Dr John McGrath, of the Queensland Brain Institute, University of Queensland, Australia, and colleagues performed the study, and have written a paper about it that will be published in the May print issue of the JAMA/Archives journal Archives of General Psychiatry.
This is not the first study to link cannabis use with psychosis risk, say the authors, but previous evidence has been questioned because earlier studies have not dealt adequately with confounding variables (eg other factors the participants could have had in common and might partially explain the results).
For this study, McGrath and colleagues looked at data on 3,801 young adults born between 1981 and 1984 who underwent follow up assessment when they were an average age of 20.1 years.
At the follow up assessment the participants answered questions about cannabis use and took part in various tests to assess psychotic outcomes, including giving diagnostic interviews and completing questionnaires that assessed the extent to which they experienced delusions and/or hallucinations. One of the tests they completed was the Peters et al Delusions Inventory.
The results showed that:
- 17.7 per cent of the participants reported using cannabis for three years or less, 16.2 per cent for four to five years, and 14.3 per cent for six years or more.
- 65 of the participants received a diagnosis of "non-affective psychosis" (eg schizophrenia), and 233 had at least one diagnostic interview item that came out positive for hallucination.
- Taking all participants into account, a longer duration of cannabis use was linked to multiple psychosis-related outcomes.
- Those who had used cannabis for six years or more, and started at around 15 years of age, were twice as likely to develop a non-affective psychosis, and four times more likely to have high scores on the Peters et al Delusions Inventory than participants who reported never having used cannabis.
In a separate analysis, the researchers also looked at the link between cannabis use and psychotic symptoms in 228 sibling pairs among the participants.
They found that the link persisted, thus showing that it was unlikely to be "due to unmeasured shared genetic and/or environmental influences".
However, the researchers cautioned that we should not interpret these findings as meaning there is a simple one-way link between cannabis use and psychosis. For instance, participants who had experienced hallucinations early in life were also more likely to have used cannabis longer and more frequently.
The researchers said this shows that the relationship is complex:
"Those individuals who were vulnerable to psychosis (ie those who had isolated psychotic symptoms) were more likely to commence cannabis use, which could then subsequently contribute to an increased risk of conversion to a non-affective psychotic disorder," they wrote.
The researchers hope their findings will prompt further studies exploring the underlying mechanisms between psychosis and cannabis use.
The study was supported by the National Health and Medical Research Council of Australia, and one of the co-authors, Dr Alati, was sponsored by a National Health and Medical Research Council Career Development Award in Population Health.
Doctors from the Mayo Clinic said teens should not be prescribed medical marijuana for chronic pain.
Archives of General Psychiatry