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.