Hallucinogens are drugs that change thoughts, feelings and perceptions of individuals. They cause hallucinations. LSD (Lysergic acid diethylamide) is one such hallucinogen that causes horrible dreams, chronic psychosis and severe damage to the central nervous system. Sedatives give a feeling of calmness, relaxation or drowsiness in the body. Their high doses induce sleep. Tranquillisers lower tension and anxiety without inducing sleep. Stimulants are the drugs that stimulate the nervous system, make a person more wakeful, alert and active; and cause excitement.