AIMS: We estimated the prevalence of substance use disorders (SUDs) in the Norwegian, Danish and Swedish prison populations and compared the prevalence of SUDs in the national prison populations with country-specific general population prevalence rates.

DESIGN: A multi-national cohort study using data from the National Prison Registries linked to the National Patient Registries in Norway, Denmark and Sweden.

SETTING AND PARTICIPANTS: We used data from the PriSUD-Nordic study, including national prison populations aged 19 years and older in Norway (2010-19), Denmark (2010-18) and Sweden (2010-13). A total of 119 507 Individuals (108 971 men and 10 536 women) contributing to 191 507 incarcerations were included in the study (Norway: 45432 men; 5429 women, Denmark: 42 162 men; 3370 women, Sweden: 21 377 men; 1737 women).

MEASUREMENT: We calculated a study prevalence and prevalence at entry to prison for all types of SUDs before imprisonment each consecutive year of observation in each prison population. We also extracted country-specific 1-year prevalence rates from the Global Burden of Diseases database to calculate comparative national prevalence ratios.

FINDINGS: The study prevalence of any SUD was approximately 40% [Norway: 44.0%, 95% confidence interval (CI) = 43.6-44.5%; Denmark: 39.9%, CI = 39.5-40.4%; Sweden: 39.1%, CI = 38.4-39.7%] in all three countries. Women had a significantly higher study prevalence of any SUD compared with men (Norway: 55.8 versus 42.6%, P < 0.001; Denmark 43.1 versus 39.7%, P = 0.004; Sweden: 51.7 versus 38.0%, P < 0.001). Prevalence estimates were higher for SUDs among people in prison than in the general population. We observed an increasing proportion of people with SUDs entering prison in Norway (P = 0.003), while the proportion was more stable in Denmark and Sweden.

CONCLUSIONS: Substance use disorders (SUDs) appear to be highly prevalent among the Scandinavian prison populations compared with the general population, especially among women. In Norway, there was a relative increase in SUDs from 2010 to 2019.