Cannabis and Schizophrenia: Which Way Around Does Causality Run?

Does cannabis lead to schizophrenia or does schizophrenia lead to the use of cannabis? That there is a strong, dose-related, association between the use of cannabis and the development of schizophrenia is not in doubt. But association studies cannot prove causality. Furthermore, a dose response can be seen where exposure to the putative causative agent and the true causative agent are correlated.

Genes to the rescue! Power and colleagues looked to see whether genetic polymorphisms that are associated with cannabis use are also associated with schizophrenia in people not exposed to cannabis.[1] They found that genes that pre-disposed to cannabis use also pre-disposed to schizophrenia, independently of whether the person actually used cannabis. The strength of the association between cannabis use predisposing genes and schizophrenia was the same in people who used cannabis, as in those who have never used this substance. Moreover, the risk was the same irrespective of the dose of cannabis consumed. The genetic predisposition to consume cannabis explained less than one-tenth of the variance in cannabis use. Nevertheless, this finding suggests that it is the predisposition to use cannabis, rather than the cannabis itself, that causes the psychiatric symptoms. If corroborated, then this study has important implications for policy.

— Richard Lilford, CLAHRC WM Director

Reference:

1. Power RA, Verwij KJH, Zuhair M, et al. Genetic predisposition to schizophrenia associated with increased use of cannabis. Mol Psychiatr. 2014; 19: 1201-4.