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 (BMJ).
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.
Sir Robin Murray, professor of psychiatric research at the Institute of Psychiatry, said: "This study adds incremental information to the already fairly solid evidence that continued use of cannabis increases risk of psychotic symptoms and psychotic illness.
"The study is one of 10 prospective studies all pointing in this same direction.
"It adds new information by showing that it is those who show psychotic symptoms within a few years of initiating cannabis use who are especially likely to develop persistent psychotic symptoms if they persist in their use of cannabis.
"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 said the study did not answer the 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 the University of Liverpool, said: "This is an interesting and potentially important paper by a team of very impressive researchers.
"It offers more evidence that cannabis use is a risk factor for psychosis and recommends a cautious and thoughtful approach to cannabis legislation."
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