Melanotan II

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Melanotan II is a synthetic analogue of the naturally occurring hormone alpha-melanocyte-stimulating hormone. It is a cyclic peptide that contains seven amino acids.

Its purported benefits include stimulation of skin tanning by increasing melanin production. It is also described as having effects on appetite and sexual function in research settings. These reported effects come from studies examining how it interacts with melanocortin receptors.

Melanotan II has been studied for its ability to increase energy expenditure and suppress appetite. In neonatal rats, MT-II reduced food intake and slowed weight gain while elevating brown fat thermogenesis, as shown by increased uncoupling protein 1 mRNA in their adipose tissue. In a mouse model lacking pituitary adenylate cyclase-activating polypeptide (PACAP), chronic MT-II injection during cold exposure partially restored their impaired thermogenic capacity ^{1, 2}.

Researchers have also found that Melanotan II can enhance sexual function in both humans and animals. In a clinical trial with men who had erectile dysfunction, MT-II induced erections in 8 out of 10 participants without external sexual stimulation. In a larger crossover study, 17 of 20 men reported spontaneous erections and more than two-thirds said they felt increased sexual desire after MT-II compared to placebo ^{3, 4}. n female rats, MT-II increased proceptive behaviors (such as hops, darts, and ear wiggling), suggesting a brain-mediated boost in sexual motivation ⁵. All in all, Melanotan II is associated with reduced appetite and boosted energy use, and it also appears to have the potential to enhance sexual arousal and function.

Melanotan II has been associated with nausea, yawning, and stretching in clinical studies. In a phase-I trial, healthy volunteers reported mild nausea and somnolence after MT-II injections. In larger crossover studies of men with erectile dysfunction, nausea and yawning were commonly reported side-effects, with some individuals experiencing severe nausea at certain doses ^{6, 7}. In animal studies, Melanotan II caused profound, transient hypothermia in mice, likely via activation of mast cells and histamine release ⁸. In a human case report, self-administration of MT-II led to priapism (a prolonged, painful erection) requiring medical intervention ⁹. Overall, reported negative effects include nausea, yawning, flushing, hypothermia in animals, and risk of priapism in humans.

Melanotan II binds non-selectively to several melanocortin receptors (MC1R, MC3R, MC4R, and MC5R), mimicking α-MSH. When MC1R on melanocytes is activated, it stimulates adenylate cyclase to increase cAMP, which activates PKA and phosphorylates CREB; CREB then drives expression of MITF and tyrosinase, increasing melanin synthesis. In the central nervous system, MT-II’s activation of MC4R contributes to sexual behavior and appetite regulation. Furthermore, animal studies show that MC4R signaling via MT-II can modulate nociception and energy homeostasis ^{10, 11}.

Melanotan II is not FDA-approved for human use, and in fact the FDA has issued warning letters over its illegal marketing as an injectable tanning agent. The FDA also classifies it as a Category 2 bulk drug substance for compounding, meaning it carries significant safety risks for pharmacies and so should not be routinely compounded. On the athletic anti-doping front, Melanotan II falls into the broad category of peptide hormones, which are monitored or restricted under WADA/USADA rules — while it may not be explicitly listed by name, athletes would face significant risk or scrutiny for using such unapproved performance-affecting peptides ^{12, 13}.