Comorbidity of mood disorders and substance use disorders: in concert and co-travelling

This dissertation investigates why mood disorders frequently co-occur with substance use disorders and why such comorbidity persists longitudinally. Part I shows that depressive–anxiety comorbidity is highly prevalent and is characterized by greater severity, chronicity, functional impairment, and coincides with a range of poorer mental, functional and social outcomes. Key risk indicators for this comorbidity are childhood trauma, early onset, and neuroticism. Part II demonstrates that elevated smoking rates in major depressive disorder (MDD) coincide with higher exposure but not with increased susceptibility to smoking risk factors. Part III longitudinally investigates the tendency of mood and substance use disorders to co-occur over time, which is termed ‘co-travelling.’ Findings demonstrated that in bipolar disorders the relationship between substance use and mood was strong in those with few and absent in those with multiple previous mood episodes. In MDD, depression persistence – the lack of any durable clinical remission - was an important risk indicator of smoking persistence over time. These findings indicate that disease progression of mood disorders – more mood episodes in BD, more depression persistence in MDD - may be viewed as a relevant determinant of ‘co-travelling.’ Finally, findings demonstrate that the bipolarity index predicted bipolar conversion in persons with lifetime MDD. Methodological considerations and the conceptual limitations of traditional comorbidity definitions are discussed in this dissertation. Clinically, the findings highlight the need for proactive smoking interventions, integrated treatment for persistent depression and smoking, and research designs that adequately represent psychiatric comorbidity.