The term dual diagnosis is a common, broad term that indicates the simultaneous presence of two independent medical disorders. Recently, within the fields of mental health, psychiatry, and addiction medicine, the term has been popularly used to describe the coexistence of a mental health disorder and AOD problems. The equivalent phrase dual disorders also denotes the coexistence of two independent (but invariably interactive) disorders, and is the preferred term used in this Treatment Improvement Protocol (TIP). 

The acronym MICA, which represents the phrase mentally ill chemical abusers, is occasionally used to designate people who have an AOD disorder and a markedly severe and persistent mental disorder such as schizophrenia or bipolar disorder. A preferred definition is mentally ill chemically affected people, since the word affected better describes their condition and is not pejorative. Other acronyms are also used: MISA (mentally ill substance abusers), CAMI (chemical abuse and mental illness), and SAMI (substance abuse and mental illness). 

Common examples of dual disorders include the combinations of major depression with cocaine addiction, alcohol addiction with panic disorder, alcoholism and polydrug addiction with schizophrenia, and borderline personality disorder with episodic polydrug abuse. Although the focus of this volume is on dual disorders, some patients have more than two disorders, such as cocaine addiction, personality disorder, and AIDS. The principles that apply to dual disorders generally apply also to multiple disorders. 

The combinations of AOD problems and psychiatric disorders 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. For instance, some methadone treatment programs treat a high percentage of opiate-addicted patients with personality disorders. Patients with schizophrenia and alcohol addiction are frequently encountered in psychiatric units, mental health centers, and programs that provide treatment to homeless patients.

Patients with mental disorders have an increased risk for AOD disorders, and patients with AOD disorders have an increased risk for mental disorders. For example, about one-third of patients who have a psychiatric disorder also experience AOD abuse at some point (Regier et al., 1990), which is about twice the rate among people without psychiatric disorders. Also, more than half of the people who use or abuse AODs have experienced psychiatric symptoms significant enough to fulfill diagnostic criteria for a psychiatric disorder (Regier et al., 1990; Ross et al., 1988), although many of these symptoms may be AOD related and might not represent an independent condition.

Compared with patients who have a mental health disorder or an AOD use problem alone, patients with dual disorders often experience more severe and chronic medical, social, and emotional problems. Because they have two disorders, they are vulnerable to both AOD relapse and a worsening of the psychiatric disorder. Further, addiction relapse often leads to psychiatric decompensation, and worsening of psychiatric problems often leads to addiction relapse. Thus, relapse prevention must be specially designed for patients with dual disorders. Compared with patients who have a single disorder, patients with dual disorders often require longer treatment, have more crises, and progress more gradually in dual diagnosis treatment.