Sat Feb 27, 11:48 pm ET
SATURDAY, Feb. 27 (HealthDay News) -- Long-term use of marijuana can lead to
increased risk of developing hallucinations, delusions and psychosis, a new
study shows.
Australian researchers asked nearly 3,100 young adults averaging about 20 years
of age about marijuana use. They found that almost 18 percent reported using
the drug for three or fewer years, about 16 percent for four to five years, and
just over 14 percent for six or more years.
Among the participants, 65 had been diagnosed with a "non-affective psychosis"
such as schizophrenia, and 233 had at least one positive item for hallucination
on a diagnostic interview conducted for the study.
The researchers found there was an association between length of marijuana use
and mental health.
"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 the Peters et al Delusions
Inventory [a measure of delusion]," wrote Dr. John McGrath, of the Queensland
Centre for Mental Health Research, Park Centre for Mental Health in Wacol, and
colleagues. "There was a 'dose-response' relationship between the variables of
interest: the longer the duration since the first cannabis use, the higher the
risk of psychosis-related outcomes."
The study appears online March 1 and in the May print issue of the Archives of
General Psychiatry.
But the association between psychosis and marijuana use is not simple, the
researchers noted. They found that people who'd experienced hallucinations
earlier in life were also more likely to have used marijuana longer and to use
it more frequently.
"This demonstrates the complexity of the relationship: those individuals who
were vulnerable to psychosis [i.e., 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," wrote the study authors.
Further research is needed to learn more about the mechanisms underlying the
association between psychosis and marijuana use, they concluded.
More information
The U.S. National Institute on Drug Abuse has more about marijuana.