Ipamorelin vs Other GHRPs: Selectivity Compared
Ipamorelin
80 nmol/kg
The ED50 for ipamorelin-induced growth hormone release in rats, comparable to GHRP-6 at 115 nmol/kg but without cortisol or prolactin elevation.
Raun et al., European Journal of Endocrinology, 1998
Raun et al., European Journal of Endocrinology, 1998
View as imageGrowth hormone releasing peptides (GHRPs) all do the same basic thing: they bind to the ghrelin receptor (GHS-R1a) on pituitary somatotroph cells and trigger growth hormone (GH) release. But they differ in what else they do. GHRP-6 makes you hungry. GHRP-2 raises cortisol and prolactin. Hexarelin elevates ACTH. Ipamorelin, developed by Novo Nordisk in the 1990s, was specifically designed to release GH without these off-target hormonal effects. The pillar article on ipamorelin covers its full research profile. This article compares the selectivity data head-to-head across the GHRP class and explains why these differences matter. For comparisons between GHRP-2 and GHRP-6 specifically, see the dedicated article on how those two GHRPs differ.
Key Takeaways
- Ipamorelin released GH with an ED50 of 80 nmol/kg in rats, comparable to GHRP-6 (ED50 of 115 nmol/kg), but did not elevate ACTH or cortisol at doses up to 1 mg/kg intravenously (Raun et al., 1998)
- GHRP-2 and hexarelin both significantly increased plasma ACTH, cortisol, and prolactin in human subjects alongside GH release (Arvat et al., 1997)
- GHRP-6 activates ghrelin signaling in hypothalamic arcuate nucleus neurons, explaining its potent appetite-stimulating effect that ipamorelin lacks (Hewson and Bhatt, 1999)
- Ipamorelin produced dose-dependent GH release without affecting FSH, LH, TSH, or PRL in swine models, making it the most hormonally selective GHRP tested (Raun et al., 1998)
- All GHRPs in this class remain unapproved for clinical use as GH secretagogues; ipamorelin reached phase 2 trials for postoperative ileus but not for GH-related indications
- The selectivity advantage is established in animal models but unconfirmed in large human trials
The Landmark Selectivity Study
The defining paper for ipamorelin's selectivity profile is Raun et al. (1998), published in the European Journal of Endocrinology. This study, conducted by researchers at Novo Nordisk, systematically compared ipamorelin against GHRP-6 and growth hormone-releasing hormone (GHRH) across multiple hormonal endpoints.[1]
In pentobarbital-anesthetized rats, ipamorelin released GH with an ED50 of 80 +/- 42 nmol/kg and a maximum efficacy (Emax) of 1,545 +/- 250 ng GH/mL. GHRP-6 had a comparable profile: ED50 of 115 +/- 36 nmol/kg and Emax of 1,167 +/- 120 ng GH/mL. The two peptides were roughly equivalent at stimulating GH release.
The difference appeared in other hormones. At GH-releasing doses, GHRP-6 elevated ACTH and cortisol. Ipamorelin did not. Even at intravenous doses of 1 mg/kg in swine (well above the GH-releasing ED50), ipamorelin produced no detectable changes in plasma ACTH, cortisol, follicle-stimulating hormone (FSH), luteinizing hormone (LH), thyroid-stimulating hormone (TSH), or prolactin (PRL). The only hormonal change observed was GH elevation.
Raun et al. titled their paper "Ipamorelin, the first selective growth hormone secretagogue" because no previously studied GHRP had demonstrated this degree of hormonal selectivity. The researchers proposed that ipamorelin's selectivity results from specific structural features of the pentapeptide that allow tight binding to GHS-R1a on somatotrophs while avoiding activation of receptors or signaling pathways in corticotrophs and lactotrophs.
GHRP-2: Potent But Less Selective
GHRP-2 (pralmorelin) is considered the most potent GHRP for GH release on a per-dose basis. However, this potency comes with broader hormonal activation.
