Several causes of drug addiction in people remain unknown. It is known that only a small percentage of those who use potent dependent drugs, such as methamphetamine and heroin, develop addiction. In addition, some studies suggest that the choice of preferred drugs among drug addicts is made proactively according to criteria other than the strength of pharmacological dependence. Therefore, this paper discusses the pathogenesis of drug dependence, focusing on the "self-medication hypothesis" of Khantzian et al.