For the study, 2,437 individuals aged 14-24 were surveyed about their marijuana use. Researchers then tracked them for four years.

According to lead study author Professor Jim van Os of Maastricht University in the Netherlands, marijuana-using teenagers and young adults with a family history or pre-existing susceptibility to mental illness were even more vulnerable to psychotic symptoms, such as bizarre behavior and delusions, later in life.

"Cannabis does not act in the same fashion on psychosis risk for everybody. There is a group that is particularly susceptible," said Van Os. "The results show that in the group without vulnerability to psychosis, there was a small effect of cannabis on the onset of psychotic symptoms four years later. But this risk was four times bigger in individuals who had a personal vulnerability to psychosis."

The study's findings were reported online by the British Medical Journal.

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