Longtime Pot Smoking May Raise Psychosis Risk
Study Shows Longtime Marijuana More Likely to Report Hallucinations
WebMD News Archive
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
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
"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.
People with non-affective psychosis "do not have a prominent mood element"
such as bipolar disorder or mania, he writes.
"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."