The University of Washington Alcohol and Drug Abuse Institute stated the following in an online fact sheet titled "Mental Health and Marijuana," based on information from the National Cannabis Prevention and Information Centre (acessed June 14, 2012):

“Evidence suggests that marijuana may somehow trigger schizophrenia in those who are already at risk of developing the disorder. Those with a vulnerability to develop schizophrenia, such as having a family history of the illness, should be strongly advised against using marijuana for this reason…

Marijuana may seem to help ease depression before the effects of the drug wear off; however after that, smoking marijuana may make depression worse. Those who use marijuana have been shown to have higher levels of depression and depressive symptoms than those who do not use marijuana. Although results are mixed, there is a substantial amount of evidence to suggest that marijuana use, particularly frequent or heavy use, predicts depression later in life. Young women appear to be more likely to experience this effect.

Marijuana can lead to symptoms of anxiety, such as panic, in the short-term, but there is a lack of evidence pointing to marijuana as an important risk factor for chronic anxiety disorders…

Again, if someone has a genetic vulnerability or has an existing mental health issues, marijuana should be avoided.”

June 14, 2012