A drug made from the hemoglobin of vampires, Fang is dangerous and (thankfully) rare. Created by a 2000 year old vampire named Negus (who apparently once defeated Dracula in hand to hand combat), it was brought to New York City by him. Here, he abducted homeless people, dosed them with the drug and forced them to battle each other in cage match fights, until Spider-Man and Blade intervened.
The effects of Fang are temporary, but massive while they last. They effectively give the human user the physical traits of a vampire – fangs, super strength, really bad skin – for a few hours, plus a berserker rage that lasts for roughly three days even after the other effects wear off. Intense doses of UV light can make the effects wear off sooner, and there appears to be little lasting damage to the minds or bodies of the users afterwards (other than whatever injuries they may receive in the cage fights).