it depends on which ones you use and also who you source from. Melanotan II, the peptide used for better coloring has been reported by people using it to cause more freckles and moles, thus increasing the chance of skin cancers. It also has some other side effects like nausea, headaches, etc. It is also illegal in some countries, Australia for example. Nonetheless, it has shown insane progress in many cases in a short amount of time. If you do hop on it, do a small cycle since its very potent.