Stimulants are drugs that excite neural activity and speed up body functions. They increase alertness, elevate mood, and raise heart rate and blood pressure.

Caffeine, nicotine, amphetamines, cocaine, and methamphetamine all belong to the class, though they differ in potency, duration, and risk. Most act by increasing the availability of dopamine, norepinephrine, or both. Low to moderate doses can enhance attention and performance; higher doses produce euphoria, agitation, and, with some substances, dangerous physiological strain. Tolerance and dependence develop with repeated use, and withdrawal typically includes fatigue, irritability, and depressed mood.

Medical uses include treatment of attention-deficit/hyperactivity disorder and narcolepsy. Recreational and nonmedical use carries substantial risk of addiction, cardiovascular events, and, for some drugs, long-term changes in brain reward systems. Public-health efforts focus on regulation, education, and treatment of dependence.

When the nervous system is chemically pushed into higher gear, the agent is a stimulant.