People who have multiple disorders of severe mental illness, drug addiction and alcoholism "dual diagnosis" have the same severity of addictive disorders as do people who have addictive disorders alone. They also experience exacerbation of both their mental illness and their addictive disorder due to interaction effects.

The step-by-step approach to integrated treatment for multiple disorders described in this chapter can be replicated at minimal cost in any type of mental health treatment program or service.

As some attention began to focus on clients with both substance abuse problems and mental illnesses, it quickly became apparent that new methods and interventions were necessary.