Cannabis use is not associated with the development of psychosis in an 'ultra' high-risk group

The association between cannabis use and the development of a first psychotic episode was studied in a group of 100 young people identified as being at very high risk for the onset of psychosis.

The 'ultra' high risk cohort was identified by the presence of subthreshold psychotic symptoms, or a combination of first-degree relative with a psychotic disorder and recent functional decline. Thirty-two per cent of the cohort developed an acute psychotic episode over the 12-month period after recruitment. As a component of a larger research study, the level of cannabis use by participants in the year prior to enrollment in the study was assessed at intake.

Cannabis use or dependence in the year prior to recruitment to this study was not associated with a heightened risk of developing psychosis over the following 12-month period and therefore did not appear to contribute to the onset of a psychotic disorder.

The results of this study suggest that cannabis use may not play an integral role in the development of psychosis in a high-risk group. While this study does not support a role for cannabis in the development of first-episode psychosis, we cannot conclude that cannabis use should be completely ignored as a candidate risk factor for onset of psychosis. A number of weaknesses of the study (the low level of cannabis use in the current sample, the lack of monitoring of cannabis use after intake) suggest that it may be premature to dismiss cannabis use as a risk factor for the development of psychosis and further research is urged in this area.