Any substance which when absorbed into a living organism may modify one or more of its functions.
Comment: The term is generally accepted for a substance taken for a therapeutic purpose, but is also commonly used for substances of abuse. Just as any substance can be a toxicant, so any substance can be a drug. The term carries with it the implication of use for medical purposes, but also the potential for abuse to produce an effect desired by the abuser, but which is ultimately harmful.