Arvat et al. (1997) published a direct comparison in Peptides, testing GHRP-2 and hexarelin against GHRH, TRH, and hCRH in healthy human volunteers. Both GHRP-2 and hexarelin significantly increased not only GH but also ACTH, cortisol, and prolactin.[2]
The cortisol elevation from GHRP-2 is clinically relevant because chronic cortisol elevation promotes visceral fat deposition, insulin resistance, muscle breakdown, and immune suppression. For someone using a GHRP to enhance body composition (a common off-label motivation), elevating cortisol simultaneously could partially counteract the anabolic effects of increased GH.
The prolactin increase is also notable. Prolactin elevation can cause gynecomastia in men, menstrual irregularities in women, and reduced libido in both sexes. While the acute prolactin increase from a single GHRP-2 dose is transient, the effects of repeated daily dosing on prolactin homeostasis have not been studied in long-term human trials.
Beyond hormonal effects, GHRP-2 has shown activity at other targets. Yamamoto et al. (2008) demonstrated that GHRP-2 directly acted on myocytes to attenuate dexamethasone-induced expression of muscle-specific ubiquitin ligases (Atrogin-1 and MuRF1), suggesting anti-catabolic effects independent of GH release.[3] Zeng et al. (2014) found that GHRP-2 produced antinociceptive (pain-reducing) effects at the supraspinal level via opioid receptors in mice.[4] These extra-GH actions illustrate that GHRP-2's lack of selectivity is a double-edged quality: it affects more systems, which means both more side effects and potentially more therapeutic applications.
GHRP-6: The Hunger Peptide
GHRP-6 is the oldest and most studied GHRP. It releases GH effectively but is best known for its potent appetite-stimulating effect, which is absent with ipamorelin.
The appetite mechanism was clarified by Hewson and Bhatt (1999), who demonstrated that GHRP-6 induced changes in electrical activity of neurons in the hypothalamic arcuate nucleus, ventromedial nucleus, and periventricular nucleus in a brain slice preparation. These are the brain regions that regulate hunger and energy balance.[5] The effect occurs because GHRP-6 is a ghrelin receptor agonist, and ghrelin is the primary endogenous hunger hormone. Activating the ghrelin receptor in hypothalamic feeding centers triggers appetite as a direct pharmacological consequence.
Ipamorelin also binds the ghrelin receptor but does not produce the same appetite stimulation. The likely explanation involves receptor binding kinetics and signaling bias: different ligands can bind the same receptor but activate different downstream signaling cascades. Ipamorelin may preferentially activate the GH-releasing signaling pathway while having lower efficacy at the appetite-signaling pathway. This phenomenon, called "biased agonism" or "functional selectivity," is well documented at other G-protein coupled receptors but has not been definitively characterized for ipamorelin at GHS-R1a.
For a deep dive into why GHRP-6 causes hunger and what that means for its use profile, see the dedicated article.
GHRP-6 also has non-GH activities. Sabatino et al. (2011) investigated the structure-activity relationships of GHRP-6 at the CD36 scavenger receptor, finding that GHRP-6 and related azapeptides could bind CD36. This receptor is involved in fatty acid uptake and foam cell formation in atherosclerosis, representing a completely separate biological activity from GH release.[6]
Hexarelin: Strong GH Release, Full Hormonal Package
Hexarelin (examorelin) is a synthetic hexapeptide that produces robust GH release but with the broadest off-target hormonal profile of any GHRP.
Deghenghi et al. (1994) published the original characterization in Life Sciences, demonstrating potent GH-releasing activity in both infant and adult rats.[7] The Arvat et al. (1997) human study confirmed that hexarelin significantly elevated ACTH, cortisol, and prolactin alongside GH.[2]
Hexarelin has one distinguishing feature: neuroprotective activity. Brywe et al. (2005) demonstrated that hexarelin reduced neonatal brain injury in a rat model and altered Akt/glycogen synthase kinase-3beta phosphorylation, a key cell survival signaling pathway. This neuroprotective effect was independent of GH release, suggesting hexarelin has direct central nervous system activity.[8]
Baser et al. (2021) showed that hexarelin attenuated antinociceptive tolerance to morphine in rats, suggesting potential applications in pain management that are unrelated to its GH-releasing properties.[9]
Head-to-Head Comparison Table
| Property | Ipamorelin | GHRP-2 | GHRP-6 | Hexarelin |
|---|---|---|---|---|
| GH release potency | High | Highest | High | High |
| ACTH/cortisol elevation | None at GH doses | Yes | Moderate | Yes |
| Prolactin elevation | None | Yes | Mild | Yes |
| Appetite stimulation | Minimal | Moderate | Strong | Moderate |
| Unique extra-GH effects | GI motility (prokinetic) | Anti-catabolic, analgesic | CD36 binding | Neuroprotective |
| Human trial data | Phase 2 (ileus) | Limited | Limited | Limited |
| Structure | Pentapeptide | Hexapeptide | Hexapeptide | Hexapeptide |
Ipamorelin Beyond GH: The Gut Motility Connection
While ipamorelin's selectivity for GH release is its primary differentiator, it has another studied property: prokinetic activity in the gastrointestinal tract.
