In reference to the central nervous system, any agent that activates, enhances, or increases neural activity; also called psychostimulant.

Included are the amphetamines, cocaine, caffeine and other xanthines, nicotine, and synthetic appetite suppressants such as phenmetrazine or methylphenidate. Other drugs have stimulant actions that are not their primary effect, but which may be manifest in high doses or after chronic use; they include antidepressants, anticholinergics, and certain opioids.

Stimulants can give rise to symptoms suggestive of intoxication, including tachycardia, pupillary dilatation, elevated blood pressure, hyperreflexia, sweating, chills, nausea or vomiting, and abnormal behavior such as fighting, grandiosity, hypervigilance, agitation, and impaired judgment. Chronic misuse commonly induces personality and behavior changes such as impulsivity, aggressivity, irritability, and suspiciousness. A full-blown delusional psychosis may occur.
Cessation of intake after prolonged or heavy use may produce withdrawal syndrome, with depressed mood, fatigue, sleep disturbance, and increased dreaming.

World Health Organization. Lexicon of alcohol and drug terms published by the World Health Organization. Internet. Accessed August 20, 2009.