A stimulant is a drug which increases the activity of the sympathetic nervous system and produces a sense of euphoria or awakeness. Stimulants can be used as recreational drugs, or therapeutically to increase alertness. They are also used and sometimes abused to boost endurance and productivity as well as to suppress appetite. Examples of stimulants are caffeine, nicotine, amphetamines, and cocaine.

Some stimulants, as for example Ritalin, have been shown to help with ADHD. This is often called a "paradoxical effect", since ADHD is commonly thought of as "hyperactivity" and stimulants would be expected to increase activity, but another effect of sympathetic nervous system stimulation is an increased ability to concentrate on mental tasks. However, long-term stimulant abuse can impair mental function and lead to psychotic symptoms.

Antidepressants are not considered stimulants, as they do not act directly on the sympathetic nervous system and generally do not produce an immediate effect on mood. A possible exception is bupropion, whose chemical and pharmacological properties are similar to stimulants.

Recently, there have been improvements in the area of stimulant pharmacology, producing a class of chemicals known as "eugregorics", or "good arousal". These stimulants tend to increase alertness without the peripheral (body) effects or addiction/tolerance/abuse potential of the traditional stimulants. They have minimal effect on sleep structure, and do not cause rebound hypersomnolence or "come down" effects. Currently, there are two stimulants in this class being used: modafinil and adrafinil, marketed as Provigil and Olmifon, respectively.