Dom Geert - Co-occurring Addictive and Psychiatric Disorders A Practice-Based Handbook from a European Perspective

Division of Clinical Psychology and Epidemiology, Department of Psychology, University of Basel, Basel, Switzerland

Roselind Lieb

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The goal of this chapter is to discuss the comorbidity between substance use disorders and other mental disorders from an epidemiological perspective. An extensive literature has documented worldwide a strong association of problematic substance use (use and use disorders) with other mental disorders (for review see, e.g., Lieb and Isensee ). Therefore, the chapter will not repeat these worldwide comorbidity findings but will instead focus on selected findings from European epidemiological studies that have examined comorbidity between substance use disorders and other mental disorders in European community samples. The aim is to demonstrate how epidemiological research can help us understand the phenomenon of comorbidity.

The term comorbidity was introduced by Feinstein () to refer to any distinct additional clinical entity that has existed or that may occur during the clinical course of a patient who has the index disease under study (pp 456457). Or in other words: When two or more clinical conditions occur simultaneously or sequentially in the same person, they are said to be comorbid.

But why is it interesting to study comorbidity? There are clinical concerns (i.e., diagnostic issues, treatment strategy) that have to be reconsidered in comorbid patients, but the phenomenon of comorbidity is also of scientific interest. Comorbidity of substance use disorders and other mental disorders may, for example, reveal gaps in current knowledge and become the basis for interesting hypotheses for future studies.

To explain comorbidity of substance use and other mental disorders, two basic scenarios can be used (assuming that the comorbidity findings cannot be explained by methodological shortcomings):

Comorbidity seems to be a very complex phenomenon that may help researchers detect etiological pathways to disorders or consequences of diseases. In evaluating the complexity of comorbidity, epidemiologists focus first on determining if there is an association (cross-sectional) between substance use disorders and other mental disorders. If such an association can be shown, the next question addresses the longitudinal or predictive association: Does one (the primary) disorder (i.e., the disorder with the earlier onset) prospectively increase the risk (in terms of incidence) for the other (secondary) disorder? If yes, the primary disorder is said to be a risk factor for the development of the secondary one (see Kraemer et al.) and must be reviewed for the each association (e.g., biological plausibility, coherence of existing empirical knowledge, doseresponse relationship.

In the following, we focus on selected epidemiological studies that have addressed cross-sectional and longitudinal (predictive) associations between substance use/disorders and other mental disorders. We start with a brief look at the size of the problem, that is, on the prevalence of mental and substance use disorders in Europe.

The best and most valid information about the prevalence of mental and substance use disorders can be taken from a comprehensive review on the 12-month prevalence and disability burden estimate of a broad range of mental disorders in the European Union (EU) that was conducted by Wittchen et al. (). The authors systematically reviewed the existing literature, reanalyzed existing data and national surveys, and consulted experts. They included studies and data from all member states of the EU (EU-27) plus Switzerland, Iceland, and Norway. Using this method, they estimated that each year, 38.5 % of the adult EU population suffer from at least one mental disorder. Adjusted for age and comorbidity, this corresponds to 164.8 million people affected per year. Anxiety disorders (14.0 %; 61.4 million people affected) and major depression (6.9 %; 30.3 million people affected) are the most frequent mental disorders. Almost 15 million people (3.4 %) are affected by alcohol dependence. More than one million adult Europeans are affected by drug dependence (opioid or cannabis dependence; prevalence rates: 0.11.8 %). These findings underline impressively that neither mental nor substance use disorders affect only few people. Rather, they must be considered an important care challenge for the EU in the twenty-first century.