The compound in this catalogue with the clearest record of causing harm, and it is not close.
what it is
Melanotan II is a synthetic analogue of alpha-MSH that activates melanocortin receptors non-selectively. Because those receptors govern pigmentation, appetite, and sexual arousal among other things, it produces all three effects at once. It was developed at the University of Arizona and never brought to approval; a related, selective compound was later approved for a specific rare condition.
what the evidence shows
It does what it is claimed to do — it darkens skin. What accompanies that is well documented in case reports and poison centre data: nausea, flushing, spontaneous erections, and changes in existing moles. Multiple published case reports describe new or changed melanocytic lesions in users, including melanoma diagnoses, and dermatology literature has repeatedly flagged the pattern.
The compound in this catalogue with the clearest record of causing harm, and it is not close.
what stays uncertain
Causality in individual cases is difficult to establish, and the reporting is retrospective. That uncertainty cuts in an uncomfortable direction: the mechanism, stimulating melanocytes systemically for cosmetic purposes, is exactly the kind of thing that would produce this signal if the association were real.
the practical note
Health authorities in several countries have issued specific warnings about this compound. If you take one thing from this catalogue, take the fact that the tanning peptide is the one with the actual body of harm reports behind it.