Study: Marijuana use and the risk of psychosis

Does smoking pot over long periods of time increase the risk of hallucinations, delusions or psychosis? Or are people prone to such problems more likely to use marijuana? An Australian study looking into the complex relationship between psychosis and marijuana use was scheduled to appear in the Archives of General Psychiatry in May, but made public early yesterday by the medical journal.

Researchers from the University of Queensland and other Australian schools studied 3,801 young adults born between 1981 and 1984. The study was funded by the National Health and Medical Research Council of Australia. At follow up interviews 21 years into the study, the participants were asked about their use of marijuana and assessed using several methods to measure psychosis including diagnostic interviews.

Overall, 65 of the participants had been diagnosed with schizophrenia or another form of "non-affective psychosis." Among all the study participants longer term use of marijuana was associated with hallucinations, delusions or psychosis.

"Compared with those who had never used cannabis, young adults who had six or more years since first use of cannabis (i.e., who commenced use when around 15 years or younger) were twice as likely to develop a non-affective psychosis and were four times as likely to have high scores" on measures for delusion," the study's authors wrote.

"The nature of the relationship between psychosis and cannabis use is by no means simple," added the researchers.  Those participants who had hallucinations early in life were more prone to start smoking pot sooner and use it more often, they noted.

"This demonstrates the complexity of the relationship: those individuals who were vulnerable to psychosis … were more likely to commence cannabis use, which could then subsequently contribute to an increased risk of conversion to a non-affective psychotic disorder."