The development of the number of new schizophrenia cases in the UK does not support the hypothesis that cannabis use increases schizophrenia risk.
According to research of scientists at Keele University in Staffordshire, UK, the incidence (the number of new diagnosed cases) of schizophrenia in the years 1996 to 2005 does not support the hypothesis that cannabis use increases the risk for the development of schizophrenia or psychosis. For this study an analysis of data from 183 practices in England,Wales,Scotland and Northern Ireland was conducted. The study cohort comprised almost 600,000 patients each year, representing approximately 2.3 per cent of the UK population aged 16 to 44. Between 1996 and 2005 the incidence of schizophrenia and psychoses were either stable or declining.
A recently published study found that cannabis use increased fourfold between 1972 and 2002 and 18 fold among people under 18 years of age. If the risk of schizophrenia is elevated 1.8-fold among light users and 3.1-fold among heavy users as suggested by another study, and assuming an elevated risk for 20 years, an increase in schizophrenia incidence of 29 per cent would have been expected between 1996 and 2005. Researches concluded that “the causal models linking cannabis with schizophrenia/psychoses are not supported by this study” and that “the underlying causes of schizophrenia/psychoses remained stable/declined over the study period.”
(Source: Frisher M, Crome I, Martino O, Croft P. Assessing the impact of cannabis use on trends in diagnosed schizophrenia in the United Kingdom from 1996 to 2005. Schizophr Res. 2009 Jun 26. [Electronic publication ahead of print])