1. Annual incidence of substance-induced psychoses in Scandinavia from 2000 to 2016

Substance-induced psychosis (SIP) is a serious condition and may predispose for schizophrenia. Little is know about SIP incidence over time and across countries, including substance-specific SIPs. We estimated the annual incidence rate of SIP in Denmark, Norway, and Sweden according type of substance, age, gender, and socioeconomic background.

Data were drawn from national registries covering the whole population in the countries. Annual incidence rate per 100,000 persons of any SIP and substance-specific SIPs was estimated for Danish and Swedish data from 2000 to 2016 and for Norwegian data from 2010 to 2015. Conflict of interest: none.

The annual incidence rate of any SIP fluctuated between 9.3 and 14.1 in all countries. The most commonly occurring SIPs were those induced by alcohol, cannabis, amphetamines, and multiple substances. There was a steady decrease in the incidence rate of alcohol-induced psychosis in Denmark (from 4.9 to 1.5) and Sweden (from 4.5 to 2.2). Simultaneously, the incidence rate of cannabis-induced psychosis increased in all countries, from 2.6 to 5.6 in Denmark, from 0.8 to 2.7 in Sweden, and from 1.8 to 3.0 in Norway. Median age of any SIP decreased. Incidence rates were higher in men and in individuals on disability pension and increased among those with high parental education.

In this study of annual incidence of SIP in Scandinavian countries, we found similar and stable incidence rates of any treated SIP in all countries through the observation period. The incidence of alcohol-induced psychosis decreased, and the incidence of cannabis-induced psychosis increased.