Melanotan II mimics the action of alpha-MSH (alpha-Melanocyte Stimulating Hormone). Alpha-MSH is a peptide in our bodies that binds to melanocortin receptors on melanocytes (the pigment-producing cells in skin). When these receptors (particularly MC-1R) are activated, melanocytes ramp up production of melanin, the pigment responsible for tan or brown skin coloration. The melanin is then distributed to skin cells, which results in a darker skin tone. By increasing melanin, Melanotan II essentially provides an “internal sunscreen” – melanin absorbs UV radiation and dissipates it, protecting skin cells from DNA damage. This was the original goal: protect fair-skinned individuals from skin cancer by giving them a way to tan without burning. In parallel, Melanotan II binds to MC-4 receptors in the brain (and MC-3 to a lesser extent), which are involved in sexual function. This is why during clinical trials, an unexpected effect was observed: men got spontaneous erections and women experienced increased sexual arousal. Mechanistically, Melanotan II’s activation of MC-4R in the hypothalamus increases dopamine release and sexual desire. (This side effect was so pronounced that it led to developing PT-141 specifically for sexual dysfunction.) Additionally, MC-4R and MC-3R activation can have an appetite-suppressing effect, which some users notice as mild nausea or reduced hunger. On the tanning front, Melanotan II effectively bypasses the usual sequence (UV -> DNA damage -> melanin production) and directly tells melanocytes to make melanin. Users still need some UV exposure to oxidize and distribute the melanin for a visible tan, but vastly less than normal. The result is a true tan that can be several shades darker than one’s baseline, providing not only cosmetic benefits but also physical protection against sunburn.