A complete comparison of the three most-researched growth hormone releasing peptides — selectivity, GH pulse amplitude, side effect profiles, and stacking protocols.
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Growth hormone releasing peptides (GHRPs) are synthetic peptides that stimulate pituitary GH secretion by binding the ghrelin receptor (GHS-R1a). Unlike GHRH analogues — which mimic the hypothalamic signal that tells the pituitary to release GH — GHRPs act through a separate receptor pathway, making them synergistic with GHRH analogues when stacked. Ipamorelin, GHRP-2, and GHRP-6 are the three most extensively studied GHRPs in preclinical and clinical research.
The key distinction between the three compounds is selectivity. GHS-R1a is expressed not only in the pituitary (where GH release occurs) but also in the hypothalamus, adrenal glands, and gut. Compounds that bind GHS-R1a with high affinity across all tissues produce off-target effects: cortisol and ACTH release from the adrenal axis, prolactin elevation, and appetite stimulation via hypothalamic and gut ghrelin pathways. Ipamorelin was specifically engineered to minimize these off-target effects while preserving GH-stimulating potency.
| Parameter | Ipamorelin | GHRP-2 | GHRP-6 |
|---|---|---|---|
| Compound Class | GHRP / GHS-R1a agonist | GHRP / GHS-R1a agonist | GHRP / GHS-R1a agonist |
| Amino Acids | 5 (pentapeptide) | 6 (hexapeptide) | 6 (hexapeptide) |
| Half-Life | ~2 hours | ~1–2 hours | ~1–2 hours |
| GH Pulse Amplitude | Moderate (3–7× baseline) | High (7–15× baseline) | Moderate (3–6× baseline) |
| GH Selectivity | High — GH only | Low — GH + cortisol + prolactin | Moderate — GH + mild cortisol |
| Cortisol Elevation | Minimal | Significant (+50–100%) | Mild–moderate |
| Prolactin Elevation | Minimal | Significant | Mild |
| Appetite Stimulation | Minimal | Mild | Pronounced (ghrelin-mediated) |
| IGF-1 Elevation | Moderate | High | Moderate |
| Best Stack Partner | CJC-1295 (gold standard) | CJC-1295 (max GH output) | CJC-1295 (less studied) |
| Research Cycle Length | 8–16 weeks | 4–8 weeks (cortisol concern) | 8–12 weeks |
| Best For | Long-term GH optimization, lean mass | Short-term max GH pulse research | Appetite stimulation, recovery |
The most clinically significant difference between these three GHRPs is their selectivity for GH release versus co-stimulation of cortisol, prolactin, and appetite pathways. This distinction was the primary engineering goal when ipamorelin was developed — researchers at Novo Nordisk specifically designed it to dissociate GH-stimulating activity from the adrenal and appetite side effects seen with earlier GHRPs like GHRP-2 and GHRP-6.
High Selectivity
Low Selectivity
Moderate Selectivity
The cortisol co-stimulation of GHRP-2 is particularly relevant for research focused on body composition. Cortisol is catabolic — it promotes muscle protein breakdown and fat storage, directly opposing the anabolic effects of elevated GH and IGF-1. Research cycles using GHRP-2 therefore carry an inherent hormonal tension that ipamorelin avoids entirely.
| Compound | Key Finding | Notable Limitation |
|---|---|---|
| Ipamorelin | Dose-dependent GH release without cortisol or prolactin co-stimulation in multiple human studies; GH pulses 3–7× baseline at 200 mcg SubQ | No large Phase III RCTs; most data from Phase I/II and preclinical studies |
| GHRP-2 | Strongest GH pulse amplitude of the three (7–15× baseline); significant IGF-1 elevation in GH-deficient adults; used as a GH stimulation test agent | Consistent cortisol and prolactin co-stimulation limits long-term research utility |
| GHRP-6 | Reliable GH stimulation; pronounced appetite stimulation via ghrelin pathway confirmed in multiple studies; used in cachexia and appetite research | Hunger side effects limit compliance in body composition research; less studied than GHRP-2 for GH endpoints |
| Protocol | Ipamorelin | GHRP-2 | GHRP-6 |
|---|---|---|---|
| Standard Dose | 100–300 mcg | 100–300 mcg | 100–300 mcg |
| Frequency | 2–3× daily | 2–3× daily | 2–3× daily |
| Route | SubQ injection | SubQ injection | SubQ injection |
| Timing | Fasted (2+ hr post-meal) | Fasted (2+ hr post-meal) | Fasted (2+ hr post-meal) |
| Cycle Length | 8–16 weeks | 4–8 weeks | 8–12 weeks |
| GHRH Stack Dose | 100 mcg + CJC-1295 100 mcg | 100 mcg + CJC-1295 100 mcg | 100 mcg + CJC-1295 100 mcg |
| Research Criterion | Ipamorelin | GHRP-2 | GHRP-6 |
|---|---|---|---|
| Selective GH release (no cortisol) | |||
| Maximum GH pulse amplitude | |||
| Long research cycles (12–16 wk) | |||
| Lean body composition focus | |||
| Appetite stimulation research | |||
| GHRH analogue stacking | |||
| Minimal hormonal side effects | |||
| Short-term GH max output |
Favorable Partial Unfavorable
Combining a GHRP with a GHRH analogue produces synergistic GH release — the two compounds act on different receptor pathways (GHS-R1a and GHRH-R respectively) and their effects are additive to supra-additive. The most studied and widely referenced combination is CJC-1295 + ipamorelin, which is considered the gold-standard GHRH+GHRP research stack.
