Dynamics between cannabis use and experience of psychotic symptoms: a follow-up study

Abstract

Various lines of evidence suggest an association between cannabis use and psychotic symptoms, although the causational direction is still not clear. The current study investigated the longitudinal relationship between cannabis use and psychotic symptoms by measuring cannabis use and level of symptoms at two points in time (with an average of 18.8 months between measurement points), and using ‘other drugs used in the past 12 months’ as a co variable. A significant relationship between cannabis use and psychotic symptoms over time was found; however, this was only the case when the co variable was introduced into the analysis. This could be explained by the idea that cannabis is a proxy of overall drug use i.e. difference in symptoms over time could be due to any drug, not just cannabis. Also, selection bias could be a factor here. In addition, there was no significance found for the coefficients, which means direction of causality can not be determined. These results imply a less straight forward association between cannabis and psychotic symptoms than suggested by the literature on this topic. However, this discrepancy could be due to limitations of the current study, like the categorization of cannabis use, a small N, or the dichotomous co variable.