March 2, 2010 -- Young adults who are longtime pot smokers are more likely to have hallucinations, delusions, or to display signs of psychosis than short-term smokers or people who've never smoked marijuana.
The finding comes from a study of 3,801 young adults who were asked about their pot use and then evaluated to determine if they'd experienced "psychotic outcomes."
The study appears online in advance of print in the May issue of the Archives of General Psychiatry.
Youths who had experienced hallucinations early in life were more likely to have used marijuana longer, and more frequently, the study shows.
The study also suggests that more research is needed to determine whether people who are predisposed to psychosis might be more likely to smoke marijuana earlier in life and for longer periods.
Pot and Psychosis
The researchers followed young people born between 1981 and 1984, up to age 21. They were asked about their use of cannabis.
The findings show:
- 17.7% reported using pot for three or fewer years.
- 16.2% used it for four to five years.
- 14.3% had smoked for six or more years.
Overall, 233 had at least one "positive" report for hallucination on their interviews, and 65 received a diagnosis of "non-affective psychosis," such as schizophrenia.
"Compared with those who had never used cannabis, young adults who had six or more years since first use of cannabis were twice as likely to develop a non-affective psychosis," the researchers write.
Non-affective psychosis "is a broad category that includes schizophrenia and a handful of less common disorders like delusional disorder," study researcher John McGrath, MD, of the University of Queensland in Brisbane, Australia, tells WebMD in an email.
"Think of depression," McGrath writes. "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."
The study also shows that those who had at least six years since the first time they used marijuana were four times as likely to have high scores on an accepted measure of delusionary experiences.
Results were similar among a subgroup of 228 siblings, "thus reducing the likelihood that the association was due to unmeasured shared genetic and/or environmental influences," the researchers write, adding that "the nature of the relationship between psychosis and cannabis use is by no means simple" but rather very complex.
The researchers also found that "the longer the duration since first cannabis use, the higher the risk of psychosis-related outcomes."