Prevalence of co‐occurring mental illness and substance use disorder and association with overdose: a linked data cohort study among residents of British Columbia, Canada

AIMS: To estimate the treated prevalence of mental illness, substance use disorder (SUD) and dual diagnosis and the association between dual diagnosis and fatal and non-fatal overdose among residents of British Columbia (BC), Canada. DESIGN: A retrospective cohort study using linked health, income assistance, corrections and death records. SETTING: British Columbia (BC), Canada. PARTICIPANTS: A total of 921 346 BC residents (455 549 males and 465 797 females) aged 10 years and older. MEASUREMENTS: Hospital and primary-care administrative data were used to identify a history of mental illness only, SUD only, dual diagnosis or no history of SUD or mental illness (2010-14) and overdoses resulting in medical care (2015-17). We calculated crude incidence rates of non-fatal and fatal overdose by dual diagnosis history. Andersen-Gill and competing risks regression were used to examine the association between dual diagnosis and non-fatal and fatal overdose, respectively, adjusting for age, sex, comorbidities, incarceration history, social assistance, history of prescription opioid and benzodiazepine dispensing and region of residence. FINDINGS: Of the 921 346 people in the cohort, 176 780 (19.2%), 6147 (0.7%) and 15 269 (1.7%) had a history of mental illness only, SUD only and dual diagnosis, respectively; 4696 (0.5%) people experienced 688 fatal and 6938 non-fatal overdoses. In multivariable analyses, mental illness only, SUD only and dual diagnosis were associated with increased rate of non-fatal hazard ratio (HR) = 1.8, 95% confidence interval (CI) = 1.6-2.1; HR = 9.0, 95% CI = 7.0-11.5, HR = 8.7, 95% CI = 6.9-10.9, respectively and fatal overdose (HR = 1.6, 95% CI = 1.3-2.0, HR = 4.3, 95% CI = 2.8-6.5, HR = 4.1, 95% CI = 2.8-6.0, respectively) compared with no history. CONCLUSIONS: In a large sample of residents of British Columbia (Canada), approximately one in five people had sought care for a substance use disorder or mental illness in the past 5 years. The rate of overdose was elevated in people with a mental illness alone, higher again in people with a substance use disorder alone and highest in people with a dual diagnosis. The adjusted hazard rates were similar for people with substance use disorder only and people with a dual diagnosis.