With ongoing debate over the legalisation of cannabis and the potential health risks it poses, understanding the association between its use and schizophrenia is a priority. It has previously been suggested that, even assuming an entirely causal relationship, the required reduction in the number of cannabis users to prevent one case of schizophrenia is in the thousands44. Our findings here highlight the possibility that this association might be bidirectional in causation, and that the risks of cannabis use could be over-estimated. This is an important subtlety to consider when calculating the economic and health impact of cannabis use.
