Our results show that to some extent the association between cannabis and schizophrenia is due to a shared genetic aetiology across common variants. They suggest individuals with an increased genetic predisposition to schizophrenia are both more likely to use cannabis and to use it in greater quantities. This is not to say there is no causal relationship between use of cannabis and risk of schizophrenia, but it does establish that at least part of the association may be due to causal relationship in the opposite direction. While the variance in cannabis use explained by schizophrenia polygenic risk profiles is small, it is in line with other cross-phenotype analyses, largely due to the polygenic risk scores for schizophrenia predicting only ~7% of the variation for schizophrenia itself. Previous associations between polygenic risk scores for schizophrenia and other psychiatric illnesses, such as bipolar disorder, major depression and autism39, have shown effects of similar sizes. Further research will be needed to see if the genetic overlap observed here is specific to cannabis use or is present across illicit drug use and addiction phenotypes,
