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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.
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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.
-- Robert Preidt
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