Dual diagnosis (also called co-occurring disorders (COD) or dual pathology) is the condition of having a mental illness and a comorbid substance use disorder. Several US based surveys suggest that about half of those with a mental illness will also experience a substance use disorder, and vice versa. There is considerable debate surrounding the appropriateness of using a single category for a heterogeneous group of individuals with complex needs and a varied range of problems. The concept can be used broadly, for example depression and alcohol use disorder, or it can be restricted to specify severe mental illness (e.g. psychosis, schizophrenia) and substance use disorder (e.g. cannabis use), or a person who has a milder mental illness and a drug dependency, such as panic disorder or generalized anxiety disorder and is dependent on opioids. Diagnosing a primary psychiatric illness in people who use substances is challenging as substance use disorder itself often induces psychiatric symptoms, thus making it necessary to differentiate between substance induced and pre-existing mental illness.
Those with co-occurring disorders face complex challenges. They have increased rates of relapse, hospitalization, homelessness, and HIV and hepatitis C infection compared to those with either mental or substance use disorders alone.
