Marijuana (cannabis) inside a dispensary in Colorado, U.S. Photographer: Matthew Staver/Bloomberg
© Bloomberg

While I agree with Simon Kuper that cannabis is associated with harm, it is difficult to know exactly the extent of the drug’s potential to harm health (“ High society”, Life & Arts, October 27). Decades of research have tried to establish whether cannabis is a causal factor in psychosis but this is a messy and complicated area of research, as among many factors most people combine tobacco with cannabis. It is possible that this combination or even tobacco on its own is the culprit.

Estimates suggest that we would need to prevent 20,000 young people ever using cannabis to prevent one case of psychosis; comparing that with the risks of tobacco and cancer makes cannabis seem almost benign, assuming we can decouple it from tobacco.

Ian Hamilton
Associate Professor in Addiction,
University of York, UK

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