Cannabis use once or twice a week almost doubles the risk of suffering psychotic symptoms in later life, a study has found.

More frequent use further increased the risk of symptoms such as hallucinations, hearing voices and paranoid thoughts. People with a family history of psychosis or mental illness were at the greatest risk of all.

The latest research, published in BMJ.com backs up earlier studies that have linked cannabis to schizophrenia, delusions and other mental illness.

Professor Jim van Os and a team at the Maastricht University in the Netherlands studied 2,437 young people aged 14 to 24, asking them subtle questions to determine whether they were vulnerable to psychosis.

After four years they were questioned again and asked about their use of cannabis.

Professor van Os said: "The results showed in the group without vulnerability there was a small increase in risk of psychosis.

"But this risk was four times bigger in those who had a vulnerability for psychosis. This is a group that is particularly susceptible."

He said that among those who were not predisposed to psychosis and did not use cannabis, 15 per cent suffered some psychotic symptoms. But 21 per cent of those who had ever used cannabis suffered such symptoms.

But the difference was most striking in those who were already vulnerable to psychotic symptoms. In this group, 26 per cent of those who did not use cannabis suffered symptoms, but for those who used cannabis it increased to 51 per cent.

"There is a public health message here because it is going to be difficult to tell the whole population to stop using cannabis," he said.

"But it may be better to say that if you have a family history of mental instability you are particularly at risk of the negative effects of cannabis use."

One possible explanation of the findings might be that people who are prone to psychosis are more likely to experiment with cannabis, perhaps in order to combat the symptoms. But the team eliminated this by finding that a predisposition to psychosis did not significantly predict cannabis use during the four years.

Cannabis was downgraded from a class B to class C drug in the UK in January, meaning that in most cases those found carrying the drug will not be arrested.

Last week figures showed that two in five 15-year-olds in the UK have tried cannabis - more than anywhere else in Europe.

The European Monitoring Centre for Drugs and Drug Addiction also showed that about one in 10 in this age group had smoked pot at least 40 times in the last year.

Professor Robin Murray, from the Institute of Psychiatry at King’s College London, said since the late 80s and early 90s doctors had begun to see a link between patients suffering psychotic symptoms and the use of cannabis.

"The initial view first of all was that these poor souls were suffering hallucinations and other symptoms and if a few puffs of marijuana helped them why shouldn’t we let them smoke" he said.

"In retrospect this is rather like an alcoholic feeling bad in the morning but thinking they know what the right medicine is and having another drink."

Martin Barnes, chief executive of DrugScope, said that the new research underlined the potentially serious health risks of cannabis use, particularly for young people.

"Frequent use, a predisposition to mental health problems, and starting at an early age, all increase the risk of adverse affects on mental health" he said.

Marjorie Wallace, chief executive of the mental health charity SANE, said: "Britain’s position as the cannabis capital of Europe could have hidden and disturbing consequences.

"The evidence is mounting that cannabis can be the trigger to lifelong mental illness such as schizophrenia, and we know that for some already suffering it exacerbates symptoms such as delusions and paranoia.

"We need to make these risks known, clamp down on drug dealing in such places as playgrounds and hospital wards, and change perceptions of cannabis from being a recreational relaxant to a dangerous substance for those who are vulnerable."

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