26(3):346–360.