A new study has found that teens who smoke marijuana are at a greater risk of developing schizophrenia and psychotic symptoms in the future.
After observing more than 3800 youngsters, researchers learnt that people who used the drug for six or more years were twice as likely to suffer from delusional disorders than those who never used it.
Researchers from Queensland Brain Institute, at the University of Queensland, quizzed 3801 young adults who were born in Brisbane between 1981 and 1984.
Among the 1272 participants who had never used marijuana, 26 (2 per cent) were diagnosed with psychosis, while the 322 people who had used marijuana for six or more years, 12 (3.7 per cent) were diagnosed with the illness.
The average age of the participants was about 20.
According to the authors, the study was the first to look at sibling pairs to discount genetic or environmental influence.
"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," the Sydney Morning Herald quoted Dr McGrath, a professor at the institute, as saying in a statement.
McGrath added: "The message for teenagers is: if they choose to use cannabis they have to understand there's a risk involved."
The study noted: "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 research has been published online by the Archives of General Psychiatry.