Ipamorelin

Ipamorelin's pentapeptide sequence Aib-His-D-2-Nal-D-Phe-Lys-NH₂ contains three non-standard elements that contribute to its pharmacological properties: (1) Aib (alpha-aminoisobutyric acid) at position 1: a non-coded amino acid with methyl groups on both α-carbons; creates steric protection against aminopeptidase cleavage at the N-terminus (same protease-resistance logic as the Ala⁸→Gln modification in Mod GRF 1-29); also constrains the peptide backbone conformation, improving receptor selectivity. (2) D-2-Nal at position 3: D-stereoisomer of 2-naphthylalanine (naphthyl ring aromatic system); the D-configuration and the large aromatic ring system are key contributors to GHS-R1a selectivity — this residue makes van der Waals contact with a hydrophobic pocket in the GHS-R1a binding site that GHRP-6 and GHRP-2 do not optimally engage. (3) C-terminal amide (-Lys-NH₂): protects against carboxypeptidase degradation; improves receptor fit at the C-terminal position. Together these features produce a compound with both enhanced metabolic stability and the receptor selectivity profile that defines ipamorelin.

Ipamorelin's pharmacokinetics after subcutaneous injection: absorption is rapid; peak serum levels within 15-30 minutes; the GH pulse peaks at 30-60 minutes post-injection. Plasma half-life approximately 2 hours — longer than GHRP-6 (~30 min distribution phase) due to the structural features providing protease resistance. The GH pulse itself resolves to baseline within 2-3 hours post-injection. IGF-1 elevation follows more slowly (reflecting hepatic IGF-1 synthesis in response to GH) and can be measured for 12-24 hours after injection at standard doses. The longer half-life vs GHRP-6 does not change the pulsatile protocol design — once daily or twice daily injection still produces discrete GH pulses rather than continuous elevation.

Ipamorelin binds GHS-R1a (ghrelin receptor) on pituitary somatotrophs with high affinity (Ki in the nM range). GHS-R1a is a Gq-coupled GPCR: binding activates phospholipase C → IP3 generation → intracellular Ca²⁺ release from ER → calcium influx through voltage-gated Ca²⁺ channels → GH granule exocytosis. Simultaneously, GHS-R1a on hypothalamic somatostatin neurons (when activated) reduces somatostatin release, removing the tonic inhibitory brake on GH release from the pituitary. The combination of direct pituitary somatotroph activation and somatostatin brake release produces a clean, robust GH pulse.

GHRP-6 and GHRP-2 activate GHS-R1a broadly — including on hypothalamic CRH neurons (which drive ACTH → cortisol) and pituitary lactotrophs (which produce prolactin). Whether GHS-R1a is the direct receptor for these HPA axis effects or whether the off-target activity involves secondary receptors is still debated, but the net result is clear: GHRP-6 and GHRP-2 reliably elevate cortisol and prolactin in animal and human studies. Ipamorelin does not. Raun 1998 demonstrated this dose-independently in rats: at doses producing maximal GH release, ipamorelin produced zero ACTH elevation; at the same doses, GHRP-2 produced significant ACTH elevation. The structural changes in ipamorelin (particularly the D-2-Nal at position 3 and the Aib at position 1) appear to eliminate the binding interaction responsible for HPA axis activation while preserving GHS-R1a/pituitary somatotroph activation.

When ipamorelin is co-injected with CJC-1295 (no-DAC), the two compounds activate separate receptor systems on the same somatotroph: GHRHR (via CJC-1295) → Gs-cAMP pathway; GHS-R1a (via ipamorelin) → Gq-Ca²⁺ pathway. The two signals converge on GH exocytosis through non-overlapping intracellular mechanisms, producing a supraadditive GH pulse (3-5x larger than either alone; Bowers 1991). Ipamorelin's clean profile means the GH Stack (CJC + Ipamorelin) produces the amplified GH pulse without adding cortisol or prolactin elevation from the GHRP side.

