Teenagers and young adults who use cannabis face increased risk of psychosis, says research published in the British Medical Journal.
Experts from Germany, the Netherlands and London's Institute of Psychiatry studied 1,900 people aged between 14 and 24 over a period of eight years.
The study found that those who started using cannabis only after the experiment had begun and those who used it before and after both had a higher risk of psychotic symptoms than those who had never used it.
"Cannabis use is a risk factor for the development of incident psychotic symptoms," the report concluded.
"Continued cannabis use might increase the risk for psychotic disorder by impacting on the persistence of symptoms."
Robin Murray, professor of psychiatric research at the Institute of Psychiatry, said the latest results backed up claims that the drug caused long-term psychological effects.
"This study adds incremental information to the already fairly solid evidence that continued use of cannabis increases risk of psychotic symptoms and psychotic illness," he said.
"In short, this study adds a further brick to the wall of evidence showing that use of traditional cannabis is a contributory cause of psychoses like schizophrenia," he added.
(Sapa, March 2011)