© 2005 The Royal College of Psychiatrists
Cannabis-induced psychosis and subsequent schizophrenia-spectrum disorders: follow-up study of 535 incident cases
Centre for Basic Psychiatric Research, Aarhus University Hospital, Risskov
Unit for Psychiatric Research, Aalborg Psychiatric Hospital, Aarhus University Hospital, Aalborg, Denmark
Correspondence: Mikkel Arendt,Centre for Basic Psychiatric Research, Aarhus University Hospital, Skovagervej 2, DK-824 0 Risskov, Denmark. E-mail: mca{at}psykiatri.aaa.dk
Background Few studies have examined samples of people with cannabis-induced psychotic symptoms.
Aims To establish whether cannabis-induced psychotic disorders are followed by development of persistent psychotic conditions, and the timing of their onset.
Method Data on patients treated for cannabis-induced psychotic symptoms between 1994 and 1999 were extracted from the Danish Psychiatric Central Register. Those previously treated for any psychotic symptoms were excluded. The remaining 535 patients were followed for at least 3 years. In a separate analysis, the sample was compared with people referred for schizophrenia-spectrum disorders for the firsttime, but who had no history of cannabis-induced psychosis.
Results Schizophrenia-spectrum disorders were diagnosed in 44.5% of the sample. New psychotic episodes of any type were diagnosed in 77.2%. Male gender and young age were associated with increased risk. Development of schizophrenia-spectrum disorders was often delayed, and 47.1% of patients received a diagnosis more than a year after seeking treatment for a cannabis-induced psychosis. The patients developed schizophrenia at an earlier agethanpeople in the comparison group (males, 24.6 v. 30.7 years, females, 28.9 v. 33.1 years).
Conclusions Cannabis-induced psychotic disorders are of greatclinical and prognostic importance.
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