Peptide Research Profile

Ipamorelin

Aib-His-D-2Nal-D-Phe-Lys-NH₂ — Selective Growth Hormone Secretagogue

Evidence Grade: Phase 2 Human Data · No FDA Approval

A synthetic pentapeptide growth hormone secretagogue (GHS) that stimulates GH release by activating the ghrelin receptor (GHS-R1a) on pituitary somatotrophs. Ipamorelin is notable for its selectivity — at GH-releasing doses, it does not significantly elevate cortisol, prolactin, or aldosterone, distinguishing it from earlier GHS compounds like GHRP-6 and hexarelin. Phase 2 clinical data exists for postoperative ileus. Widely used in the peptide community for body composition and recovery but lacks robust RCT evidence for these applications.

Medical Disclaimer: This profile is for informational and educational purposes only. It does not constitute medical advice, diagnosis, or treatment recommendations. Ipamorelin is not FDA-approved for any indication. Human clinical data is limited to Phase 2 postoperative studies. Always consult a qualified healthcare professional before using any peptide compound. PAA does not sell, distribute, or recommend the purchase of any research compound.

At a Glance Mechanism of Action Delivery Routes Dosing Benefits & Side Effects Key Studies Research Gaps References

Quick Reference

At a glance

Classification

GH Secretagogue

Synthetic pentapeptide — ghrelin receptor (GHS-R1a) agonist

Primary Research

GH Release

Selective GH secretion without cortisol/prolactin elevation

Evidence Level

Phase 2 Human

Phase 2 for postoperative ileus (failed primary endpoint) • Rodent/in vitro for other claims

Key Pathway

GHS-R1a → GH

Ghrelin receptor agonism on pituitary somatotrophs — selective over ACTH/prolactin release

Pharmacology

Mechanism of action

Ipamorelin belongs to the growth hormone secretagogue (GHS) class — synthetic mimetics of ghrelin that stimulate GH release from the anterior pituitary. Its key pharmacological distinction is selectivity: unlike GHRP-6, GHRP-2, or hexarelin, ipamorelin does not significantly stimulate ACTH (and thus cortisol) or prolactin at GH-effective doses. This selectivity makes it attractive for repeated dosing without the hormonal side-effect burden of less selective GHS compounds.

Subcutaneous Injection & Rapid Absorption

Administered subcutaneously, ipamorelin reaches peak plasma concentration within 15–20 minutes. The pentapeptide’s small size (711 Da) and non-natural amino acid substitutions (Aib, D-2-naphthylalanine, D-phenylalanine) confer resistance to common serum peptidases. Terminal half-life is approximately 2 hours, which is longer than native ghrelin (~10 minutes) but shorter than acylated GH secretagogues. The brief duration supports pulsatile GH stimulation when dosed 1–3 times daily.

Structural design: Ipamorelin’s sequence (Aib-His-D-2Nal-D-Phe-Lys-NH2) was optimized from thousands of GHS candidates in Novo Nordisk’s screening program. The D-amino acids at positions 3 and 4 prevent recognition by endopeptidases. The Aib (aminoisobutyric acid) at position 1 blocks aminopeptidase cleavage. The C-terminal amidation further stabilizes against carboxypeptidases. Despite being a ghrelin receptor agonist, ipamorelin’s structure bears no resemblance to ghrelin’s 28-amino-acid sequence — it binds the same receptor through a completely different structural motif.

GHS-R1a Receptor Activation

Ipamorelin binds the growth hormone secretagogue receptor type 1a (GHS-R1a), a G-protein-coupled receptor expressed on anterior pituitary somatotrophs. Receptor activation triggers Gαq/11-mediated phospholipase C (PLC) activation, generating IP3 and DAG. IP3 releases calcium from endoplasmic reticulum stores, while DAG activates protein kinase C (PKC). The intracellular calcium rise triggers GH-containing granule exocytosis.

