Drug

A drug may be defined as a chemical substance (usually of known structure), other than a nutrient or essential dietary ingredient, which, when administered to a living organism, produces a biological effect. As such, environmental chemicals and toxins, insect venoms, chemical warfare agents, bioactive dietary components and drugs of abuse, whether legal or illegal, may all be classed as drugs, although students in professional clinical science programs are most interested in therapeutic medicines.

The study of drug action is the focus of the subject of pharmacology. Pharmacology has been broadly defined as the chemical control of physiology.