Melanotan 1 vs 2 — selectivity is the difference that explains the different side effect profiles

The melanocortin receptor family has several members with quite different roles, which is the whole story here.

Melanotan 1 (afamelanotide) is comparatively selective for MC1R, the receptor most associated with pigmentation. It has an approved use in a rare photosensitivity condition — erythropoietic protoporphyria — so there's a genuine regulatory dossier for that narrow indication.

Melanotan II is less selective and hits other melanocortin receptors, including MC4R, which is involved in appetite and sexual function. That non-selectivity is the reason its reported effect profile is broader.

So the difference isn't potency, it's specificity. A less selective compound doing more things is not a bonus feature, it's what off-target activity looks like.

Worth being clear on evidence tier: MT-1 approved for one specific rare indication, MT-II not approved anywhere and studied far less formally.