Some of the major differences between an illicit drug and a medicine are the factor of dependence and the purpose of use. Illicit drugs can be further divided into different categories according to usage history，chemical properties，origin，and the activation of neurochemical mechanisms. It is suggested that chronic use of illicit drugs could lead to compulsory consumption，constant craving and high rates of relapse. In fact，the concept of drug addiction varies from time to time. The changes in the conceptualization of drug addiction have also significantly influenced drug policies worldwide. Different illicit drugs，depending upon variable such as drug abuse history and population groups，influence public health in different ways，