title: If cannabis caused schizophrenia--how many cannabis users may need to be prevented in order to prevent one case of schizophrenia? England and Wales calculations.
abstract: BACKGROUND
  We consider how many cannabis users may need to be prevented in order to prevent one case of schizophrenia or psychosis [defined as number needed to prevent (NNP)].
  
  
  METHODS
  Calculation for England and Wales using best available estimates of: incidence of schizophrenia; rates of heavy and light cannabis use; and risk that cannabis causes schizophrenia.
  
  
  RESULTS
  In men the annual mean NNP for heavy cannabis and schizophrenia ranged from 2800 [90% confidence interval (CI) 2018-4530] in those aged 20-24 years to 4700 (90% CI 3114-8416) in those aged 35-39. In women, mean NNP for heavy cannabis use and schizophrenia ranged from 5470 (90% CI 3640-9839) in those aged 25-29 to 10 870 (90% CI 6786-22 732) in 35-39-year-olds. Equivalent mean NNP for heavy 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 be prevented in order to prevent one case of schizophrenia or psychosis per year are four to five times greater than among heavy users.
  
  
  CONCLUSIONS
  The number of young people who need to be exposed to an intervention to generate NNP and prevent one case of schizophrenia will be even larger. The public health importance of preventing cannabis to reduce schizophrenia or psychosis remains uncertain. More attention should be given to testing the hypothesis that cannabis is related causally to psychotic outcomes, and to considering what strategies will be the most effective in reducing heavy cannabis use among young people.
authors.affiliation: Social Medicine, University of Bristol, Canynge Hall, Bristol BS8 2PR, UK. matthew.hickman@bristol.ac.uk
authors.forename: Matt
authors.initials: M
authors.lastname: Hickman
authors.forename: Peter
authors.initials: P
authors.lastname: Vickerman
authors.forename: John
authors.initials: J
authors.lastname: Macleod
authors.forename: Glyn
authors.initials: G
authors.lastname: Lewis
authors.forename: Stan
authors.initials: S
authors.lastname: Zammit
authors.forename: James
authors.initials: J
authors.lastname: Kirkbride
authors.forename: Peter
authors.initials: P
authors.lastname: Jones
mesh_terms: D000293:Adolescent; D000328:Adult; D004812:Epidemiologic Methods; D005260:Female; D006801:Humans; D008297:Male; D002189:Marijuana Abuse / Q000150:complications / Q000453:epidemiology / Q000517:prevention & control*; D011605:Psychoses, Substance-Induced / Q000453:epidemiology / Q000517:prevention & control*; D012107:Research Design; D012559:Schizophrenia / Q000453:epidemiology / Q000209:etiology / Q000517:prevention & control*; D012737:Sex Factors; D006113:United Kingdom / Q000453:epidemiology; D055815:Young Adult