The term co-occurring disorder replaces the terms dual disorder and dual diagnosis when referring to an individual who has a co-existing mental illness and a 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 as mental disorders and intellectual disability).

Clients with co-occurring disorders (COD) typically have one or more disorders relating to the use of alcohol and/or other drugs as well as one or more mental disorders. A client can be described as having co-occurring disorders when at least one disorder of each type can be established independent of the other and is not simply a cluster of symptoms resulting from another disorder.

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