CJC-1295 + Ipamorelin
CJC-1295 + GHRP-2
CJC-1295 + GHRP-6
| Side Effect | Ipamorelin | GHRP-2 | GHRP-6 |
|---|---|---|---|
| Injection site reaction | Mild | Mild | Mild |
| Headache (post-injection) | Occasional | Occasional | Occasional |
| Flushing / warmth | Mild | Moderate | Mild |
| Cortisol elevation | None | Significant | Mild–moderate |
| Prolactin elevation | None | Significant | Mild |
| Appetite stimulation | Minimal | Mild | Pronounced |
| Water retention | Mild | Moderate | Mild |
| Fatigue / lethargy | Rare | Occasional | Rare |
All three are growth hormone secretagogues (GHS) that bind the ghrelin receptor (GHS-R1a) to stimulate pituitary GH release, but they differ critically in selectivity and side effect profiles. Ipamorelin is the most selective: it stimulates GH release without meaningfully elevating cortisol, prolactin, or ACTH. GHRP-2 produces strong GH pulses but also significantly elevates cortisol and prolactin. GHRP-6 produces moderate GH pulses with the most pronounced appetite stimulation and the highest ghrelin-mediated hunger side effects of the three.
GHRP-2 produces the largest GH pulse amplitude of the three, with studies demonstrating GH elevations 7–15× above baseline in dose-dependent fashion. Ipamorelin produces moderate but highly selective GH pulses (3–7× baseline), and GHRP-6 produces pulses broadly similar to ipamorelin but with greater inter-individual variability. For raw GH output, GHRP-2 leads — but at the cost of cortisol and prolactin co-stimulation.
For most research applications, ipamorelin is considered superior to GHRP-2 due to its selectivity profile. Ipamorelin produces GH release without the cortisol, prolactin, or ACTH co-stimulation seen with GHRP-2. Elevated cortisol is catabolic and counteracts the anabolic goals of GH stimulation. GHRP-2 may be preferred in short-term research contexts where maximum GH pulse amplitude is the primary endpoint and hormonal side effects are acceptable.
GHRP-6 stimulates the ghrelin receptor (GHS-R1a) with high potency and also has significant activity at the ghrelin receptor in the hypothalamus and gut, triggering the orexigenic (appetite-stimulating) pathway. This results in pronounced hunger, particularly in the 30–60 minutes following injection. Ipamorelin and GHRP-2 also bind GHS-R1a but with different receptor kinetics that produce less pronounced appetite stimulation — ipamorelin in particular has the lowest hunger side effect burden of the three.
Yes — combining a GHRP with a GHRH analogue produces synergistic GH release through complementary receptor pathways. The most studied combination is CJC-1295 + ipamorelin, which is considered the gold standard GHRH+GHRP stack. GHRP-2 + CJC-1295 produces higher peak GH but with cortisol co-elevation. GHRP-6 + CJC-1295 is less commonly studied. For research stacking protocols, ipamorelin is the preferred GHRP partner due to its selectivity.
GHRP-2 produces the highest GH pulse amplitude, which drives the greatest lipolytic (fat-mobilizing) response through IGF-1 and direct GH receptor activation on adipocytes. However, its cortisol co-stimulation can partially offset fat loss benefits. Ipamorelin's selective GH stimulation without cortisol elevation may produce a more favorable net body composition effect over longer research cycles. GHRP-6's appetite stimulation makes it the least favorable for fat loss research.
Standard research doses are: Ipamorelin 100–300 mcg per injection, 2–3× daily; GHRP-2 100–300 mcg per injection, 2–3× daily; GHRP-6 100–300 mcg per injection, 2–3× daily. All three are typically administered subcutaneously in a fasted state (2+ hours post-meal) to maximize GH pulse amplitude. When stacked with a GHRH analogue, the GHRP dose is typically at the lower end of the range (100 mcg) as the synergistic effect amplifies GH output.
Purgo Labs supplies COA-verified Ipamorelin, GHRP-2, and GHRP-6 for research purposes.
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