The terms suggest that there are only two disorders occurring at the same time, when in fact there may be more. Clients with co-occurring disorders (COD) have one more disorders relating to the use of alcohol and/or other drugs of abuse as well as one or more mental disorders. A diagnosis of co-occurring disorders occurs 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 one disorder.

The combinations of COD problems and psychiatric disorder vary along important dimensions, such as severity chronicity, disability, and degree of impairment in functioning. For example, the two disorders may each be severe or mild, or one may be more severe than the other, indeed, the severity of both disorders may change over time. Levels of disability and impairment in functioning may also vary.

Thus, there is no single combination of dual disorders; in fact, there is great variability among them. However, patients with similar combinations of dual disorders are often encountered in certain treatment settings.