The GHRP family question is not which peptide releases the most growth hormone — all three release it robustly. It is which one releases only that. Ipamorelin's founding paper answered by title: the first selective growth hormone secretagogue.
Ipamorelin's defining, literature-anchored property is selectivity: GH release without the ACTH/cortisol and prolactin co-release documented for earlier GHRPs at comparable doses. GHRP-6 is the classic hunger-signal secretagogue; GHRP-2 the potent middle child; neither is an approved drug and all three remain research compounds.
One receptor, three peptides
GHRP-6, GHRP-2 and ipamorelin all agonize the ghrelin receptor (GHS-R1a) — the same receptor the stomach's hunger hormone uses — triggering pulsatile growth-hormone release upstream of the GHRH system. The family emerged in sequence: hexapeptides first (GHRP-6, then the more potent GHRP-2), then ipamorelin, a pentapeptide engineered specifically to strip away the off-target endocrine effects the earlier compounds carried.
The selectivity data
Ipamorelin's characterization work put numbers on the family's separation. In the founding studies, ipamorelin released growth hormone with potency comparable to GHRP-6 — but unlike GHRP-2 and GHRP-6, it did not meaningfully elevate ACTH or cortisol at GH-releasing doses; substantially larger exposures were required before those axes moved. The earlier GHRPs co-release ACTH/cortisol and prolactin at the same doses that release GH — an endocrine side-channel, not a rounding error.
Human pharmacology followed: ipamorelin's PK/PD modeling work characterized clean dose-dependent GH release, and the molecule progressed as far as a phase-2 clinical program (postoperative ileus) — making it one of the few GHRPs with formal clinical-trial history.
Hunger: GHRP-6's signature
Acting on the hunger hormone's receptor has predictable consequences, and the older hexapeptides show them most strongly — GHRP-6 in particular is the family's appetite-stimulating workhorse in the literature. Ipamorelin's selectivity extends here too: the hunger signal is attenuated relative to GHRP-6 at comparable GH-releasing exposures.
Ipamorelin
The selective growth hormone secretagogue discussed here, supplied as research-use-only material with a batch-matched certificate of analysis.
Side-by-side
| GHRP-6 | GHRP-2 | Ipamorelin | |
|---|---|---|---|
| Structure | Hexapeptide | Hexapeptide | Pentapeptide |
| GH release | Robust | Most potent of the three | Comparable to GHRP-6 |
| Cortisol/ACTH at GH doses | Co-released | Co-released | Not meaningfully elevated |
| Prolactin | Co-released | Co-released | Not meaningfully elevated |
| Hunger signal | Strongest | Present | Attenuated |
| Clinical-trial history | Mechanistic studies | Mechanistic studies | Phase-2 program (ileus) |
| Approval status | None approved; all are unapproved research compounds, prohibited in sport | ||
The honest frame for all three
Selectivity is a within-family comparison, not a safety clearance. Growth-hormone secretagogues as a class carry the reviewed caution that long-term human safety data are thin, the compounds are detectable and prohibited in sport, and none of the three is an approved medicine. The literature's verdict is narrow and useful: if the research question is GH release with minimal endocrine side-channel, ipamorelin is the family's purpose-built answer — that is what it was made for, and what its founding data show.
Ipamorelin
Research-use-only material, sold by the vial with batch documentation. Check the certificate of analysis against the batch you receive.
What to know now
- All three agonize GHS-R1a, the ghrelin receptor — GH release is the shared property, not the differentiator.
- Ipamorelin's founding data: GH potency comparable to GHRP-6 with no meaningful ACTH/cortisol or prolactin rise at GH-releasing doses.
- GHRP-2 and GHRP-6 co-release cortisol and prolactin at the same doses that release GH; GHRP-6 carries the family's strongest hunger signal.
- Ipamorelin is the only one of the three with a formal phase-2 clinical program on record.
- Class-wide caution stands: thin long-term human safety data, sport-prohibited, none approved.
What we're watching
Whether any GHS returns to formal clinical development — the class's pharmacology is well mapped, and a single funded program would reset how these comparisons are written.
Frequently asked questions
Is ipamorelin better than GHRP-2 or GHRP-6?
For the specific property the literature measures — GH release without cortisol, prolactin, or strong hunger co-signaling — ipamorelin is the purpose-built member of the family. GHRP-2 releases more GH in absolute terms but brings the off-target column with it.
Does ipamorelin raise cortisol or prolactin?
Not meaningfully at GH-releasing doses in its characterization studies — that selectivity is the founding claim of the molecule, in contrast to GHRP-2 and GHRP-6, which co-release both.
Why does GHRP-6 cause hunger?
All three act on GHS-R1a, the receptor of the hunger hormone ghrelin. GHRP-6 activates that appetite signaling most strongly; ipamorelin's engineering attenuated it.
Are any of these peptides FDA approved?
No. None of the three is an approved medicine anywhere; all are unapproved research compounds, and growth hormone secretagogues as a class are prohibited in sport.
References
- Raun, K., Hansen, B. S., Johansen, N. L., et al. (1998). Ipamorelin, the first selective growth hormone secretagogue. European Journal of Endocrinology, 139(5), 552–561. https://doi.org/10.1530/eje.0.1390552
- Gobburu, J. V., Agersø, H., Jusko, W. J., & Ynddal, L. (1999). Pharmacokinetic-pharmacodynamic modeling of ipamorelin, a growth hormone releasing peptide, in human volunteers. Pharmaceutical Research, 16(9), 1412–1416. https://doi.org/10.1023/a:1018955126402
- ClinicalTrials.gov. Phase II double-blind placebo-controlled dose-finding study to evaluate safety/efficacy of ipamorelin compared to placebo for recovery of gastrointestinal function following bowel resection (NCT01280344). No results posted. https://clinicaltrials.gov/study/NCT01280344
- Ghigo, E., Arvat, E., Gianotti, L., et al. (1994). Growth hormone-releasing activity of hexarelin, a new synthetic hexapeptide, after intravenous, subcutaneous, intranasal, and oral administration in man. Journal of Clinical Endocrinology and Metabolism, 78(3), 693–698. https://doi.org/10.1210/jcem.78.3.8126144
- Arvat, E., Gianotti, L., Grottoli, S., et al. (1994). Arginine and growth hormone-releasing hormone restore the blunted growth hormone-releasing activity of hexarelin in elderly subjects. Journal of Clinical Endocrinology and Metabolism, 79(5), 1440–1443. https://doi.org/10.1210/jcem.79.5.7962341
- Sigalos, J. T., & Pastuszak, A. W. (2018). The safety and efficacy of growth hormone secretagogues. Sexual Medicine Reviews, 6(1), 45–53. https://doi.org/10.1016/j.sxmr.2017.02.004
- Thomas, A., Delahaut, P., Krug, O., Schänzer, W., & Thevis, M. (2012). Metabolism of growth hormone releasing peptides. Analytical Chemistry, 84(23), 10252–10259. https://doi.org/10.1021/ac302034w
- Thomas, A., Walpurgis, K., & Thevis, M. (2024). Chromatographic-mass spectrometric analysis of peptidic analytes in doping control urine samples. Journal of Mass Spectrometry, 59(1), e4996. https://doi.org/10.1002/jms.4996