A psychoactive drug, psychopharmaceutical, or psychotropic is a chemical substance that crosses the blood–brain barrier and acts primarily upon the central nervous system where it affects brain function, resulting in changes in perception, mood, consciousness, cognition, and behavior. These substances may be used recreationally, to purposefully alter one's consciousness, as entheogens, for ritual, spiritual, and/or shamanic purposes, as a tool for studying or augmenting the mind, or therapeutically as medication.

Because psychoactive substances bring about subjective changes in consciousness and mood that the user may find pleasant (e.g. euphoria) or advantageous (e.g. increased alertness), many psychoactive substances are abused, that is, used excessively, despite the health risks or negative consequences. With sustained use of some substances, psychological and physical dependence ("addiction") may develop, making the cycle of abuse even more difficult to interrupt. Drug rehabilitation aims to break this cycle of dependency, through a combination of psychotherapy, support groups and even other psychoactive substances such as acamprosate or naltrexone in the treatment of alcoholism, or methadone or buprenorphine maintenance therapy in the case of opioid dependency. However, the reverse is also true in some cases, that is certain experiences on drugs may be so unfriendly and uncomforting that the user may never want to try the substance again. This is especially true of the deliriants (e.g. Jimson weed) and powerful dissociatives (e.g. Salvia divinorum). Most purely psychedelic drugs are considered to be non-addictive (LSD, psilocybin, mescaline etc.); "psychedelic amphetamines" (such as MDA, MDMA etc.) may produce an additional stimulant and/or euphoriant effect, and thus have an addiction potential.