The term co-occurring disorder refers to the condition in which an individual has a co-existing mental illness and substance use disorder. While commonly used to refer to the combination of substance use and mental disorders, the term also refers to other combinations of disorders, such a mental disorder and an intellectual disability. (The terms dual disorder and dual diagnosis were previously used to describe the same condition.)
When a substance use disorder and a psychiatric disorder co-occur, they may differ in severity, and the severity of each can change over time. Compared to individuals who have a single disorder, those with a combination of disorders may experience more severe medical and mental health challenges and may also require longer periods of treatment.