A psychoactive substance is a chemical substance, other than a nutrient or essential dietary ingredient, that affects brain function to produce alterations in sensation, perception, mood, consciousness, cognition, and behavior.
Psychoactive substances are used for a number of purposes; for example, they can be taken recreationally for their novelty and euphoric effects, as entheogens for ritual or spiritual purposes, or as tools for augmenting and studying the mind. Many psychoactive substances have demonstrable therapeutic utility and are used in routine medical practice (e.g. anaesthetics, analgesics, anxiolytics, antidepressants, etc.).