As a result of the availability of new and exotic drugs and a diversifying medical profession to serve burgeoning towns, some late thirteenth-century physicians began to inquire more explicitly about the difference between a medicine and a poison. Were some substances simply too dangerous to be used in general medical practice? That medical authorities had never agreed on a single definition of poison complicated progress on this front. Fifteenth-century physicians brought significantly new energy to bear on this question through a rigorous examination of the properties of poison and the extent to which it was defined and thought to operate by some kind of occult virtue. Fifteenth-century writing about poison thus provides an important new and early site for examining the historical development of occult influence, because the notion of specific form had become integral to poison's definition and its ontological status as a category of substance. These discussions were instrumental to the development of modern toxicology as well, as it helped further distinguish between the classical notion of pharmaka, in which no substance was singled out as uniquely different in kind from other substances, and that of venenum, which was increasingly defined as fundamentally harmful to the human body.