Could Cannabis Use Cause Schizophrenia?

Individuals who smoke cannabis have a slightly higher risk of developing schizophrenia, according to the results of a recent study.

The researchers used Mendelian randomization on data from 2 genome-wide association studies to assess the causal relationship between cannabis initiation and schizophrenia, and used the MR-Egger regression to investigate any pleiotropic effects of single nucleotide polymorphisms on cannabis use and schizophrenia. They included education as a positive control, and height as a negative control to support any causal interpretations.  
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Their analysis showed that individuals at risk for developing schizophrenia are more likely to use cannabis. Likewise, they found individuals who smoke cannabis have a slightly higher risk of developing schizophrenia (24%) than those who do not smoke cannabis (10%). In addition, there was no association with height and cannabis initiation, as predicted by the researchers. Lower education levels were associated with cannabis initiation, although there was weaker evidence than the researchers predicted.

“Our results provide some evidence that cannabis initiation might be causally associated with odds of developing schizophrenia, using 2-sample MR to support stronger causal inference, although the size of the association is small. The results provide stronger evidence that schizophrenia risk predicts likelihood of cannabis initiation,” the researchers concluded.

—Melissa Weiss

Reference:

Gage SH, Jones HJ, Burgess S, et al. Assessing causality in associations between cannabis use and schizophrenia risk: a two-sample Mendelian randomization study [published online December 8, 2016]. Psychological Medicine. doi:https://doi.org/10.1017/S0033291716003172.