Hexarelin: A Potent GH Trigger That Stalled at Early Human Trials

Hexarelin is a lab-made peptide (a short chain of amino acids) that copies what the hunger hormone ghrelin does: it tells the body to release a quick, short burst of growth hormone (GH). Small human studies in the 1990s showed it does this dependably. But every one of those trials was tiny, brief, and measured stand-in lab numbers rather than real-world results. It was never approved anywhere, and it is banned in sport.

What it is

Hexarelin is a synthetic hexapeptide, meaning it is built from six amino acids. Its sequence is H-His-D-2-methyl-Trp-Ala-Trp-D-Phe-Lys-NH2, its molecular formula is C47H58N12O6, it weighs about 887 g/mol, and its CAS number (a unique chemical ID) is 140703-51-1. Scientists made it by tweaking an older peptide called GHRP-6, swapping one tryptophan building block for 2-methyl-tryptophan. That change makes it tougher against the body’s enzymes and lets it work when taken by mouth.

Its main trick is to switch on the growth-hormone-secretagogue receptor (GHS-R1a) — a docking site on cells that the natural hormone ghrelin also uses — found in the hypothalamus and pituitary (two control centers in the brain). Flipping that switch tells the body to release a pulse of GH, even though hexarelin’s chemical sequence looks nothing like ghrelin’s. At higher doses it also gives a modest nudge to ACTH, cortisol, and prolactin (three other hormones). On a separate track, hexarelin latches onto a docking site called CD36 in heart muscle, which researchers think may explain heart effects that seem to happen without any help from GH. It clears the bloodstream fairly quickly: its plasma half-life (the time for half a dose to leave the blood) is roughly 55 minutes.

The claims

In the research world, hexarelin was studied for two legitimate purposes: diagnosing and treating GH deficiency (in both children and adults), and protecting or supporting the heart after a heart attack or in heart failure.

On the gray market, it is sold for very different reasons — muscle growth, fat loss, faster recovery, anti-aging, and better sleep or skin. Those selling points are stretched from one fact (hexarelin causes a sharp, brief spike in GH) plus some rodent experiments. None of them has been tested in a controlled human trial that actually measured those specific results.

The evidence

The human evidence is real but early and small, and it leans almost entirely on stand-in measurements — hormone levels and heart-imaging numbers — rather than outcomes that matter to a patient, like growth, body composition, or survival.

In a 1994 double-blind, placebo-controlled study (Imbimbo et al.) — meaning neither the volunteers nor the researchers knew who got the real drug, and some got a dummy for comparison — 12 healthy men received hexarelin by IV (straight into a vein). GH went up in step with the dose, topping out around 55 ng/mL at the highest dose versus about 3.9 on placebo, peaking near the 30-minute mark. A 1996 dose-response study (Massoud, Hindmarsh & Brook) confirmed the strong GH release, but also showed that prolactin and cortisol creep up at higher doses while barely moving at low doses.

On the heart side, a 2001 study (Broglio et al.) gave a single IV dose of hexarelin to small groups — 7 healthy people, 7 with severe GH deficiency, and patients with cardiomyopathy (a diseased heart muscle). It raised left-ventricular ejection fraction (a measure of how much blood the heart pumps out with each beat) in the healthy and GH-deficient subjects through a direct effect on the heart muscle that seemed to work without GH. But these were tiny, one-time doses measuring an imaging number, not patient outcomes.

A big catch showed up with longer dosing. In a 16-week study (Rahim, O’Neill & Shalet, 1998), the GH response roughly halved over time — a partial, reversible desensitization (the body stops responding as strongly) that bounced back after people stopped. A companion 1999 paper found that long-term dosing did not over-rev the adrenal hormones or prolactin. Lab work backs up the fading-response story: in cells engineered to carry the human receptor, the response blunted within just 2 to 5 minutes of a first dose (Orkin et al., 2003), and the way the receptor was regulated changed with age in rats (Bresciani et al., 2004).

The largest pile of research is animal heart work, all done in rodents — better heart function after an experimentally induced heart attack, and heart protection that did not depend on GH in rats and mice. That hints at something, but it is not proof in humans.

The skeptical read: there is no Phase III, no repeated randomized controlled trial, and no hard data on real clinical outcomes. The human heart signal comes from single doses and is small. A lot of the early human and discovery work was done by the developer’s own scientists — normal for early-stage drug companies, but worth keeping in mind. And the fast, reversible drop in response is a real pharmacological snag that helped kill development. The popular muscle and anti-aging claims, in particular, have no controlled human trials behind them at all.

Safety and side effects

In the short, low-dose controlled studies, hexarelin was generally well tolerated. The acute effects people reported include brief flushing or warmth and mild hunger — both of which fit with switching on the ghrelin receptor.

