LAUSR

Abstract Background A large body of evidence shows that comorbidity is pervasive among individuals being treated for mental disorders, including schizophrenia. Those with mental disorders have been found to have higher rates of comorbidity and associated mortality due to a range of general medical conditions. However much of the evidence relies on surveys, cross-sectional studies and retrospectively collected data. Methods Using the Danish registers, we identified those born in Denmark (1900–2015) and who resided there during the study period (2000–2016). Information was obtained on ten groups of mental disorders (MDs), including “Schizophrenia spectrum disorders” (corresponding to ICD-10 F20-F29 codes). We compared the rate of diagnosis with nine general medical conditions (GMC) categories between those exposed and unexposed to each MD. Overall and lagged hazard ratios, with 95% confidence intervals, were calculated using Cox proportional hazards models. Absolute risks were estimated by competing risks survival analyses. Results The cohort followed 5.9 million individuals for 83.9 million person years. For the majority of prior MD-later GMC pairs, receiving a MD diagnosis increased the risk of subsequent GMC diagnosis; however, there were some exceptions. Focusing on those diagnosed with schizophrenia, elevated rates of diagnosis were observed for circulatory, endocrine, pulmonary, gastrointestinal, urogenital and hematological GMCs, and cancers. Various patterns were seen for lagged HRs. Results for schizophrenia generally showed hazard ratios for GMC diagnoses were most elevated the first couple of years after schizophrenia diagnosis. Cumulative incidence proportions for each GMC category 15 years after diagnosis with schizophrenia varied between 3.4% (for urogenital GMCs, 95% CI 3.2, 3.6) and 34.0% (for circulatory GMCs, 95% CI 33.51, 34.57). Discussion This study provides a comprehensive picture of GMC comorbidity among those with diagnosed with MDs in Denmark. It is the first study, to our knowledge, to provide both relative and absolute measures of the risk of such comorbidity. We hope that our findings related to specific MD-GMC comorbidity pairs will stimulate future research to explore mechanisms of action underlying of the observed associations. The provision of absolute risk estimates may aid in identifying those at greater need of primary prevention of GMC comorbidity. As MDs typically have a younger age of onset than many GMCs, natural critical windows exist for the reduction of secondary comorbidity.