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Estimates of how many cannabis users need to be prevented in order to prevent one case of schizophrenia

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If cannabis causes schizophrenia, how many cannabis users need to be prevented (number needed to prevent (NNP)) in order to prevent one case of schizophrenia in England and Wales?


Model estimates the NNP for England and Wales, that is number of cannabis users to be prevented; estimate of schizophrenia or psychosis incidence from the Aetiology and Ethnicity in Schizophrenia survey conducted between 1997 and 1999.


England and Wales.

Risk factors:

Cannabis use; estimate of risk of developing psychosis with heavy and light cannabis use (compared with non-user) based on data available in a recent meta-analysis.


NNP is defined as the number of heavy or light cannabis users who would need to be prevented in that year in order to prevent one incident case of schizophrenia or psychosis per year (both for men and women by age).



Aetiology: Modelling study.

Follow-up period:

Twelve-month exposure data.

Main results

In heavy cannabis users, the annual mean NNP for …

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  • Source of funding: No specific external funding provided.


  • Competing interests: None.

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