Substance abuse still remains one of the major problems in the world today, with millions of people abusing legal and illegal drugs. In addition, a billion people may also be infected with one or more infections. Both drugs of abuse and infections are associated with enormous burden of social, economic, and health consequences. This article briefly discusses a few medical consequences of drugs of abuse and infections such as human immunodeficiency virus, hepatitis C virus, psychiatric complications in hepatitis C infection, pharmacokinetic drug-drug interactions among medications used in the treatment of addiction and infections, and new drugs in development for the treatment of infections. Research is encouraged to study interactions between infections, drugs of abuse, and underlying pathophysiologic and molecular/genetic mechanisms of these interactions.