Venkova et al. (2009) tested ipamorelin in a rodent model of postoperative ileus (the gut paralysis that commonly follows abdominal surgery). Ipamorelin significantly improved gastric emptying and intestinal transit, demonstrating prokinetic effects comparable to established treatments. The mechanism likely involves ghrelin receptor activation on enteric neurons, similar to how ghrelin itself promotes gut motility.[10]
This finding led to ipamorelin's only significant clinical development program. It entered phase 2 trials for postoperative ileus, though the development was ultimately discontinued. No GHRP, including ipamorelin, has received FDA approval for any indication.
Johansen et al. (1999) demonstrated that ipamorelin induced longitudinal bone growth in rats at doses that stimulated GH release, confirming that the GH elevated by ipamorelin is biologically active and capable of downstream anabolic effects.[11]
Pharmacokinetic Differences
Johansen et al. (1998) published a pharmacokinetic evaluation of ipamorelin alongside other peptidyl growth hormone secretagogues. Ipamorelin showed rapid absorption and clearance with a half-life of approximately 2 hours after subcutaneous administration. The study also evaluated nasal absorption, finding that intranasal bioavailability was low (approximately 7%) but detectable.[12]
All GHRPs share similar pharmacokinetic limitations: short half-lives requiring multiple daily injections, poor oral bioavailability (peptides are degraded in the gut), and limited tissue penetration. These properties are why MK-677 (ibutamoren), a non-peptide oral GH secretagogue, gained attention despite having its own selectivity issues. The GHRH pathway, represented by peptides like CJC-1295 and sermorelin, offers a complementary approach that works upstream at the hypothalamic level. The common practice of combining ipamorelin with CJC-1295 is covered in the cluster article on why they are paired together.
What Selectivity Means in Practice
Ipamorelin's selectivity advantage is real but requires context:
The data is primarily from animal models. The Raun et al. (1998) study was conducted in rats and swine. While the Arvat et al. (1997) study confirms that GHRP-2 and hexarelin elevate cortisol and prolactin in humans, the corresponding human data for ipamorelin's lack of these effects is limited to small studies.
Selectivity does not mean safety. Ipamorelin still activates the ghrelin receptor, and long-term consequences of chronic ghrelin receptor activation (even selectively) are unknown. GH elevation itself carries risks including insulin resistance, fluid retention, joint pain, and theoretical concerns about cancer promotion. These risks apply regardless of which GHRP produces the GH elevation.
The clinical development gap is large. None of these peptides has been approved for GH augmentation. Tesamorelin (a GHRH analog, not a GHRP) is the only FDA-approved peptide in this space, and its approval is limited to HIV-associated lipodystrophy. The selectivity comparison, while scientifically valid, is comparing unapproved compounds against each other.
Anti-doping considerations. All GHRPs are prohibited by WADA (World Anti-Doping Agency) under the category of GH secretagogues. Ipamorelin's selectivity does not make it any less detectable or any less prohibited in competitive sport.
The Bottom Line
Ipamorelin is distinguished from other GHRPs (GHRP-2, GHRP-6, hexarelin) by its selective stimulation of GH release without concurrent elevation of cortisol, ACTH, or prolactin. This was demonstrated in the landmark 1998 study by Raun et al. GHRP-2 and hexarelin both raise cortisol and prolactin in humans. GHRP-6 causes strong appetite stimulation that ipamorelin avoids. All these peptides remain unapproved for GH-related indications, and the selectivity data comes primarily from animal models with limited human confirmation.