Nationwide time trends in co-occurring mental illness and substance use disorders among adolescents and young adults: differences in diagnoses, age, gender, and educational attainment between 2010 and 2022

Abstract This study aims to describe time trends in dual diagnosis occurrence among adolescents and young adults (ages 12–25) between 2010 and 2022 in Denmark. It also aims to describe how incident cases are distributed by age, gender, diagnoses, educational attainment, and enrollment in substance use treatment services. The study used diagnostic data on every admission to public mental health hospitals in Denmark linked with data from other national social service- and educational registers. Our analyses show a substantial increase in dual diagnosis occurrence in 2010-17, followed by a declining trend. When distributed by gender, age and diagnoses, dual diagnosis was most frequently occurring among males; young adults (ages 22–25); adolescents and young adults with schizophrenia, schizotypal and delusional disorders; neurotic, stress-related and somatoform disorders; as well as cannabis related disorders across the study period. Our analyses also show an increase in the proportion who were enrolled in substance use treatment at the time of their first dual diagnosis contact with mental health services, but a decreasing trend in the proportion who enrolled within a year after their first dual diagnosis contact. Finally, our analyses show a slight increase in the number of adolescents and young adults with dual diagnosis above 16 years of age who had completed primary school (9th grade) during the study period.