An association between the diagnosis of schizophrenia and the risk for subsequent development of substance abuse has been suggested; and a prospective cohort study on the subject was conducted in Denmark with the use of a longitudinal study design. Results of the analysis were published in the journal Addiction.

The investigators sought to explore whether a diagnosis of schizophrenia increases an individual’s risk of being diagnosed with a substance abuse disorder. Individuals who were born in Denmark (between 1955 and 1999) and registered in the Danish registers from January 1, 1968, through July 1, 2013, participated in the study. The links between schizophrenia and diagnosis of substance abuse, according to the International Classification of Diseases, were determined through a variety of Danish registers. The Cox regression model was used and adjusted based on calendar year, gender, urbanicity, other psychiatric diagnoses, substance abuse, and psychiatric history of parents, immigration status of parents, and socioeconomic status of parents. Moreover, patients who had been diagnosed with substance abuse <1 year following a diagnosis of schizophrenia were classified as not being diagnosed with schizophrenia.

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The patient cohort comprised 3,133,968 individuals. During the follow-up period (ie, 103,212,328 person-years at risk), 14,007 persons developed schizophrenia with 2885 of these participants subsequently being diagnosed with substance abuse (incidence rate, 137.25/10,000 person-years at risk). In fact, a diagnosis of schizophrenia was positively associated with the risk for development of substance abuse disorder after Cox regression adjustments (hazard ratio [HR], 3.69; 95% CI, 3.56-3.83). More men were diagnosed with both schizophrenia (56/8%) and subsequent substance abuse (65.6%) compared with women.

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When various types of substances were examined individually, according to unadjusted results, the highest associations between schizophrenia and substance abuse were shown with stimulants (HR, 22.56; 95% CI, 20.60-24.70) and hallucinogens (HR, 18.94; 95% CI, 14.24-25.18).

The presence of co-abuse was common, with 16.9% of individuals who were diagnosed with substance abuse being diagnosed with >1 type of substance abuse during follow-up. Further adjustments for co-abuse had an effect on the associations as well, with a prior diagnosis of schizophrenia significantly increasing the risk for abuse of cannabis (HR, 2.48; 95% CI, 2.34-2.64), alcohol (HR, 1.94; 95% CI, 1.87-2.02), stimulants (HR, 1.77; 95% CI, 1.61-1.95), and other types of substances (HR, 1.36; 95% CI, 1.20-1.54). Although the risk decreased over time, the association remained significant at 10 to 15 years following a diagnosis of schizophrenia (HR, 2.50; 95% CI, 2.26-2.76).

The investigators concluded that in Denmark, a diagnosis of schizophrenia is significantly associated with an elevated risk of being diagnosed with a substance abuse disorder. These findings underscore the importance of integrating treatment of schizophrenia and substance abuse, along with prevention of substance abuse in persons diagnosed with schizophrenia, prior to the onset of a substance abuse disorder.


Petersen SM, Toftdahl NG, Nordentoft M, Hjorthøj C. Schizophrenia is associated with increased risk of subsequent substance abuse diagnosis – a nationwide population based register study [published online July 13, 2019]. Addiction. doi:10.1111/add.14746