If cannabis caused schizophrenia—how many cannabis users may need to be prevented in order to prevent one case of schizophrenia?

ABSTRACT:Background We consider how many cannabis users may need to be prevented in order to prevent one case ofschizophrenia or psychosis [defined as number needed to prevent (NNP)]. Method Calculation for England and Walesusing best available estimates of: incidence of schizophrenia; rates of heavy and light cannabis use; and risk thatcannabis causes schizophrenia. Results In men the annual mean NNP for heavy cannabis and schizophrenia rangedfrom 2800 [90% confidence interval (CI) 2018–4530] in those aged 20–24 years to 4700 (90% CI 3114–8416) inthose aged 35–39. In women, mean NNP for heavy cannabis use and schizophrenia ranged from 5470 (90% CI3640–9839) in those aged 25–29 to 10 870 (90% CI 6786–22 732) in 35–39-year-olds. Equivalent mean NNP forheavy cannabis use and psychosis were lower, from 1360 (90% CI 1007–2124) in men aged 20–24 and 2480 (90%CI 1408–3518) in women aged 16–19. The mean and median number of light cannabis users that would need to beprevented in order to prevent one case of schizophrenia or psychosis per year are four to five times greater than amongheavy users. Conclusions The number of young people who need to be exposed to an intervention to generate NNPand prevent one case of schizophrenia will be even larger. The public health importance of preventing cannabis toreduce schizophrenia or psychosis remains uncertain. More attention should be given to testing the hypothesis thatcannabis is related causally to psychotic outcomes, and to considering what strategies will be the most effective inreducing heavy cannabis use among young people.Keywords Cannabis, models, number needed to prevent, prevention, psychosis, schizophrenia.Correspondence to: Matthew Hickman, Social Medicine, University of Bristol, Canynge Hall, Whiteladies Road, Bristol BS8 2PR, UK.E-mail: matthew.hickman@bristol.ac.ukSubmitted 5 March 2009; initial review completed 12 May 2009; final version accepted 3 July 2009