The research follows last year’s decision by the UK government to reclassify the drug from class C to class B, partly out of concerns that cannabis, especially the more potent varieties, may increase the risk of schizophrenia in young people.  However, the evidence for the relationship between cannabis and schizophrenia or psychosis remains controversial.

The Bristol scientists, with colleagues from the University of Cambridge and the London School of Hygiene and Tropical Medicine, took the latest information on numbers of cannabis users, the risk of developing schizophrenia, and the risk that cannabis use causes schizophrenia to estimate how many cannabis users may need to be stopped to prevent one case of schizophrenia.

The study found it would be necessary to stop 2,800 heavy cannabis users in young men and over 5,000 heavy cannabis users in young women to prevent a single case of schizophrenia.  Among light cannabis users, those numbers rise to over 10,000 young men and nearly 30,000 young women to prevent one case of schizophrenia.

That’s just part of the story.  Interventions to prevent cannabis use typically do not succeed for every person who is treated.  Depending on how effective an intervention is at preventing cannabis use, it would be necessary to treat even higher numbers of users to achieve the thousands of successful results necessary to prevent a very few cases of schizophrenia.

Dr Matthew Hickman, one of the authors of the report published last week in the journal Addiction, said: “Preventing cannabis use is important for many reasons, including reducing tobacco and drug dependence and improving school performance.  But our evidence suggests that focusing on schizophrenia may have been misguided.  Our research cannot resolve the question whether cannabis causes schizophrenia, but does show that many people need to give up cannabis in order to have an impact on the number of people with schizophrenia.  The likely impact of re-classifying cannabis in the UK on schizophrenia or psychosis incidence is very uncertain.”

Paper:

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; 104: 1856-1861