A stimulant is any of a group of 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)