Using cannabis as a teenager or into young adulthood increases the risk of psychosis, experts have warned.
There has long been a debate over whether the link between cannabis and psychosis is causal or whether early psychotic experiences lead people to "self-medicate" with cannabis.
Researchers examined data for more than 1,900 people who were aged 14 to 24 at the start of the study.
They were followed up at three, and then eight years later, to assess the link between cannabis use and psychotic symptoms.
Those who were not cannabis users at the start of the study, but who went on to become so, had a higher risk of psychotic symptoms later on.
In those who were using cannabis at the start of the study and carried on, there was also an increased risk of psychotic experiences.
The experts, including from Germany, the Netherlands and the Institute of Psychiatry in London, published their findings in the British Medical Journal.
They concluded: "Cannabis use is a risk factor for the development of incident psychotic symptoms.
"Continued cannabis use might increase the risk for psychotic disorder by impacting on the persistence of symptoms."
The experts found no evidence that psychotic symptoms led to later cannabis use.