The common co-occurrence of substance misuse and other psychiatric disorders and their intricate relationships have led to major community-based epidemiological studies in the US which showed high rates of current and lifetime comorbidity. Moreover, studies of clinical populations conducted in North America, Europe and Australia, showed even higher rates of comorbidity. The aetology of this comorbidity has also been investigated and important models have emerged with findings that inform its assessment and treatment. Future epidemiological studies should focus on the study of concurrent conditions rather than lifetime ones using research diagnostic instruments with high reliability providing information on a number of key outcomes.

The common occurrence of comorbid substance misuse and other psychiatric disorders has challenged the diagnostic and therapeutic skills of professionals concerned with the care of patients with these dual disorders. Combined pharmacological and psychological treatment approaches have evolved empirically drawing upon standard treatments with emphasis on psychosocial approaches to substance misuse for psychotic disorders and pharmacological approaches for mood disorders. Advances in the biology of both disorders have started to inform their psychopharmacology. The specific role of atypical antipscychotics is highlighted. Further studies of the biology of comorbidity will impact the use of effective pharmaceuticals such as clozapine with dual effects on schizophrenia and substance misuse.

In order to more effectively treat substance use disorders in persons with severe mental illness, integrated treatment models have been developed for these ‘co-occurring disorders.’ The nature of clinical services provided in these programs is described in this article, with brief attention to research on these programs. Clinical services are generally guided by a stage model of change, in which the client's motivation to change (or stage of treatment: engagement, persuasion, active treatment, relapse prevention) informs the optimal selection of appropriate interventions. Psychotherapeutic services for co-occurring disorders include a range of individual (motivational interviewing, cognitive-behavioral counseling), group (stage-wise groups, social skills training, self-help), and family (single family, multifamily formats) interventions. Supplementary interventions are also commonly provided, including psychopharmacological, residential, coerced and involuntary, and vocational rehabilitation. Access to a range of different interventions for co-occurring disorders may improve the ability of clinicians to engage and tailor treatment to meet clients' unique needs. The research base supporting integrated treatment for co-occurring disorders is limited at this time owing to the small number of methodologically rigorous studies from which to draw conclusions. Nevertheless, the available evidence suggests that integrated treatment programs improve outcomes compared with nonintegrated approaches, and that more research is needed to evaluate the benefits of these programs.

Co-occurring severe mental illness and substance use disorder has been recognized as a common problem in the U.S. since the early 1980s (1–3). For these individuals with co-occurring disorders, research demonstrates the effectiveness of various forms of combining, blending, or integrating mental health and substance abuse treatments (4). The evolving U.S. service model for integrated dual disorders treatment emphasizes several key elements: implementation, leadership, training, engagement, assessment, counseling for all patients, ancillary treatments for those with multiple needs, secondary treatments for patients who are nonresponders, and quality assurance regarding process and outcomes.

The co-occurrence of substance misuse and other psychiatric disorder (dual diagnosis) has been increasingly recognized in the UK. Clinical studies of patients with severe mental disorders showed high rates of substance misuse with poor clinical and social outcome. These patients often fall ‘between the cracks’ of the separate general psychiatric and addiction services. This has necessitated the development of a national policy advocating the provision of integrated care within mainstream psychiatric services. There are emergent models of good practice that require evaluation of effectiveness and cost effectiveness.

Trends in prevalence in substance misuse in young people in the UK are described: increased use over the last decade, the tendency to polydrug use, the narrowing of the gender gap. The complex issues relating to the description, definition and classification, and diagnosis of psychiatric comorbidity and substance misuse are outlined. The fact that there is no uniformly accepted definition of what constitutes ‘dual diagnosis’ or ‘psychiatric comorbidity’ in young people is highlighted: this variability impacts upon clinical assessment and study design. Despite these considerable drawbacks, a degree of consistency is beginning to emerge in the co-occurrence of some conditions, e.g. disruptive and suicidal behaviours. It is recognized that psychological distress and substance misuse are also associated with multiple social and physical complications. The rapid advances in treatment options are presented. At present the ‘best practice’ is implementation of what works for adults with addiction and young people with psychiatric disorder. This includes psychosocial interventions such as motivational enhancement techniques and cognitive behavioural treatment, as well as appropriate safe (usually), short-term use of a range of pharmacological agents. Very few comprehensive designated adolescent addiction services exist, while child and adolescent mental health services are under serious pressure. Thus the need for a longitudinal and multidisciplinary approach, with appropriate assessment instruments in young people, is required to further explore diagnostic classification which will classify the degrees and patterns of relationships between disorders. In this way it may be possible to build up a picture of the nature and extent of numerous complex, and sometimes overlapping, problems and needs in children and adolescents in a variety of settings: primary and secondary care, the criminal justice system and educational establishments. This might strengthen the development of innovative treatment services where novel interventions are tested as a priority.