Drug misuse is the improper use of common medications in ways that lead to 
acute and chronic toxicity.Physiologic dependence: is due to biochemical changes in body tissues, especially 
the nervous system.Laxatives, antacids, vitamins, headache remedies, and cough and cold 
medications are often self-prescribed and overused.Illicit drugs are of 
two types: (a) drugs unavailable for purchase under any circumstances, such as 
heroin, and (b) drugs normally available with a prescription that is being obtained 
through illegal channels.The two 
types of dependence, physiologic and psychologic, may occur separately or 
together.Psychologic dependence: is emotional reliance on a drug to maintain a sense of 
well-being, accompanied by feelings of need or cravings for that drug.Drug habituation: means a mild form of psychologic dependence.