Wake-up call for teen pot smokers
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YOUNG adults who used marijuana as teens were more likely than those who didn't to develop schizophrenia and psychotic symptoms, a seven-year Australian study found.
Those who used the drug for six or more years were twice as likely to develop a psychosis such as schizophrenia or to have delusional disorders than those who never used it.
Research involving more than 3800 young adults, released online by the Archives of General Psychiatry, found long-term users were also four times more likely to have psychotic-like experiences.
The findings, by the Queensland Brain Institute at the University of Queensland, build on previous research and shows that marijuana use is not as harmless as some people think, lead study author John McGrath said yesterday in an email.
The researchers quizzed 3801 young adults who were born in Brisbane between 1981 and 1984. The participants, whose average age was about 20, were asked about marijuana use. The researchers also measured whether those in the study had psychotic symptoms.
The study was the first to look at sibling pairs to discount genetic or environmental influence and still find marijuana linked to later psychosis, the authors said.
''This is the most convincing evidence yet that the earlier you use cannabis, the more likely you are to have symptoms of a psychotic illness,'' said Dr McGrath, a professor at the institute, in a statement. ''The message for teenagers is: if they choose to use cannabis they have to understand there's a risk involved.''
Researchers were looking for causes of schizophrenia, Dr McGrath said.
Of the 1272 participants who had never used marijuana, 26 (2 per cent) were diagnosed with psychosis. Of the 322 people who had used marijuana for six or more years, 12 (3.7 per cent) were diagnosed with the illness. Overall, 65 people were diagnosed with psychosis, the study said.
The researchers also found those who used marijuana the longest were four times more likely than those who didn't to have the highest scores derived from a list of psychotic-like experiences.
Dr McGrath said even those who used marijuana for fewer than three years still had an increased risk of scoring higher than those who had not.
''Apart from the implications for policy makers and health planners, we hope our findings will encourage further clinical and animal-model research to unravel the mechanisms linking cannabis use and psychosis,'' the authors wrote.
Those in the study were interviewed at the ages of 14 and 21, so the symptoms emerged between those two study periods, Dr McGrath said.
The study also showed that among 228 sibling pairs, those who didn't use marijuana reported fewer psychotic-like delusions compared with those who used cannabis. That difference was statistically significant and reduces the likelihood that the psychotic problems were caused by genetics or environment, the authors said.
The study was funded by the National Health and Medical Research Council of Australia.
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