Hexarelin: The Forgotten Growth-Hormone Peptide
A potent GH secretagogue sidelined by tachyphylaxis and its cortisol and prolactin release — and what it taught the field.
Hexarelin rarely comes up in current peptide discussions, which is itself telling. It was once among the more potent growth-hormone-releasing peptides studied in humans, and then it largely faded. That arc — strong initial promise followed by quiet retreat — makes it a useful case study in why “more potent” doesn’t automatically win.
This is a look back at a sidelined compound, not a pitch to revive it.
What the human dose-response data showed
Hexarelin is a synthetic hexapeptide GH secretagogue. Its acute potency is well documented. In a 1996 dose-response study in healthy adult males by Massoud, Hindmarsh, and Brook, published in The Journal of Clinical Endocrinology & Metabolism, intravenous hexarelin produced a dose-dependent GH surge — a plateau near 140 mU/L at 1.0 µg/kg, with a half-maximal dose (ED50) of about 0.48 µg/kg.
The problem was what came with it. In the same study, hexarelin was not GH-selective: it drove prolactin up to a plateau of roughly 180% above baseline at 1.0 µg/kg (ED50 ~0.39 µg/kg) and produced a step increase in cortisol of about 40% at 0.5 µg/kg. A peptide meant to release GH was also pushing prolactin and the cortisol axis.
| Hormone | Effect (Massoud et al., 1996) |
|---|---|
| GH | Plateau ~140 mU/L at 1.0 µg/kg; ED50 ~0.48 µg/kg |
| Prolactin | Plateau ~180% above baseline; ED50 ~0.39 µg/kg |
| Cortisol | Step increase ~40% at 0.5 µg/kg |
Hexarelin’s potency was real, but it wasn’t clean: at GH-releasing doses it also raised prolactin and cortisol. Raw acute potency was never the right design target.
The tolerance problem
The second issue is tachyphylaxis. Among GH secretagogues, hexarelin is notably prone to it: the GH-releasing effect blunts with repeated exposure as the receptor desensitizes, undercutting any sustained use. Strong first pulse, diminishing returns after.
There has also been scattered interest in hexarelin’s effects outside GH release, including exploratory work on cardiac tissue, but that remains preliminary and far from established clinical use.
The takeaway
Hexarelin was a potent early GH secretagogue undone by a combination of tachyphylaxis and its off-target prolactin and cortisol release at the same doses that released GH. The well-documented part of its story is the acute dose-response data; the human evidence for any sustained benefit is thin. As a research lesson it is valuable — it helped clarify that the target is a sustainable, GH-selective release pattern, not the biggest possible single pulse.