Straight chain, narrow aim

This molecule is linear and it leans toward MC1R, the pigment receptor, more than its cyclic cousin does. Melanotan II is a ring-closed molecule that hits the whole melanocortin family, including the MC3 and MC4 receptors that govern appetite and sexual arousal. Same family, different shape, different spread of effects. That is the single most useful thing to hold in mind when these two names are used as though they were one product.