Growth Hormone Releasing Peptides (GHRPs): GHRP-2, GHRP-6, Hexarelin & Ipamorelin Compared

The Four GHRPs Compared

How GHRPs Work: The Ghrelin Receptor Pathway

All four GHRPs bind the growth hormone secretagogue receptor type 1a (GHSR-1a), the same receptor that ghrelin — the body's endogenous hunger hormone — uses to trigger GH release from pituitary somatotroph cells.

When GHSR-1a is activated, it triggers an intracellular calcium cascade that causes somatotroph cells to release stored GH granules. This is mechanistically distinct from how GHRH works — GHRH binds its own receptor (GHRH-R) and stimulates GH synthesis and release via cAMP signaling. Because the two pathways converge on the same cell through different mechanisms, combining a GHRP with a GHRH analog produces synergistic GH release far greater than either alone.

The selectivity difference between GHRPs comes down to receptor specificity. GHRP-6 and GHRP-2 activate GHSR-1a but also cross-react with receptors on corticotroph cells (triggering ACTH and cortisol release) and lactotroph cells (triggering prolactin). Ipamorelin, by contrast, was specifically engineered to bind only the GH-releasing component of GHSR-1a without activating these off-target pathways — making it, as the original 1998 Novo Nordisk study described it, "the first selective growth hormone secretagogue."

GHRP-6: First Generation — Strong but Crude

GHRP-6 was the first synthetic GHRP developed (Bowers et al., 1984) and remains the most widely studied. It produces reliable GH release at doses of 100–150 mcg subcutaneously, but its strong ghrelin receptor activation causes intense hunger — often within 15–20 minutes of injection. This makes it useful for underweight patients or bodybuilders in a gaining phase, but problematic for anyone trying to lose fat.

At standard doses, GHRP-6 elevates cortisol and prolactin in a dose-dependent manner. A 1999 study in the Journal of Neuroendocrinology (Frieboes et al.) found that 100 mcg GHRP-6 administered subcutaneously significantly increased both ACTH and cortisol alongside GH. For most users, these elevations are transient and clinically insignificant, but chronic use may matter for individuals already dealing with elevated cortisol or prolactin.

GHRP-2: Higher Potency, Moderate Side Effects

GHRP-2 produces more intense GH release than GHRP-6 with a less extreme hunger response. A study published in the European Journal of Endocrinology (Arvat et al., 1997) compared GHRP-2 and hexarelin head-to-head and found both to be potent GH secretagogues with "slight stimulatory effect on PRL, ACTH and cortisol levels" — though GHRP-2 shows slightly more cortisol/prolactin elevation than hexarelin at equivalent GH-releasing doses.

Standard dosing is 100–300 mcg subcutaneously, 2–3 times daily. GHRP-2 is often chosen over GHRP-6 by users who want stronger GH output without extreme hunger. However, the cortisol and prolactin effects mean it's not ideal for long-term use without monitoring.

Hexarelin: Strongest GH Output, but Desensitizes

Hexarelin produces the highest raw GH output of any GHRP — exceeding GHRP-2 in direct comparisons. However, it has a critical limitation: receptor desensitization. After 4–8 weeks of continuous use, hexarelin's efficacy drops significantly as GHSR-1a receptors downregulate. This makes it unsuitable for long-term protocols.

Hexarelin also raises prolactin (~80% increase) and cortisol (~40% at 0.5 μg/kg) more than other GHRPs. Age-related research (Arvat et al., 1997) found that the prolactin and cortisol responses to hexarelin vary significantly with age — younger subjects show less off-target hormone stimulation. Dosing is typically 100–200 mcg subcutaneously, used in short 4–6 week bursts rather than sustained protocols.

Ipamorelin: Third Generation — Selective and Sustainable

Ipamorelin represents the third generation of GHRPs, developed by Novo Nordisk with a specific goal: selective GH release without cortisol, prolactin, or appetite effects. The landmark 1998 study by Raun et al. in the European Journal of Endocrinology demonstrated that "unlike GHRP-6 and GHRP-2, ipamorelin does not release ACTH or cortisol, and is highly selective for inducing the secretion only of GH" — even at doses up to 100 times the effective GH-releasing dose.

This selectivity makes ipamorelin the safest and most sustainable GHRP for long-term use. It doesn't desensitize the way hexarelin does, doesn't cause the hunger spikes of GHRP-6, and doesn't elevate stress hormones. The tradeoff: it produces less raw GH output per dose than GHRP-2 or hexarelin. For most optimization purposes, this is an acceptable tradeoff — especially when combined with CJC-1295 for synergistic release.

Stacking GHRPs with GHRH Analogs

The most effective way to use any GHRP is in combination with a GHRH analog. The two peptide classes act on different receptors on the same pituitary cells, producing synergistic GH release 3–5x greater than either alone:

• GHRP + CJC-1295 (with DAC): CJC-1295 with DAC has a half-life of 6–8 days, providing sustained baseline GHRH stimulation. Combined with a GHRP (usually ipamorelin), this creates continuous GH elevation with natural pulsatility. Typical protocol: CJC-1295 DAC 2 mg/week + ipamorelin 200–300 mcg 2–3x/day.
• GHRP + Mod GRF 1-29 (CJC-1295 no DAC): Mod GRF 1-29 has a short half-life (~30 min), so it must be injected alongside the GHRP. This produces sharper GH pulses — closer to natural physiology. Typical protocol: 100 mcg Mod GRF + 100–200 mcg ipamorelin, 2–3x/day.
• GHRP + Sermorelin: Sermorelin is the original GHRH analog (29 amino acids). Effective but requires more frequent dosing than CJC-1295. Typical protocol: 200–300 mcg sermorelin + 200 mcg ipamorelin before bed.