A number of studies, using different methodologies, have examined causal relationships between depression and smoking with smoking increasing the risk of depression 40, a bi-directional effect 41 and no effect 42 reported. In the current analysis there was an insufficient number of instrumental variants to test the effect of smoking on depression, but there was evidence of a genetic correlation between depression and cigarette smoking as well as a causal effect of depression on smoking. Smoking has been reported to have an anxiolytic and antidepressant effect 43 and this may explain why we observe a causal effect of depression on smoking; however, the relationship is likely to be complex and requires further investigation.
