Melanotan I (afamelanotide) is an alpha-MSH analogue with relative selectivity for the melanocortin-1 receptor, the receptor governing melanogenesis. Melanotan II is a non-selective analogue active across MC1R, MC3R, MC4R and MC5R, which is why its reported effects extend beyond pigmentation into appetite and sexual function. That breadth is also why Melanotan II carries explicit safety warnings from the MHRA and TGA, and why PT-141 was developed from it by narrowing the receptor profile back down.
Both descend from alpha-melanocyte-stimulating hormone, the endogenous peptide that drives melanogenesis. Melanotan I — afamelanotide — is a linear analogue with relative selectivity for the melanocortin-1 receptor, the receptor that governs pigment production.
Melanotan II is a cyclic analogue that binds across the melanocortin family: MC1R, MC3R, MC4R and MC5R. It is not a refined version of Melanotan I. It is a broader one, and the breadth is the entire story.
MC4R is centrally involved in appetite regulation and in sexual arousal pathways. MC3R contributes to energy homeostasis. Binding them is why Melanotan II reports effects — appetite suppression, spontaneous erections, nausea — that a compound acting mainly on MC1R does not produce.
The strongest evidence that this breadth was considered a liability is PT-141. Bremelanotide was developed from Melanotan II by narrowing the receptor profile toward MC3R and MC4R and abandoning the pigmentation activity — deliberately undoing the promiscuity rather than building on it.
Continue reading:PT-141 vs Melanotan II
Melanotan II has drawn explicit public warnings from national regulators. The MHRA in the United Kingdom and the TGA in Australia have both issued statements about unlicensed supply, citing the absence of safety evaluation and the risks of unregulated injectable products sold for cosmetic purposes.
Separately, the literature includes case reports of changes in melanocytic naevi following Melanotan use. The causal picture is not settled, but a compound that stimulates melanocytes and is used without dermatological oversight is not a neutral proposition.
Both arrive lyophilised, both require reconstitution with bacteriostatic water, and both need the reconstituted vial refrigerated and used within its working window. Neither tolerates repeated freeze-thaw cycles.
Receptor selectivity. Melanotan I acts with relative selectivity at MC1R, the pigmentation receptor. Melanotan II acts across MC1R, MC3R, MC4R and MC5R, which is why it reports effects on appetite and sexual function that Melanotan I largely does not.
No. It is a broader one, not a stronger one. The additional effects come from binding additional receptors, not from greater potency at the same receptor.
By narrowing the receptor profile toward MC3R and MC4R and dropping the pigmentation activity. That development history is itself evidence that the broad binding was treated as a problem to be engineered out.
Yes. The MHRA and the TGA have both issued public warnings about unlicensed Melanotan II supply, citing absent safety evaluation and the risks of unregulated injectables sold for cosmetic use.
Educational content. Not medical advice.
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