Using cannabis in one's youth almost doubles the risk of having psychotic episodes like paranoia and hallucinations, a study has found.
A team of international researchers followed the lives of almost 2,000 young Germans, who began with no history of psychosis, for more than a decade.
They discovered that youngsters who started smoking the drug at college age were 90pc more likely to have psychotic symptoms in their mid-20s.
In the study, cannabis users were defined as those who had used the drug at least five times between 18 and 21.
Rates of psychotic episodes between users and non-users at the age of 26 were then compared. Other factors known to affect likelihood of psychosis, such as childhood trauma, class, and growing up in an urban environment, were accounted for.
The authors, led by Rebecca Kuepper, a research psychologist at Maastricht University in The Netherlands, concluded: "This 10 year follow-up study showed that incident cannabis use significantly increased the risk of incident psychotic experiences."
The scientists believe that exposure to THC, the main psychoactive component in cannabis, makes the brain more susceptible to psychotic episodes.
The study also found that a rise in psychotic episodes followed cannabis use, rather than the other way around, disproving the theory that the relationship between the two was nothing more than people prone to psychosis self-medicating.
Sir Robin Murray, professor of psychiatric research at Kings College London, said: "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."
But he said it did not "address the important question of whether skunk and other potent types of cannabis carry a higher risk of psychosis than traditional resin and marijuana".
Peter Kinderman, professor of clinical psychology at Liverpool University, said the study "offers more evidence that cannabis use is a risk factor for psychosis and recommends a cautious and thoughtful approach to cannabis legislation".