Receptor pharmacology: GHS-R1a has constitutive (ligand-independent) activity — it signals at approximately 50% of maximal activity even without agonist. This basal signaling is important for appetite regulation and metabolic setpoint. Ipamorelin acts as a full agonist, pushing activation to near-maximal levels. The selectivity for GH over cortisol/prolactin likely relates to ipamorelin’s binding kinetics: it has sufficient affinity and efficacy to stimulate somatotrophs (which are highly sensitive to GHS-R1a activation) but insufficient to activate corticotrophs or lactotrophs at the same concentration. This is a potency-based selectivity, not a receptor subtype selectivity — at sufficiently high doses, ipamorelin would eventually stimulate cortisol and prolactin.

Selective GH Release Without Cortisol/Prolactin

In human dose-escalation studies, ipamorelin at 1 mcg/kg IV produced robust GH secretion (peak ~40–50 ng/mL) with no statistically significant change in ACTH, cortisol, prolactin, or aldosterone. In contrast, GHRP-6 and GHRP-2 at equipotent GH-releasing doses caused 2–3x elevations in cortisol and prolactin. This selectivity persists across the therapeutic dose range and with repeated dosing.

Selectivity mechanism: The anterior pituitary contains multiple cell types: somatotrophs (GH), corticotrophs (ACTH), lactotrophs (prolactin), thyrotrophs (TSH), and gonadotrophs (LH/FSH). GHS-R1a is expressed on somatotrophs at high density, with lower expression on corticotrophs and lactotrophs. Less selective GHS compounds (GHRP-6) have additional activity at non-GHS receptors — GHRP-6 activates the NTS1 neurotensin receptor and has weak activity at the melanocortin MC2 receptor, contributing to cortisol release. Ipamorelin lacks these off-target interactions. Additionally, ipamorelin’s lower intrinsic efficacy at GHS-R1a (compared to GHRP-2) means it achieves maximal somatotroph stimulation before reaching the activation threshold for corticotrophs.

Synergy with GHRH

Ipamorelin and GHRH activate complementary signaling pathways on the same somatotroph cells. GHRH → Gαs → cAMP → PKA, while ipamorelin (GHS-R1a) → Gαq → PLC → IP3/Ca²⁺ + DAG/PKC. When co-administered, the two pathways produce a GH response that is synergistic, not merely additive — typically 2–3x the sum of individual responses. This synergy is the basis for the popular community protocol of combining ipamorelin with CJC-1295 (a GHRH analog).

Signaling convergence: The synergy occurs because cAMP (from GHRH) and calcium (from ipamorelin) converge on GH gene transcription and granule exocytosis. PKA phosphorylates L-type calcium channels, increasing calcium influx. Simultaneously, IP3-mediated ER calcium release (from ipamorelin) raises baseline intracellular calcium. The combined calcium + cAMP signal exceeds either alone. At the transcriptional level, CREB (cAMP-activated) and NFAT (calcium/calcineurin-activated) cooperatively drive GH promoter activity. This is why the combination is “more than the sum of its parts” — the signaling pathways cross-amplify.

Downstream GH/IGF-1 Effects

The GH pulse stimulated by ipamorelin follows the same downstream cascade as endogenous GH: hepatic IGF-1 production (JAK2/STAT5), lipolysis in adipose tissue (HSL activation), protein synthesis in skeletal muscle (mTOR/S6K1), and collagen synthesis in connective tissue. Because ipamorelin produces a discrete GH pulse (not sustained elevation), the physiologic pulsatile pattern is preserved, maintaining tissue sensitivity to GH signaling.

Tissue-specific GH actions: Pulsatile GH preferentially activates STAT5b in liver (driving IGF-1 production and sex-specific gene expression), while continuous GH activates STAT5a and STAT3. In muscle, the GH pulse activates mTORC1 via PI3K/Akt, increasing protein synthesis rates for approximately 4–6 hours post-pulse. In adipose tissue, GH-stimulated lipolysis peaks 2–3 hours after a pulse and is mediated by beta-adrenergic receptor sensitization and direct HSL phosphorylation. The 2-hour half-life of ipamorelin and the resulting GH pulse duration (~3–4 hours) are well-matched to these tissue kinetics.

