A drug is any substance that can be used to treat an illness, relieve a symptom or modify a chemical process in the body for a specific purpose.
The term is necessarily a vague one, being defined by intent: for example, foods consumed for normal metabolism are not generally considered "drugs", but the same foods consumed for a more specific purpose (such as the use of alcohol as a depressant or caffeine as a stimulant) may be.
Pharmaceutical medications (for example, to treat injury or disease). Please see Pharmacology.
