Exploring dual diagnosis in opioid agonist treatment: A comparative study between the Czech Republic and Norway

Background: Mental health comorbidities are common in individuals with opioid use disorder (OUD). The co-occurrence of mental disorders with OUD (also referred to as dual diagnosis) has been associated with increased mortality, negative physical health outcomes, and poor psychosocial functioning. Opioid agonist treatment (OAT) is considered a state-of-the-art intervention effective in reducing illicit substance use and improving the physical as well as mental health of patients with OUD, yet the research on dual diagnosis in large-scale OAT patient populations remains limited.
Aim: This study aimed to (1) estimate the sex-specific prevalence and structure of dual diagnoses in two national cohorts of OAT patients and (2) compare mental disorders between OAT patients and the general population.
Design: A registry-linkage study utilizing national health registers analyzed OAT patient data in the Czech Republic (N = 4,280) and Norway (N = 11,389) from 2010 to 2019. 
Methods: Mental disorders were identified using ICD-10 codes from health registers linked to OAT patient records. Sex-specific age-standardized morbidity ratios (SMR) for 2019 were calculated to compare mental disorders between OAT patients and the two general populations. Sex-stratified analyses were performed.
Results: Overall, we found a large burden of dual diagnosis across sex groups in OAT patients and large country-specific differences in the main categories of mental disorders. The prevalence of dual diagnosis was 57.3% for the Czech Republic and 78.3% for Norway. Anxiety, personality disorders, and depression were the most frequent. The largest country-specific differences were observed for ADHD (0.5% in the Czech Republic and 15.8% in Norway). The one-year prevalence of dual diagnosis among OAT patients in 2019 was 19.6% for the Czech Republic and 32.9% for Norway. In the Czech Republic, the SMR for all mental disorders (excl. substance use disorders) was 5.1 for males and 3.1 for females. In Norway, the corresponding estimates were 8.2 for males and 5.6 for females. OAT females had a significantly higher prevalence of co-occurring mental disorders, whereas SMRs were higher in OAT males. 
Conclusions: Our findings support the need for early mental health screening, treatment of mental health comorbidities, and the integration of health promotion activities to reduce psychiatric morbidity in OAT patients. The sex differences underline the need for a tailored approach to address specific medical conditions. Country-specific differences may stem from variations in diagnostics and care, coding and reporting practices, OAT provision, or substance use patterns.