Raun K, Hansen BS, Johansen NL, Thøgersen H, Madsen K, Ankersen M, Andersen PH. (1998). Ipamorelin, the first selective growth hormone secretagogue. European Journal of Endocrinology. 139(5):552-561. Design: in vivo GH release studies in anesthetized pigs; direct pituitary GH release; ACTH measurement; dose-response characterization; comparison with GHRP-6 and GHRP-2; pharmacological receptor characterization. Key findings: ipamorelin produced potent GH release in pigs equivalent to GHRP-2 and GHRP-6; at all doses tested, ipamorelin produced zero ACTH elevation (GHRP-2 and GHRP-6 elevated ACTH dose-dependently); the selectivity was absolute at pharmacological doses; ipamorelin was named 'the first selective growth hormone secretagogue' based on this dissociation of GH release from ACTH elevation. This paper is the pharmacological foundation for all ipamorelin use.

Multiple subsequent studies have characterized ipamorelin's mechanism in detail. In vitro: GHS-R1a binding affinity confirmed (Ki ~nM range); Gq-PLC-Ca²⁺ pathway activation confirmed; no significant binding to GHRHR, MC receptors, or opioid receptors at pharmacological concentrations. Animal body composition: GH-releasing and IGF-1-elevating effects confirmed across multiple species at doses consistent with the therapeutic window. Sleep quality: consistent with all GHRH-adjacent compounds, ipamorelin use is associated with improved slow-wave sleep (SWS) in community experience, consistent with GH-mediated SWS enhancement.

Unlike sermorelin (FDA-approved for GH deficiency; extensive human Phase 2/3 data) and tesamorelin (FDA-approved; Phase 3 NEJM data), ipamorelin does not have a published Phase 2/3 human RCT for the GH optimization use case. The human evidence is: (1) the class-level GHRH+GHRP synergy from Bowers 1991 (which used GHRP-6 but establishes the principle); (2) ipamorelin's selectivity profile from Raun 1998 (pigs, not humans, but the receptor pharmacology is fully established); (3) community evidence from years of use (Grade E). The Raun 1998 paper is the primary evidence document; it is a pharmacology characterization paper, not a human efficacy trial.

Evidence

Grade

Finding

Limitation

Raun 1998 (Eur J Endocrinol; pigs; in vivo GH release; ACTH measurement)

B (animal/pharmacology)

GH release equivalent to GHRP-2/GHRP-6; zero ACTH elevation at all doses; first selective GHS; selectivity absolute within pharmacological range

Pig model; not human RCT; receptor pharmacology directly applicable to humans but clinical endpoints not measured

GHS-R1a binding characterization (in vitro)

D (in vitro)

High-affinity GHS-R1a binding; Gq-PLC-Ca2+ pathway; no significant off-target receptor binding at pharmacological concentrations

In vitro; receptor binding does not equal clinical efficacy

Bowers 1991 (GHRH+GHRP synergy; human)

B (human; different GHRP)

GHRH+GHRP synergy: 2-3x larger GH pulse; principle applies to all GHRH+GHRP combinations including CJC + Ipamorelin

Used GHRP-6, not ipamorelin; class-level principle extrapolation

Community experience (GH Stack; years of use)

E

Consistent GH pulse, IGF-1 elevation, sleep quality improvement, body composition effects; ipamorelin universally described as clean (no cortisol/hunger effects)

No controls; no blinding; community consensus only

Ipamorelin alone produces a GH pulse via GHS-R1a without the GHRH receptor component. Some users run ipamorelin alone when they specifically want the GHS-R1a mechanism (somatostatin suppression + pituitary GH trigger) without adding another peptide to the protocol. The standalone pulse is real but smaller than the GH Stack combination.

Parameter

Recommendation

Notes

Dose

100-300 mcg per injection

200 mcg is the standard community dose; 100 mcg for conservative start; 300 mcg for maximum pulse

Frequency

1-3x daily

1x before bed (primary); 2x (AM fasted + PM bedtime); 3x rarely needed

Timing

Before bed primary; AM fasted secondary

Bedtime amplifies natural sleep-phase GH pulse; empty stomach mandatory

Empty stomach

Mandatory: 2+ hours post-meal; 20-30 min before eating

Insulin suppresses GH release; same rule as all GH secretagogues

Stacking

Always with CJC-1295 (no-DAC) for GH Stack synergy

Same syringe acceptable; same injection timing; 3-5x GH pulse amplitude

Reconstitution

5 mg vial + 2 mL BAC water = 2.5 mg/mL (2,500 mcg/mL)

