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A new study in the UK has cast doubt on the supposed link between cannabis use and schizophrenia.
In terms of the model set out in the Introduction, the expected rise in diagnoses of schizophrenia and psychoses did not occur over a 10 year period. This study does not therefore support the specific causal link between cannabis use and the incidence of psychotic disorders based on the 3 assumptions described in the Introduction. This concurs with other reports indicating that increases in population cannabis use have not been followed by increases in psychotic incidence ([Macleod et al., 2006], [Arsenault et al., 2004] and [Rey and Tennant, 2002]). However, it is not in line with findings of a rise in first admission rates for psychotic disorders among young people in Zurich following increases in cannabis availability and consumption (Ajdacic-Gross et al., 2007). One factor involved in this discrepancy may be the potency of the cannabis consumed, which varies considerably within Europe (EMCDDA, 2008). In addition, a Netherlands study found that high-potency cannabis obtained from ‘coffee shops’ led to higher levels of tetrahydrocannabinol (THC) in the blood, with young males aged 18–45 at particular risk for excessive consumption (Mensinga et al., 2006 T. Mensinga, I. de Vries, M. Kruidenier, C. Hunault, C. van den Hengel-Koot, M. Fijen, M. Leenders and J. Meulenbelt, Dubbel-blind, geramndomiseerd, placebogecontroleered, 4-weg gekruist onderzoek naar de farmacokinetik en effecten van cannabis [A double-blind, randomized, placebo-controlled, cross-over study on the pharmacokinetics and effects of cannabis], RIVM rapport 267002001/2006, Bilthoven (2006).Mensinga et al., 2006)."