Morbidity and mortality in schizophrenia with comorbid substance use disorders

Objective Schizophrenia is highly comorbid with substance use disorders (SUD) but large epidemiological cohorts exploring the prevalence and prognostic significance of SUD are lacking. Here, we investigated the prevalence of SUD in patients with schizophrenia in Finland and Sweden, and the effect of these co-occurring disorders on risks of psychiatric hospitalization and mortality. Methods 45,476 individuals with schizophrenia from two independent national cohort studies, aged <46 years at cohort entry, were followed during 22 (1996-2017, Finland) and 11 years (2006-2016, Sweden). We first assessed SUD prevalence (excluding smoking). Then, we performed Cox regression on risk of psychiatric hospitalization and all-cause and cause-specific mortality in SUD compared with those without SUD. Results The prevalence of SUD ranged from 26% (Finland) to 31% (Sweden). Multiple drug use (n = 4164, 48%, Finland; n = 3268, 67%, Sweden) and alcohol use disorders (n = 3846, 45%, Finland; n = 1002, 21%, Sweden) were the most prevalent SUD, followed by cannabis. Any SUD comorbidity, and particularly multiple drug use and alcohol use, were associated with 50% to 100% increase in hospitalization (aHR any SUD: 1.53, 95% CI = 1.46-1.61, Finland; 1.83, 1.72-1.96, Sweden) and mortality (aHR all-cause mortality: 1.65, 95% CI = 1.50-1.81, Finland; 2.17, 1.74-2.70, Sweden) compared to individuals without SUD. Elevated mortality risks were observed especially for suicides and other external causes. All results were similar across countries. Conclusion Co-occurring SUD, and particularly alcohol and multiple drug use, are associated with high rates of hospitalization and mortality in schizophrenia. Preventive interventions should prioritize detection and tailored treatments for these comorbidities, which often remain underdiagnosed and untreated.