The relationship between psychopathology and criminal offending has been the subject of many studies. Co-occurring substance use seems to increase the risk of offending in those with mental illness.

To present data on the prevalence of mental disorders and demographics of prisoners with mental illness, and investigate associations between diagnoses and substance use from a network perspective.

Data used in this study are part of a cohort study within the four penitentiary psychiatric centres in The Netherlands. It includes data of 4956 incarcerated male patients. Prevalence rates of mental disorders and demographic variables were compared between individuals with and without problematic substance use. A network of diagnoses, including three categories of substance use, was constructed with regression coefficients.

Most patients showed prior problematic substance use (72.2%) in more than one category of substances (58.7%). Problematic substance use was associated with diagnoses of schizophrenia spectrum disorders (χ2(1) = 37.52, P < 0.001, V = 0.09) and cluster B personality disorders (χ2(1) = 56.39, P < 0.001, V = 0.11). Three major findings of the network are discussed in detail: the role of antisocial personality disorder, impulsivity and psychotic disorders in combination with problematic substance use.

Problematic substance use is highly prevalent among prisoners with mental illness, and should always be taken into account in research on this topic. Treatment should target substance use to reduce the risk of recidivism. Further differentiation in categories of substances is needed for the development of risk profiles.