We review the evaluation and treatment of patients with drug addictions complicated by other acute or chronic diseases from the perspective of the hospital setting. The spectrum of drug addiction's complications is broad and in many instances it is predetermined by the abuse substance and its administration route. Some complications of intravenous drug addiction have dramatically decreased in the last few years as a result of a better knowledge of hygienic customs and after the implementation of some health interventions such as the provision of sterile injectable devices. Two highly prevalent infections --HIV/AIDS and hepatitis C-- remain from the period in which most HIV infections owed to the intravenous use of heroin. Of note, these two infections largely account for the survival and quality of life of those who quit their addiction. On the other hand, it is still common the hospitalization of patients with both alcohol dependence and intercurrent diseases in whom their drug addiction may pass unnoticed. Other common situations include the treatment of acute patients with cocaine addiction and psychiatric comorbidity, patients under methadone therapy and, in general, all those cases in which, in emergency, ordinary hospital wards and specialized units, a wide differential diagnosis is raised when there is a coexistence of signs and symptoms common to an addiction, infection and/or intoxication. An integral vision of drug addiction and its complications, as well as the clinical evaluation of all health problems, is fundamental for the prognosis and treatment of these patients.