Substance use disorders are chronic, relapsing illnesses that are associated with serious medical, economic, and social consequences. Substance use disorders are associated with loss of control over the amount of substance used, continued use despite negative consequences, preoccupation with use, and dysphoria during abstinence. In heavy and long-term users, abrupt cessation or reduction in substance use results in substance-specific withdrawal syndrome. Risk of substance use disorders is influenced by genetic, psychosocial, and environmental factors.