200 mcg = 8 units on U-100 syringe; 300 mcg = 12 units

Cycle

8-12 weeks on; 4-8 weeks off

Allows GHS-R1a recovery; consistent with GH Stack protocol

IGF-1 monitoring

Baseline + 6-8 weeks into cycle

Target upper half of age-appropriate reference range

Ipamorelin's zero cortisol/ACTH elevation is not just a theoretical advantage — it has practical safety implications. Users sensitive to cortisol elevation (adrenal conditions, existing high-stress cortisol baseline, users on protocols that already increase cortisol) can use ipamorelin without concern about HPA axis compounding. Users concerned about gynecomastia risk from prolactin elevation (from concurrent anabolic protocols) do not need to consider ipamorelin's prolactin profile as a variable. The hormonal footprint is GH and IGF-1 only.

Water retention: GH-mediated sodium/water retention; mild peripheral edema in the first 2-4 weeks; resolves with stable dosing or cycle end. Transient insulin resistance: GH antagonizes insulin; fasting glucose monitoring appropriate on extended cycles. Carpal tunnel symptoms: water retention and IGF-1 effects on soft tissues; dose-dependent; usually mild at community doses. These are class-level GH axis effects, not ipamorelin-specific.

Unlike GHRP-6, ipamorelin produces minimal to no appetite stimulation. The hypothalamic hunger signal (NPY/AgRP arcuate nucleus activation) that makes GHRP-6 useful for bulking is not produced by ipamorelin at standard doses. For users in caloric restriction, this is an advantage: ipamorelin enhances GH signaling without making dietary adherence harder. For users trying to increase caloric intake, this is the reason to consider GHRP-6 instead.

Different compounds entirely. GHRP-6 is a hexapeptide (His-D-Trp-Ala-Trp-D-Phe-Lys-NH₂; MW 872 Da) discovered in 1984. Ipamorelin is a pentapeptide (Aib-His-D-2-Nal-D-Phe-Lys-NH₂; MW 711 Da) characterized in 1998. Both activate GHS-R1a, but GHRP-6 also activates off-target receptors producing cortisol, prolactin, and strong appetite stimulation. Ipamorelin does not. They are in the same class (GHRPs) but have completely different secondary effect profiles.

The GH dose-response curve for ipamorelin, like all GHRPs, has a saturation point beyond which additional dose produces more off-target stimulation without proportionally more GH. At very high doses (beyond the community range of 100-300 mcg), some partial cortisol or appetite stimulation may emerge even with ipamorelin. The 'clean' profile holds firmly at standard doses (100-300 mcg); it is not guaranteed to hold at arbitrarily high doses.

GH potency is similar; ipamorelin is slightly less potent per mcg than GHRP-2 but comparable to GHRP-6 per-mcg at equivalent doses. The potency comparison should not be used to conclude GHRP-6 is 'better' — the cortisol and prolactin elevation from GHRP-6 are real adverse effects that counteract GH's anabolic benefits in some contexts (cortisol is catabolic). 'More hormones' is not inherently better. Ipamorelin's clean profile means the GH signal is produced without competing cortisol-driven catabolism.

• Default GHRP for: cutting, body recomposition, recovery, sleep enhancement, general GH axis support. Ipamorelin produces GH without cortisol/prolactin/hunger.
• Not the choice for: specifically wanting appetite stimulation for a bulking phase; use GHRP-6 instead for its hunger signal.
• Always pair with CJC-1295 no-DAC: the GH Stack synergy (GHRHR + GHS-R1a dual activation) produces 3-5x the GH pulse of ipamorelin alone; this is the correct protocol.
• Dose: 200 mcg per injection standard; 100 mcg conservative; 300 mcg higher amplitude; stay within the clean-profile range.
• Empty stomach: 2+ hours post-meal; 20-30 min before eating; mandatory; insulin suppresses GH release.
• IGF-1 monitoring: baseline and week 6-8; target upper half age-appropriate range.
• WADA S2: banned in and out of competition; career-ending for any athlete under testing.
• See also: pbghstackv4 (GH Stack: CJC + Ipamorelin); pbcjc1295v4 (CJC-1295 no-DAC standalone); pbcit_v4 (CIT Blend).

— End of Ipamorelin —

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