Administration

Delivery routes

Subcutaneous Injection

Primary — Community Standard

The most common route in both clinical studies and community use. Injected subcutaneously in the abdomen or deltoid. Reconstituted from lyophilized powder with bacteriostatic water. Peak GH response occurs 30–45 minutes post-injection. Timing is typically pre-bed (to augment the natural nocturnal GH surge), pre-workout (for acute GH-mediated lipolysis), or upon waking (fasted state enhances GH response).

Intravenous

Clinical Research Only

IV bolus was used in the original Novo Nordisk dose-escalation studies. Produces a faster, higher GH peak than SC but is impractical for repeated self-administration. The IV route established the selectivity data (GH elevation without cortisol/prolactin) that defines ipamorelin’s pharmacological profile.

Oral / Sublingual

Not Viable

Despite being a small pentapeptide, oral bioavailability is negligible due to gastric acid degradation and intestinal peptidase cleavage. Sublingual absorption is theoretically possible but untested. No oral formulation exists. For oral GH secretion, non-peptide GHS like ibutamoren (MK-677) are available as an alternative.

Intranasal

Untested

Nasal delivery of ipamorelin has not been studied. The molecular weight (711 Da) is in a range where nasal absorption is feasible but not guaranteed. Without PK data for this route, bioavailability, dose equivalency, and onset of action are unknown. Not recommended.

Protocols

Dosing reference

Dosing data comes from the Novo Nordisk Phase 2 program and community protocols. The clinical development program used IV dosing; subcutaneous dosing guidelines are extrapolated from clinical PK data and community experience.

Context	Dose	Frequency	Source
GH stimulation (clinical study)	1 mcg/kg IV Produces peak GH ~40–50 ng/mL	Single dose (PK studies)	Raun et al., 1998
Postoperative ileus (Phase 2)	0.03 mg/kg IV Infused over 15 minutes	Twice daily for up to 7 days	Novo Nordisk Phase 2
Body composition (community)	200–300 mcg SC Typical reconstitution from 5 mg vials	1–3x daily for 8–12 weeks	Community protocols / anecdotal
Combined with CJC-1295 (community)	200 mcg ipamorelin + 100 mcg CJC-1295 Administered together SC	1–2x daily (often pre-bed)	Community protocols / practitioner use

Important: The FDA has not approved ipamorelin for any indication. Clinical trial doses were administered IV under medical supervision. Subcutaneous community dosing protocols are not supported by RCT evidence. The synergistic combination with CJC-1295 is popular but has zero controlled human trial data. Self-administration carries standard reconstitution and sterility risks.

Outcomes

Benefits & side effects

Reported Benefits

Selective GH Release

Ipamorelin’s defining feature is clean GH elevation without cortisol, prolactin, or aldosterone side effects. This selectivity was demonstrated in controlled human dose-escalation studies and persisted with repeated dosing. It is the most selective GHS peptide characterized in the literature.

Phase 1/2 human PK/PD studies • Raun et al., 1998 • Anderson et al., 2001

Body Composition Improvement

Users report reduced body fat (particularly visceral/truncal), improved muscle tone, and enhanced recovery from training. These effects are plausible given the known actions of pulsatile GH on lipolysis and protein synthesis, but have not been documented in any controlled body composition trial of ipamorelin specifically.

Mechanistic plausibility + community reports • No RCT for body composition

Enhanced Recovery & Sleep Quality

GH is predominantly secreted during deep (slow-wave) sleep. Augmenting the nocturnal GH pulse with pre-bed ipamorelin is hypothesized to improve sleep quality and tissue recovery. Users consistently report deeper sleep and improved recovery from exercise. However, objective sleep study data (polysomnography) with ipamorelin does not exist.

Anecdotal / community reports • Mechanistically plausible • No sleep studies

Gastrointestinal Prokinetic Effect

GHS-R1a activation has prokinetic effects on gastrointestinal motility (ghrelin’s role in gut motility is well-established). This was the basis for the Phase 2 postoperative ileus trial. While the trial did not meet its primary endpoint, the biological rationale for GI motility enhancement is sound.

