Previous research has established a moderate relationship between marijuana abuse and depressive disorders.

Now researchers report that the association between marijuana use and major depression probably arises from a combination of shared genetic and environmental vulnerabilities.
Lead author Dr. Michael T. Lynskey and colleagues report the findings of their large study in the Archives of General Psychiatry.
To examine the role of genetics and shared childhood environment in the relationship between marijuana abuse and major depression, Lynskey, at the Washington University School of Medicine in St. Louis, and his team interviewed individuals enrolled in the Australian Twin Register, at around 30 years of age.
They identified 277 pairs in which one twin used marijuana while the other didn’t. The odds of depression, suicidal thoughts and attempted suicide were 1.3 to 3.4 times more likely in the marijuana-using twin compared with the twin who was not a user. The strength of the association was higher in fraternal twins than identical twins.
This latter finding "implies that shared or correlated genetic vulnerabilities make substantial contributions" to the association between marijuana use and depression, the authors report.
Depression onset before age 17 or early suicidal thoughts in fraternal twin pairs, but not identical pairs, was significantly associated with rates of marijuana dependence.
Altogether, these results suggest that "depression does not cause people to use (marijuana), or that (marijuana) use and dependence do not in themselves cause depression," Lynskey said in an interview with Reuters Health. "So from a clinical point of view, it is not necessarily the case that eliminating one of those will automatically get rid of the other."
In other words, he added, both problems "need to be addressed within the treatment setting."