Study Links Cannabis Use With Psychosis

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 3,800 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 build on previous research by author John McGrath Queensland Brain Institute at the University of Queensland. The study was the first to look at sibling pairs to discount genetic or envionmental 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 a the institute in a statement.

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 caused by genetics or environment, the authors said.