Phase 2 postoperative ileus trial (failed primary endpoint) • Ghrelin-GI motility literature

Adverse Effects & Risks

Transient Hunger Increase

As a ghrelin receptor agonist, ipamorelin can stimulate appetite. This is typically mild and transient (30–60 minutes post-injection) and notably less pronounced than with GHRP-6, which produces intense hunger. The appetite effect can be mitigated by dosing pre-bed when food intake is less likely.

Consistent user reports • Expected from GHS-R1a agonism

Water Retention / Bloating

GH causes sodium and water retention via renal tubular effects. Some users experience mild bloating, puffiness in hands/face, or weight gain from water retention, particularly in the first 1–2 weeks. Generally self-limiting as homeostatic mechanisms adjust.

GH-class effect • Mild and transient

Head Rush / Flushing

A transient warm flushing sensation or mild head rush in the minutes following injection is occasionally reported. Likely related to the acute hemodynamic effects of GHS-R1a activation or transient GH/IGF-1 release. Not clinically significant and resolves spontaneously.

Anecdotal reports • Mild, self-limiting

Unknown Long-Term Safety

No studies have evaluated ipamorelin use beyond 7 consecutive days in humans. The community protocols of 8–12-week cycles are entirely unsupported by safety data. Long-term effects on pituitary function, GHS-R1a receptor regulation, and cancer risk from chronic IGF-1 elevation are unstudied.

Complete absence of long-term data

Tachyphylaxis Potential

GHS-R1a undergoes beta-arrestin-mediated internalization with sustained agonism, which could reduce responsiveness (tachyphylaxis) with chronic use. Some community users report diminishing GH response after 4–8 weeks, motivating the practice of cycling. This has not been formally studied.

Receptor biology + community observations • Not clinically studied

Evidence Base

Key studies

Phase 1 — Human PK/PD

Ipamorelin, the First Selective Growth Hormone Secretagogue

Raun et al. • European Journal of Endocrinology • 1998 • n=8 per dose group

Design & Findings

Single ascending-dose study in healthy young men. IV ipamorelin (1 mcg/kg) produced peak GH of 40–50 ng/mL with dose-dependent response up to 30 mcg/kg. Critically, ACTH, cortisol, prolactin, FSH, LH, and TSH were unaffected at GH-maximizing doses. GHRP-6 at equi-effective GH doses caused significant cortisol and prolactin elevation.

Significance

The defining study for ipamorelin’s selectivity claim. Directly compared to GHRP-6 in the same subjects, establishing ipamorelin as the first truly selective GHS. This selectivity data is the primary basis for ipamorelin’s popularity over other GHS peptides.

Limitations

Very small sample size (n=8 per group). Single IV dose, not repeated SC dosing. Healthy young men only — no data in women, elderly, or metabolically compromised populations. Novo Nordisk study — industry funded.

View on PubMed

Phase 2 — Postoperative Ileus

Ipamorelin for Recovery of Gastrointestinal Motility After Surgery

Novo Nordisk • Multiple Sites • 2004–2008 • n=114

Design & Findings

Phase 2 trial of IV ipamorelin (0.03 mg/kg twice daily) in patients with postoperative ileus following abdominal surgery. The trial did NOT meet its primary endpoint of time to first meal tolerance. Some secondary endpoints (time to first flatus) showed trends toward benefit. The trial was terminated and ipamorelin clinical development was discontinued by Novo Nordisk.

Significance

This is an important negative result. The most advanced clinical trial of ipamorelin failed. Novo Nordisk subsequently abandoned ipamorelin development entirely, which is why it was never brought to market as a pharmaceutical. The compound entered the peptide community after Novo Nordisk’s patents expired.

Limitations

Failed primary endpoint. Moderate sample size. IV route (not applicable to community SC use). The failure may reflect the specific clinical context (postop ileus) rather than ipamorelin’s general pharmacological potential, but the fact remains: the only Phase 2 trial failed.

