Psychoactive drugs affect states of consciousness. They come in several categories, including depressants, stimulants, and hallucinogens. Depressants reduce and slow down neural activity, as opposed to simulants, which increase activity of the nervous system. For example, a type of depressant called an opiate reduces pain, produces a numb feeling, and is highly addictive. A stimulant like nicotine, on the other hand, causes increased heart rate, dependence, and tolerance over time. The third category of drug discussed, hallucinogens, can alter the brain's sensory input and change perception of sensory information. Some hallucinogens, like ecstasy and marijuana, do not easily fit into a category because of their dual symptoms.

Many factors contribute to drug use, including biological, psychological, and social influences.