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Résumé (anglais)

Background. Many patients with mental disorders receiving treatment in mental health centres are using illicit drugs while a large proportion of those in addiction care programs have significant mental health issues. Substance use disorder and major depression are highly prevalent in the general population. They frequently co-exist, share common biological, psychological and social risk factors and affect one another in clinically significant ways. Comorbid substance use disorder and major depression represent a major health problem globally. The primary goal of this thesis is to further our understanding of the relationship between substance use disorders and co-occurring major depressive disorder by applying different epidemiological methods. Methods. The study designs used in this thesis were cross-sectional design (Chapters 3,4 and 5), population cohort design (Chapter 6) and systematic review with meta-analysis (Chapter7). Existing datasets from Statistics Canada were used for Chapters 3,4,5 and 6 while the systematic review collected data via a computerized search for original studies. Trend (Chapter 3), multilevel logistics regression (Chapter 4), Multinomial logistics regression (Chapter 5), Poisson regression (Chapter 6) and meta-analysis (Chapter 7) were done. Descriptive analysis was also done for all chapters. Results. The pooled prevalence of substance use disorder comorbid with major depression is 3.2%. The prevalence of comorbid alcohol dependence and major depression increased from 1996 to 2012 in females, those 30 years and older, who had a household income of less than $50,000 and no post-secondary graduate level education. Individuals with substance dependence and cannabis dependence were about three times (pooled OR (95%CI): 3.62 (95%CI 2.82-4.63)) and five times (pooled OR (95%CI): 5.77 (95%CI 3.8-8.77)) more likely to have comorbid major depression. Alcohol dependence increased the risk of persistent or recurrent major depression by three-fold. Comorbid substance use disorder with major depression was significantly associated with increased disability and suicide ideation. Conclusion. Given the significant overlap in comorbid substance use disorder and major depression, it is crucial that co-occurring disorders are managed proactively and concurrently. The consensus of research evidence and clinical expertise is that the treatment of comorbid substance use and mental health disorders is insufficient if they are solely psychiatric focused or addiction focused. It is recommended that an integrated treatment approach should be adopted.