Young people who use cannabis are doubling their risk of developing psychotic symptoms, experts warn.
And mental health problems persist among those who continue using it compared with those who stop, according to research by an international team of scientists.
Their study adds to mounting evidence that smoking cannabis can trigger psychotic illnesses such as schizophrenia in vulnerable youngsters.
It appears to demolish counter-arguments that cannabis does not cause symptoms of mental illness, or that some turn to the drug as a form of self-medication to deal with them.
The research also shows a link with psychosis at a very early stage of use among young people who previously never experienced such symptoms. They include paranoid ideas, hallucinations, hearing voices or bizarre behaviour.
The study, by a team from Germany, the Netherlands and the Institute of Psychiatry in London, focused on more than 1,900 volunteers aged 14 to 24 living in Germany. It followed up with the group after three years and eight years.
Those who had not previously used cannabis but began to during the study had double the risk of developing psychotic symptoms, it found.
If they carried on using it, they were at an increased risk of psychotic experiences compared with those who did not.
There was also no evidence that suffering psychotic symptoms was likely to result in people turning to cannabis for relief.
Reporting on their findings in the British Medical Journal, the team concluded: ‘Cannabis use precedes the onset of psychotic symptoms in individuals with no history of them.’
Cannabis may also increase the risk of lasting harm to mental health by making such symptoms persist with continued use.
Last month, Australian researchers found that cannabis use accelerates the onset of full-blown mental illness almost three years earlier in people at risk.
Sir Robin Murray, professor of psychiatric research at the Institute of Psychiatry, said of the latest study: ‘It is one of ten prospective studies all pointing in this same direction.
In short, it adds a further brick to the wall of evidence showing that use of traditional cannabis is a contributory cause of psychoses like schizophrenia.
‘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.’
Previous research has shown that a quarter of the population has a genetic predisposition which makes them ten times more likely to develop psychosis and other schizophrenia-like symptoms after smoking cannabis.
Experts warn that anyone with pre-existing mental health problems or family history is at increased risk of mental illness if they use cannabis.
In a BMJ commentary, Professor Wayne Hall, from the University of Queensland, and Professor Louisa Degenhardt, from the Burnet Institute in Melbourne, say the link is biologically plausible and more information should be given to young people about the risks.
‘The case is strengthened by evidence that regular cannabis use in adolescence predicts poorer educational outcomes, increased risk of using other illicit drugs, increased risk of depression and poorer social relationships in early adulthood’, they added.
