Co-Occurring Mental Illness and Substance Use Disorders in Canadian Forensic Inpatients: Underdiagnosis and Implications for Treatment Planning

Research suggests that co-occurring substance use disorder (SUD) is prevalent among adults with psychiatric illness. Studies with forensic patients in Australia indicate that co-occurring disorders (CD) are underdiagnosed. To help determine how widespread CD underdiagnosis is in forensic populations internationally, we compared current diagnoses recorded in the clinical record with clinical evidence gathered during forensic assessment meeting DSM-IV criteria for SUD, in a Canadian sample of 638 male forensic inpatients. Among 491 with a major mental illness diagnosis, most (61%) met criteria for CD but only 19% were diagnosed as such. CD was associated with longer hospitalization, and with greater evidence of criminal history, antisociality, and risk of violent recidivism, regardless of how CD was defined. Identifying CD based on documented evidence, though, allowed for slightly greater detection of group differences. Underdiagnosing SUD has a potential impact on understanding substance use as a criminogenic treatment need in forensic mental health.