The risk of developing symptoms was much higher in young people with a pre-existing vulnerability to psychosis.
Please note, the embargo for this paper will be lifted at 11.00 hrs on Wednesday 1 December 2004, when the author will present the full findings at a press briefing at the Science Media Centre, 21 Albemarle Street, London.
The study took place in Germany and involved 2437 young people aged 14 to 24 years. Participants were assessed for substance use, predisposition for psychosis, and psychotic symptoms, and were monitored for four years.
After adjusting for influential factors, such as social and economic status and use of other drugs, tobacco, and alcohol, cannabis use moderately increased the risk of psychotic symptoms. This effect was much stronger in those with any predisposition for psychosis.