Originally a powerful, non-addictive mutagen devised by X-Man Hank McCoy (a.k.a. The Beast), Mutant Growth Hormone has since been debased into a less potent but more addictive form, and shown up on the streets of New York City, usually abbreviated to MGH.
MGH is created from the DNA of a mutant (or mutate), and grants the user with limited superpowers for a brief period. Although most sellers of the drug claim that the powers granted are related to the person who the MGH was harvested from, this is in fact untrue, and the powers granted arise from the activation of latent mutant genes in the user.
Known DNA sources for MGH include Hank McCoy (who made the original formulation from his own DNA and took it himself, which may account for both the potency and permanency of the effects), Leland Owlsley (the criminal known as The Owl) and Mattie Franklin (the third Spider-Woman). No verified sample has ever been created from the DNA of Peter Parker (Spider-Man) although dealers frequently claim him as a source of the drug.