“Some research suggests that marijuana smokers are diagnosed with depression more often than are nonsmokers — particularly regular or heavy marijuana users. However, it doesn’t appear that marijuana directly causes depression. It’s likely that the genetic, environmental or other factors that trigger depression also lead to using marijuana. For example, some people may use marijuana as a way to cope with depression symptoms.
There are also links between marijuana and other mental health conditions. Marijuana use may trigger schizophrenia or detachment from reality (psychosis) in certain people. There is also some evidence that adolescents who attempt suicide may be more likely to use marijuana than those who don’t. As with marijuana use and depression, more research is needed to better understand these associations.
The bottom line: Marijuana use and depression accompany each other more often than you might expect by chance, but there’s no clear evidence that marijuana directly causes depression.”June 11, 2012