Mayo Clinic answers:
Some research suggests a connection between marijuana and depression — particularly regular or heavy marijuana use. However, it’s not clear whether marijuana actually causes depression or whether marijuana use simply goes hand in hand with depression for some people. It’s possible that the genetic, environmental or other factors that make someone more likely to use marijuana also make the person more likely to experience depression. The reverse also may be true. The circumstances that trigger depression may make marijuana use more likely.
The bottom line: Marijuana use and depression accompany each other more often than you might expect by chance, but there’s not enough evidence to indicate that marijuana directly causes depression.
www.mayoclinic.com is an excellent resource for health information