Drug Addiction. It is defined as psychological dependence on drug for the feeling of well-being.  The person becomes so habituated to drugs and body become tolerant to drugs that only high doses are needed for response.

Tranquillisers. They slow down the function of brain and give a feeling of relaxation. They influence only the mental excitement but do not affect the working efficiency, e.g., alcohol, calmpose, diazepum.

Depressant. Depressants are used to calm anxiety. They give a feeling of  drowsiness and relaxation. e.g., Barbiturates .

Stimulants (Antidepressants). These drugs temporarily increase the mental alertness. They are also called mood elevators  e.g., Amphetamines, cocaine, coffee and tea also contain small amount of stimulant.