A study to be published in May's edition of Archives of General Psychiatry, a monthly medical journal by the American Medical Association, shows the constant use of marijuana over a long period of time beginning during adolescence makes delusions and psychosis more likely.


John McGrath, M.D., Ph.D., F.R.A.N.Z.C.P., of the Queensland Brain Institute, University of Queensland, Australia, led a team of researchers found that over 35% of their subjects used cannabis for 4-6 years beginning around age 15 and  "compared with those who had never used cannabis, ...were twice as likely to develop a non-affective psychosis and were four times as likely to have high scores on the Peters et al Delusions Inventory, a measure of delusion".

"There was a ‘dose-response' relationship between the variables of interest: the longer the duration since first cannabis use, the higher the risk of psychosis-related outcomes," continued the researchers.

"The nature of the relationship between psychosis and cannabis use is by no means simple." The AMA echoed the studies authors' sentiment and said the following with regards to McGrath's study, "concerns remain that this research has not adequately accounted for confounding variables".

Full research study is available pre-embargo to the media at http://www.jamamedia.org.