An agent, or drug, that induces temporary improvements in either mental or physical functions or both. Examples of these kinds of effects may include enhanced alertness, wakefulness, and locomotion, among others.

Stimulants are drugs that can increase alertness and awareness, usually for a short time only. Most stimulants have more side-effects than other drugs. Some are classified as illegal drugs, most can cause addiction. For this reason, most legal stimulants are only available on prescription. Stimulants act on the nerves: Stimulants cause more neurotransmitters to the synapse (this is the gap between different nerves)

Most stimulants fall into one of the following groups:

Some of the substances mentioned above can be used to treat certain diseases. Some are mostly used as illegal drugs.