Speed is the slang term for the Class B drug amphetamine sulphate. In some cases, speed is a term used in identifying other terms of amphetamines.
Speed is an excitant and people use 'speed' to make them remain conscious, energised and active.
Other important consequences and dangers of using speed are:
Feeling 'up', energetic and tending to talk a lot.
It is used because it energises people to be active participants in several activities for a very long period without exhaustion; dancing, engaging in conversations and going out are some of such activities.
It has the ability to make someone extremely active, worked up or even acutely psychotic (this is a mental condition when you see or hear things that seem real but does not really exist and have hallucinations).
The intoxication is usually followed by a long gradual comedown, making you so irascible and depressed.
Speed exerts a pressure on the heart, so can lead to heart problems; in fact, some individuals have lost their life from excessive use of speed.