Clarissa Janousch

The co-occurrence of substance use disorders (SUDs) and other psychiatric disorders (oPDs) is a major global public health and policy concern in adolescents and young adults. Past research has commonly examined single disorder pairs, with comorbidities considered discrete events. Yet, new models of psychopathology stress the need for dimensional and longitudinal perspectives that account for complex, changing patterns of mental health issues. Despite these developments, several key research gaps remain. First, few studies have investigated intricate multi-disorder comorbidity patterns (i.e., multimorbidity) beyond simple disorder pairs. Second, longitudinal studies with closely spaced measures spanning key developmental periods, such as adolescence to adulthood, are scarce, limiting insights into onset, remission, and relapse. Third, most research relies on self-reported data, which can introduce bias, and few studies have utilized large-scale, registry-based data to track comorbidity at the population level. Finally, there is a lack of research on how sociodemographic and contextual factors (e.g., sex, age, healthcare utilization) influence these patterns and their transitions over time.This project will address these gaps by examining the heterogeneity, development, and transition of co-occurring SUDs and oPDs over time. Using a large register-based cohort in Sweden (~3 million individuals born 1970-2000), the project will pursue three overall aims: (1) Identify and characterize distinct patterns of multimorbidity, (2) investigate developmental trajectories including onset, remission, and relapse across the lifespan, and (3) examine intra-individual transitions between different multimorbidity profiles. For all investigations, all mentioned factors related to these patterns and transitions will be examined. This project will substantially enhance our knowledge of multimorbidity in SUDs and oPDs and the underlying mechanisms within the general population. It will contribute to identifying population subgroups at increased risk and clarifying how multimorbidity develops across different phases of the lifespan. Ultimately, the results will help guide targeted prevention efforts and support the development of evidence-based public health policies.