Increasing evidence suggests that early and heavy cannabis exposure may increase the risk of developing a psychotic disorder. The relationship between cannabis exposure and schizophrenia fulfills some but not all of the usual criteria for causality. Despite some empirical support for a causal hypothesis between cannabis use and psychotic outcome, most people who use cannabis do not develop schizophrenia, and most people with schizophrenia have never used cannabis. This is similar to the role of genetics in schizophrenia—identical twins are not concordant for schizophrenia, and most people with schizophrenia do not have a family history of the disorder. Furthermore, there is a significant mismatch between the rates of cannabis abuse and those of schizophrenia. This might be similar to the role of dietary sodium and hypertension—the rates of salt consumption far exceed the rates of hypertension. The increase in cannabis use, the use of more potent forms of cannabis, and the earlier age of first use has not been accompanied or followed by a commensurate increase in the rates of schizophrenia or an earlier age of onset of the illness.
