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Hickman M, Vickerman P, Macleod J, Lewis G, Zammit S, Kirkbride J, Jones P. 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. Addiction . 2009 Nov 1 ; 104(11):1856-61. PubMed Abstract

Comments on Paper and Primary News
Comment by:  John McGrath, SRF Advisor
Submitted 27 October 2009 Posted 28 October 2009
  I recommend this paper

This interesting paper uses the best available data examining the association between cannabis use and schizophrenia to estimate a metric called Number Needed to Prevent (NNP). Clinicians will be familiar with the related concept of Number Needed to Treat (NNT)—the smaller the number, the stronger the effect size of an intervention. The NNP metric for cannabis and psychosis is very large. For example, for young people who are heavy users, we would need to stop use in about 2,800 individuals in order to prevent one member of this cohort from going on to develop a case of schizophrenia (that would not otherwise have occurred due to other factors).

The NNP is even higher in other groups. In other words, because cannabis does not appear to be a potent (large-effect) causal factor, the NNP is large and unimpressive. Taking into account the second-order issue that interventions to stop people from using cannabis themselves have a high NNT, leveraging cannabis use in order to stop psychosis does not look like an appealing prospect. However, cannabis use is also linked to...  Read more

View all comments by John McGrath

Comment by:  David J. Porteous, SRF Advisor
Submitted 28 October 2009 Posted 29 October 2009
  I recommend this paper

This important study demonstrates the need for evidential epidemiology and associated risk estimates as an essential balance to the sort of knee-jerk responses that all too often follow studies linking drug use and mental health problems.

View all comments by David J. Porteous

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