The clearest dose-related issue is that it nudges other hormones it is not meant to: prolactin and cortisol/ACTH go up at higher doses, while low doses mostly leave them alone. The 16-week study did not show over-activation of the adrenal hormones or prolactin. The more practical limit is tachyphylaxis — a fancy word for the response fading with repeated use (blunting within minutes in cells, GH output roughly halved by 16 weeks in people) before recovering after a break. That caps how useful it stays over time more than it points to anything toxic.

The honest caveats: there is no long-term human safety data, and broader concerns for this class of drug — such as effects on blood sugar and insulin from revving up the GH system — are not well studied for hexarelin specifically in humans. On top of that, anything sold as a “research chemical” is unregulated; what’s in the vial, how pure it is, how strong it is, and whether it’s sterile are all unverified, and that is a separate danger on its own. Nothing here is medical advice.

Legal and regulatory status

Hexarelin itself was never approved for any use by the FDA, the EMA (Europe’s drug regulator), or any other regulator. It made it only to Phase II for GH deficiency and heart failure before development was dropped, and it was never sold as a medicine.

There’s a common mix-up worth clearing up. A May 14, 2007 FDA orphan-drug designation often credited to “hexarelin” was actually given to a different compound — ARD-07 / EP-1572, an oral GH secretagogue (Ardana Bioscience, licensed from Aeterna Zentaris) for diagnosing adult GH deficiency. An orphan designation (a special status for treatments of rare conditions) is not the same as approval to sell a drug. And contrary to the idea that the program just fizzled out, that same molecule came back as macimorelin (AEZS-130) and was FDA-approved on December 20, 2017 as Macrilen for diagnosing adult GH deficiency. So an oral cousin of hexarelin did eventually reach the market — but hexarelin itself never did.

In sport, the World Anti-Doping Agency bans hexarelin at all times under class S2.2.4 (GH-releasing peptides). The 2026 Prohibited List names “examorelin (hexarelin)” outright, alongside related GHRPs. Since it has no approved medical use, there is no realistic way to get a therapeutic use exemption (special permission to use a banned substance for a genuine medical need).

In practice it is sold online as a “research chemical / not for human consumption” — a label that comes with no requirement to prove what is actually in the vial.

Bottom line

Hexarelin reliably sets off a short GH pulse in humans, and small single-dose studies hint at a direct effect on the heart muscle. But every human study is tiny, brief, and built on stand-in markers; a quick, reversible loss of response helped end its development; it was never approved anywhere; and it is banned at all times in sport. The popular muscle and anti-aging claims have no controlled human evidence behind them.

Evidence grade: 6/10 · Preliminary.

Sources

• Imbimbo BP, et al. Growth hormone-releasing activity of hexarelin in humans. Eur J Clin Pharmacol. 1994.
• Massoud AF, Hindmarsh PC, Brook CGD. Hexarelin-induced growth hormone, cortisol, and prolactin release. JCEM. 1996;81(12).
• Broglio F, et al. Cardiac effects of hexarelin. Endocrine. 2001;14(1):105.
• Rahim A, O’Neill PA, Shalet SM. Desensitization to hexarelin over 16 weeks. JCEM. 1998;83(5):1644-1649.
• Rahim A, O’Neill PA, Shalet SM. Chronic hexarelin and the pituitary-adrenal axis. Clin Endocrinol (Oxf). 1999;50(1):77-84.
• Orkin R, et al. Rapid desensitization of the GHS receptor in vitro. J Endocrinol Invest. 2003;26(8):743-747.
• Bresciani E, et al. Age-dependent GHS-R1a mRNA regulation by hexarelin. Neuroendocrinology. 2004;80(1):52-59.
• Deghenghi R. The development of “impervious peptides” as growth hormone secretagogues. Acta Paediatr Suppl. 1997;423:85-87.
• Tivesten Å, et al. Hexarelin improves cardiac function after experimental MI in rats. Endocrinology. 2000.
• Locatelli V, et al. GH-independent cardioprotective effects of hexarelin in rats. Endocrinology. 1999.
• McDonald H, et al. Hexarelin preserves function and reduces fibrosis in mouse MI. Physiol Rep. 2018.
• Agbo P, et al. Hexarelin modulates PTEN to attenuate heart failure in rats. Turk J Med Sci. 2019.
• Bodart V, et al. CD36 mediates the cardiovascular action of GH-releasing peptides in the heart. Circ Res.
• Examorelin (hexarelin) — overview.
• Medscape — New FDA Orphan Drugs (ARD-07 designation, May 14, 2007).
• Drugs.com — Macrilen (macimorelin) approval history (Dec 20, 2017).
• Macimorelin — overview.
• World Anti-Doping Agency — 2026 Prohibited List (S2.2.4; names examorelin/hexarelin).
• Drugs.com — WADA S2: Peptide Hormones, Growth Factors and Related Substances.