Hexarelin has no approved label anywhere, so there is no regulator-reviewed adverse-effect list. What follows is drawn from healthy-volunteer studies, small patient studies, and receptor pharmacology — with the source of each observation stated.
The short answer
Hexarelin's most reliably documented downside is not an adverse event in the conventional sense: it is that the drug stops working as well. Over 16 weeks of twice-daily subcutaneous dosing in 12 healthy elderly volunteers, the growth hormone area under the curve fell from 19.1 to 10.5 µg/L/h — roughly a halving — before recovering to baseline four weeks after stopping (Rahim & Shalet, Growth Horm IGF Res 1998). The second distinctive feature is that hexarelin has a documented direct action on the heart through CD36, independent of growth hormone — a genuine pharmacological difference from the rest of this class, and one with no long-term human safety data attached to it.
Desensitisation, measured over 16 weeks
The tachyphylaxis is the best-quantified property of this compound. Twelve healthy elderly subjects on twice-daily subcutaneous hexarelin had GH area under the curve measured at weeks 0, 1, 4 and 16: 19.1, 13.1, 12.3 and 10.5 µg/L/h. The decline was significant overall (p = 0.0003) and individually significant at weeks 4 and 16. Four weeks after the study ended, a further hexarelin dose produced an AUC of 19.4 — not significantly different from baseline (Rahim & Shalet 1998).
A single-day study showed how fast it starts. After two or three subcutaneous doses of 1.5 µg/kg, an intravenous hexarelin challenge produced a blunted GH response and an entirely abolished ACTH and cortisol response (Maccario et al., Eur J Endocrinol 2002). Reviews of the class describe this as partial desensitisation, more marked with continuous exposure than with intermittent dosing (Ghigo et al., Eur J Endocrinol 1997).
Fading response is not directly harmful. The safety issue is behavioural: the predictable reaction is to raise the dose, which moves exposure into a range where no human data exists — and, for hexarelin specifically, toward the acute doses at which the hormonal spillover does appear.
The hormonal picture is better than its reputation
Hexarelin is routinely described as raising prolactin and cortisol. The most detailed human measurement does not support that at subcutaneous doses. Across 24 hours of sampling every 20 minutes, in six healthy young men given hexarelin 1.5 µg/kg subcutaneously two or three times, "no change occurred in the secretion of IGF-I, PRL, ACTH and cortisol" — the GH amplification was selective (Maccario 2002).
Where the reputation comes from is acute high-dose intravenous work across the peptide family. In the original human study of the parent hexapeptide, prolactin and cortisol rose about two-fold above baseline — but only at the top dose of 1 µg/kg intravenously, not at 0.1 or 0.3 (Bowers et al., J Clin Endocrinol Metab 1990). The accurate statement is that this class can raise prolactin and cortisol in a dose- and route-dependent way, and that hexarelin at the subcutaneous doses studied did not.
The cardiovascular action nobody else has
Hexarelin binds CD36 in the heart, a target distinct from the ghrelin receptor. In rat cardiac membranes the labelled binding protein was identified as CD36 by N-terminal sequencing, and hexarelin's dose-dependent increase in coronary perfusion pressure was absent in hearts from CD36-null mice and CD36-deficient rats. The authors suggested the mechanism could be relevant to coronary vasospasm in hypercholesterolaemia and atherosclerosis (Bodart et al., Circ Res 2002).
In humans, a single 2 µg/kg intravenous dose raised ejection fraction, cardiac index and cardiac output in patients with coronary artery disease during bypass surgery, an effect absent with recombinant GH, GHRH or placebo, and accompanied by a slight rise in mean arterial pressure (Broglio et al., Eur J Pharmacol 2002). In an earlier series, ejection fraction rose in healthy adults and in GH-deficient patients but not in patients with severe dilated cardiomyopathy (Broglio et al., Endocrine 2001).
Read as a safety statement rather than a benefit: hexarelin measurably changes cardiac contractility and coronary vascular tone in humans, acutely, at microgram-per-kilogram doses. No study has examined what repeated dosing does to a heart over months, and the acute studies were carried out under anaesthesia with invasive monitoring in place.
Class risks and what was never studied
The general growth hormone secretagogue risks apply. A class review found these compounds generally well tolerated in short studies but singled out increases in blood glucose driven by decreased insulin sensitivity, and called explicitly for evaluation of cancer incidence and mortality with long-term use (Sigalos & Pastuszak, Sex Med Rev 2018). Fluid retention, joint aching and glucose drift track GH elevation itself; in the two-year MK-677 trial in older adults, fasting glucose rose about 5 mg/dL and the commonest complaints were increased appetite, transient mild lower-extremity oedema and muscle pain (Nass et al., Ann Intern Med 2008).
What does not exist for hexarelin: any carcinogenicity study, any reproductive toxicology, any exposure beyond 16 weeks, and any post-marketing surveillance, since it has never been marketed as a medicine. Material bought outside a pharmacy supply chain also carries no verified identity, purity, aggregate or endotoxin data — the attributes that drive immunogenicity risk in an injected peptide.
Hexarelin is prohibited in sport at all times under WADA section S2; it is extensively metabolised and both parent compound and metabolites are targeted in validated urine assays (Semenistaya et al., Drug Test Anal 2015).
See also hexarelin benefits and dosing education.
Sport & Anti-Doping Warning
Hexarelin is another potent GHRP-family peptide that anti-doping laboratories monitor; it appears in WADA-target lists and in research focused on detecting GH secretagogue misuse.
Although less well-known than GHRP-2 or GHRP-6 in the media, hexarelin is handled the same way under anti-doping rules and remains prohibited.