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 neuro transmitters to the synapse (this is the gap between different nerves).ome 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 neuro transmitters to the synapse (this is the gap between different nerves).