Australian research suggests that smoking cannabis predisposes people to psychosis. The study published in the Archives of General Psychiatry involved 3,800 people born in Brisbane between 1981 and 1984, which were followed to assess psychosis development such as hallucinations up to the age of 21 years old.

The findings show that 17.7 percent reported using pot for three or fewer years, 16.2 percent used it for four to five years while 14.3 percent had smoked for six or more years. Overall, a total of 223 had experienced at least one positive report for hallucination and 65 received a diagnosis of non-affective psychosis such as schizophrenia.

The researchers reported that young adults who had six or more years of using cannabis were twice as likely to develop a non-affective psychosis, as compared to those who had never used cannabis.

John McGrathe M.D., the study researcher of the University of Queensland informs that non-affective psychosis is a broad category that includes schizophrenia and a handful of less common disorders like delusional disorder. He adds that people with non-affective psychosis, do not have a prominent mood element usually seen in bipolar disorder or mania.

McGrath compares this with depression, saying that many of us get mild or moderate depression from time to time but not all of these individuals meet the criteria for full clinical depression. So too for psychosis - some otherwise well people have isolated symptoms, but no disability,' says McGrath.

The findings of the study also tells us that those who had at least six years since the first time they started using cannabis, were four times as likely to have high scores on an accepted measure of delusionary experiences.

The study reports that similar results were observed in a subgroup of 228 siblings, thus reducing the likelihood that the association was due to unmeasured shared genetic and/or environmental influences. This shows that the nature of the relationship between psychosis and cannabis use is by no means simple but very complicated.

The longer the duration since first cannabis use, the higher the risk of psychosis-related outcomes, says researchers.