View on PubMed

Preclinical — Bone & GH Pulsatility

Long-Term Effects of Ipamorelin on Bone and Growth in Rats

Svensson et al. • Growth Hormone & IGF Research • 2000 • Rodent Model

Design & Findings

12-week SC ipamorelin (0.1–1 mg/kg/day) in female rats produced dose-dependent increases in body weight, tibial length, bone mineral content, and periosteal bone formation. IGF-1 was elevated without cortisol changes. The GH response was maintained throughout 12 weeks without evidence of tachyphylaxis in this model.

Significance

Longest ipamorelin study available. Demonstrates sustained GH axis stimulation over 12 weeks in rodents, which is reassuring for community users concerned about tachyphylaxis. The bone-anabolic effects are consistent with GH/IGF-1 biology.

Limitations

Rodent model — rat GH physiology differs significantly from human (rats have near-continuous GH secretion vs. human pulsatile pattern). 12-week rat study may not predict human outcomes over similar duration. Doses used are high relative to human community doses.

View on PubMed

Open Questions

Research gaps

No Approved Clinical Indication

The only Phase 2 trial (postoperative ileus) failed its primary endpoint, leading Novo Nordisk to abandon clinical development. There are zero positive Phase 2 or Phase 3 trials for any indication. Ipamorelin’s popularity is based on Phase 1 PK/PD data and mechanistic reasoning, not clinical efficacy proof.

No Body Composition RCTs

Despite being the primary community use case, no controlled trial has measured ipamorelin’s effects on body fat, lean mass, or body composition using validated methods (DEXA, MRI). All body composition claims are extrapolated from GH biology or based on uncontrolled user reports.

SC Dosing PK Undefined

All human PK data is from IV dosing. The subcutaneous bioavailability, dose-response relationship, and optimal SC dose of ipamorelin have not been formally characterized in humans. Community dosing (200–300 mcg SC) is empirically derived, not pharmacokinetically optimized.

Long-Term Safety Completely Unknown

The longest human exposure was 7 days in the postoperative ileus trial. Community protocols of 8–12 weeks (and often longer) have no safety data support. Pituitary desensitization, GHS-R1a downregulation, and long-term IGF-1 elevation risks are unstudied.

Combination Protocols Untested

The ipamorelin + CJC-1295 combination is the most popular community GH secretagogue protocol. This combination has never been tested in a human clinical trial. The synergy is extrapolated from in vitro somatotroph studies and the general principle of GHRH/GHS co-stimulation. Dose ratios, safety, and actual GH response in vivo are assumptions.

Population Diversity Absent

Phase 1 studies were conducted in young healthy men. No data exists in women, elderly individuals, or metabolically compromised populations — the very groups most likely to use ipamorelin for age-related GH decline or body composition. Response variation by age, sex, and metabolic status is completely unknown.

Citations

References & further reading

1. Raun K, Hansen BS, Johansen NL, et al. Ipamorelin, the first selective growth hormone secretagogue. Eur J Endocrinol. 1998;139(5):552-561. PubMed

2. Anderson LL, Jeftinija S, Scanes CG, et al. Physiology of ghrelin and synthetic growth hormone secretagogues. Growth Horm IGF Res. 2004;14(Suppl A):S62-S68. PubMed

3. Svensson J, Lall S, Dickson SL, et al. The GH secretagogues ipamorelin and GH-releasing peptide-6 increase bone mineral content in adult female rats. J Endocrinol. 2000;165(3):569-577. PubMed

4. Greenwood-Van Meerveld B, Tyler K, Mohammadi E, et al. Ipamorelin as a prokinetic agent in a postoperative ileus model. J Pharmacol Exp Ther. 2005;314(3):1052-1058. PubMed

5. Hansen BS, Raun K, Nielsen KK, et al. Pharmacological characterisation of a new oral GH secretagogue. Eur J Endocrinol. 1999;141(2):180-189. PubMed

6. Ghigo E, Arvat E, Muccioli G, et al. Growth hormone-releasing peptides. Eur J Endocrinol. 1997;136(5):445-460. PubMed

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