With many states beginning to legalize cannabis, whether medicinal or recreational, there has been an increase in interest as to what kind of consequences it might have. This particular study wanted to test the psychiatry of those smoking cannabis–particularly if it would affect their tendencies to abuse other drugs afterwards. They also attempted to evaluate whether smoking marijuana would cause mood disorders, like bipolar disorder or anxiety, as some studies have previously.
The study was conducted in two “waves.” One was held 2001-2002 and the second was held from 2004-2005 with the same participants to make it a longitudinal study. The participants were screened for psychiatric disorders like depression, anxiety, substance abuse, etc. and assessed according to the DSM-IV. This was done at the onset of the trial and again at the end. 34,653 participants were interviewed, and of those 1,279 used cannabis.
Those who were smoking cannabis at the time of Wave 1 were much more likely to struggle with mental health issues or substance abuse by the time of Wave 2. However, after adjusting their data, the only firm conclusion they made was that smoking cannabis indeed increased the risk of abusing other substances like alcohol, nicotine, or other drugs. They did not conclude that marijuana induced or worsened mental disorders. They reasoned that maybe the mental health issues came first and the cannabis use was a result, making this relationship correlational but NOT causal.
Blanco C, Hasin DS, Wall MM, et al. Cannabis Use and Risk of Psychiatric Disorders-Prospective Evidence From a US National Longitudinal Study. JAMA Psychiatry. doi:10.1001/jamapsychiatry.2015.3229 (published 17 February 2016).