Although there are studies describing the prevalence of patients with drug addiction and dual diagnosis, there is a paucity of data regarding the progression from psychiatric disorders to drug addiction or vice versa. A total of 1,090 patients dependent on heroin were interviewed to examine the presence of dual diagnosis and the progression from psychiatric disorders to drug dependence or vice versa. A total of 574 patients met the criteria for a dual diagnosis. A total of 362 patients progressed from substance abuse disorders to psychiatric disorders (SUD-PR) and 144 patients progressed from psychiatric disorders to drug addiction (PSY-PR). SUD-PR patients are more frequently affected by mood disorders. PSY-PR patients were more frequently diagnosed as psychotic or affected by anxiety disorders. The authors' findings suggest that the self-medication theory of dual diagnosis was relevant only for those with existing diagnoses of schizophrenia or anxiety disorders. When treating patients with a dual diagnosis, clinicians should ensure that the same attention is given to the treatment of drug dependence as it is to the treatment of comorbid psychiatric conditions.