We also found a significant genetic correlation between cannabis use and schizophrenia (rg=0.24), which is in line with previous findings20,21, indicating that genetic risk factors for cannabis use and schizophrenia are positively correlated. As for the causal direction of this correlation, we found weak evidence for a causal link from cannabis use to schizophrenia and much stronger evidence for a causal link from schizophrenia to cannabis use. This suggests that individuals with schizophrenia have a higher risk to start using cannabis. These results are in contrast with results from a MR study by Vaucher et al.23, who found strong evidence for a causal effect from cannabis use to schizophrenia (causality in the other direction was not tested). However, our findings are in line with a MR study by Gage et al.24 who used genetic instruments similar to ours and also found weak evidence for a causal effect of cannabis use to schizophrenia and much stronger evidence for a causal effect in the other direction. Our findings may indicate that individuals at risk for developing schizophrenia experience prodromal symptoms or negative
