Does the use of cannabis increase the risk for psychosis and the development of schizophrenia?

Over the past 30 years evidence has been growing that cannabis use increases the risk for psychosis which could develop into schizophrenia in a proportion of cases. Over the past decade many studies have been published which clarify the association between cannabis use and psychosis. The aim of this review is to examine this association. A systematic search yielded 14 cohort studies carried out in 9 cohorts and 9 case-control studies. When the results of these studies are taken together they unambiguously support that cannabis use is an independent risk factor for psychosis and may also give rise to chronic psychotic disorders like schizophrenia. A dose dependent link is present because more frequent use associates with greater risk. The studies also show that cannabis-use in adolescence is associated with greater risk of developing psychosis than commencing the use of cannabis in adulthood. Further studies are needed to explain this association since psychotic disorders take years to evolve and it remains difficult to measure both the explanatory and the response variable and their complex relationship. The results emphasize the need to enhance public knowledge on the possible consequences of cannabis use and the fact that it cannot be predicted who will experience transient psychosis and who will develop a chronic psychotic disorder.