Drugs that affect a person's mental state, whether prescribed for a medical condition (for example, antidepressants) or taken for recreational purposes (such as alcohol and heroin), are called psychoactive drugs.
Psychoactive drugs affect the way a person thinks and feels—which may also affect the way they behave. The most commonly used legal psychoactive drugs, apart from drugs taken on prescription, are alcohol and tobacco. The most commonly used illegal psychoactive drug is cannabis (marijuana).
Some drugs fall into more than one category. For example, cannabis is both a depressant and hallucinogen, while ecstasy is a stimulant and hallucinogen.