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Annual incidence of cannabis-induced psychosis, other substance-induced psychoses and dually diagnosed schizophrenia and cannabis use disorder in Denmark from 1994 to 2016

Published online by Cambridge University Press:  16 December 2019

Carsten Hjorthøj Open the ORCID record for Carsten Hjorthøj [Opens in a new window] ,
Maria Oku Larsen ,
Marie Stefanie Kejser Starzer  and
Merete Nordentoft
Carsten Hjorthøj*
Affiliation:
Copenhagen Research Center for Mental Health – CORE, Mental Health Center Copenhagen, Copenhagen University Hospital, Copenhagen, Denmark Department of Public Health, Section of Epidemiology, University of Copenhagen, Copenhagen, Denmark
Maria Oku Larsen
Affiliation:
Copenhagen Research Center for Mental Health – CORE, Mental Health Center Copenhagen, Copenhagen University Hospital, Copenhagen, Denmark Department of Epidemiology Research, Statens Serum Institut, Copenhagen, Denmark
Marie Stefanie Kejser Starzer
Affiliation:
Copenhagen Research Center for Mental Health – CORE, Mental Health Center Copenhagen, Copenhagen University Hospital, Copenhagen, Denmark
Merete Nordentoft
Affiliation:
Copenhagen Research Center for Mental Health – CORE, Mental Health Center Copenhagen, Copenhagen University Hospital, Copenhagen, Denmark
*
Author for correspondence: Carsten Hjorthøj, E-mail: Carsten.hjorthoej@regionh.dk
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Abstract

Background

Worldwide, cannabis is the most used illegal substance, and the use of cannabis has increased over the years. An increase in the level of tetrahydrocannabinol (THC) in cannabis has also been seen. It is currently unclear whether this has led to an increase in the incidence of cannabis-induced psychosis. We aimed to investigate (1) the development of incidence of cannabis-induced psychosis over time compared with other substance-induced psychoses and (2) the development of incident cases of cannabis-induced psychosis over time compared with dual diagnosis defined as schizophrenia and a cannabis use disorder.

Method

Data on psychiatric diagnoses were extracted from the Danish Psychiatric Central Research Register and summarized per year as both absolute incidence (number of cases) and incidence rates per 100 000 person years.

Results

The incidence rate of cannabis-induced psychosis increased steadily from 2.8 per 100 000 person years in 2006 to 6.1 per 100 000 person years in 2016. There was a corresponding increase in dual diagnosis with schizophrenia and cannabis use disorder, but a decrease in alcohol-induced psychosis. The data showed no trend in the other substance-induced psychosis investigated in this thesis.

Conclusion

The increase in cannabis-induced psychosis follows both the increase in the level of THC in cannabis, and the increase in cannabis use. The change in diagnostic practice does not appear to explain the increase in incidence of cannabis-induced psychosis.

Keywords

Cannabis use disordercannabis-induced psychosisdual diagnosisincidenceschizophreniasubstance-induced psychosis
Type
Original Article
Information
Psychological Medicine , Volume 51 , Issue 4 , March 2021 , pp. 617 - 622
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Copyright
Copyright © Cambridge University Press 2019

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