Using cannabis as a teenager or young adult increases the risk of psychosis, a report suggests.
The study published in the British Medical Journal involved tracking 1,900 people over a period of 10 years.
Although the link between cannabis and psychosis is well-established, it had been unclear whether cannabis triggers the disorder.
This research strongly suggests that cannabis use comes first, rather than people taking it for their symptoms.
The research was led by Professor Jim van Os from Maastricht University in the Netherlands, and included researchers from the Netherlands, Germany, Switzerland and the UK.
They excluded anyone who reported cannabis use or pre-existing psychotic symptoms at the start of the study, which took place in Germany.
The participants in the study, aged between 14 and 24, were assessed for cannabis use and psychotic symptoms at three points over a 10-year period.
It found that cannabis use "significantly" increased the risk of psychotic symptoms, even when other factors such as socio-economic status, use of different drugs and other psychiatric conditions were taken into account.
Skunk
Sir Robin Murray, professor of psychiatric research at the Institute of Psychiatry, said the study added "a further brick to the wall of evidence" showing that use of traditional cannabis is a contributory cause of psychoses like schizophrenia.
He said it was one of 10 prospective studies all pointing in this same direction.
However it did not answer the question of whether skunk and other potent types of cannabis carried a higher risk of psychosis than traditional resin and marijuana, he added.
Peter Kinderman, professor of clinical psychology at the University of Liverpool, agreed the study offered more evidence of the psychotic risks from cannabis.
Three years ago the Labour government reclassified cannabis up to Class B from C - against the advice of its own drug advisers who said cannabis played only a "modest" role in the development of psychotic illnesses.
http://www.bbc.co.uk/news/uk-12616543
