Some research suggests that marijuana smokers are diagnosed with depression more often than nonsmokers are — 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 marijuana use. For example, some people may use marijuana as a way to cope with depression symptoms. Heavy users may also appear depressed as a result of the dulling effects of the drug on feelings and emotions.
There are also links between marijuana and other mental health conditions. Marijuana use may trigger schizophrenia or detachment from reality (psychosis) in people who are at higher risk of psychosis. The symptoms of diagnosed psychotic illness and its course may be aggravated if marijuana use continues. There is also some evidence that teenagers who attempt suicide may be more likely to have used marijuana than those who have not made an attempt. 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.