Drugs: A Simple Definition
The term "drugs" is an extraordinarily broad one, which can encompass such a great variety of items that the meaning of the word almost becomes lost. Even Webster's Dictionary offers a variety of definitions which distinguish between (usually legal) medications intended to treat a specific physiological or psychological problem and (usually illegal) substances which are used recreationally, and whose use may lead to addiction.
A simple, all-encompassing definition of a drug would be "a substance containing chemicals which result in a specific physical or psychological reaction or reactions." This, at least, is as much true of penicillin, a drug which has saved millions of lives, as it is of cocaine--a drug which, especially in the latter part of the twentieth century, has claimed a death toll in the hundreds of thousands if not the millions.