Various authors defined the term drug in various ways.The most widely accepted definition of the drug is any chemical agent that affects the living process.Further, it can be elaborated as the chemical and medicine meant for treatment,mitigation or prevention of diseases in human beings or animals for internal or external use.All substances other than food intended to affect the structure or any function of the human or animal body including contraceptives as well as substances used for destruction of vermin and insects,which are causing diseases in human beings or animals.Pharmaceutical chemistry is a science that makes use of the general laws of chemistry to study drugs i.e.,their preparation chemical nature,composition,structure,influence on an organism and studies the physical and chemical properties of drugs,the methods of quality control and conditions of their storage.A sharp division into two classes(according to their chemical structure or therapeutic action)is, however,not possible as many drugs having the same physiological behaviour may fall into various groups formed on the basis of structure.As a compromise the following classification of drugs may be considered.