A poll of more than 50 of the world's leading authorities on drugs and mental health confirms that most believe cannabis, and particularly its stronger variant, skunk, pose significant health risks and increase users'
susceptibility to psychosis and schizophrenia.
The Government's announcement last week of a review that could see the reclassification of the drug and harsher penalties for possession re-ignited the debate about the risks of using cannabis.
Launching the three-month consultation, Jacqui Smith, the Home Secretary, said: "Government must remain responsive - alive to new evidence, feedback and trends." Health ministry sources said that new medical evidence about the link between cannabis and mental illness, reported first in this newspaper, would form "a key part of the evidence" that the Government will consider.
It will also examine a new study published in The Lancet last week, which said that cannabis users increased their risk of suffering psychotic episodes by some 40 per cent. The findings by the team at Bristol and Cardiff Universities, led by Dr Stanley Zammit, said that some 14 per cent of psychotic episodes among young people could be prevented if they avoided the drug.
Inquiries by the IoS have drawn warnings from a wide range of organisations, such as the Royal College of Psychiatrists, and specialists. They include Professor Colin Drummond, addiction psychiatry and consultant psychiatrist at St George's Hospital, London, Professor Yasmin Hurd, from the department of psychiatry and pharmacology at New York's prestigious Mount Sinai School of Medicine, Dr Andrew Johns, consultant forensic psychiatrist, at the Maudsley Hospital in London, and Dr Raj Persaud, Gresham Professor for Public Understanding of Psychiatry.
The drug destroys lives by causing or precipitating psychosis in the vulnerable, argued Dr Persaud. "Just a little cannabis, if you have the wrong genetic make-up, will precipitate psychosis," he argues. "Many in my experience commit suicide secondary to psychosis brought on by cannabis, so it is lethal."
Reports of stabbings, murders and suicides caused by psychotic delusions after smoking cannabis have flooded the press in recent months. Perhaps most worryingly, it is Britain's teenagers who are most at risk due to the drug's effects on the developing brain, warn leading experts.
"Young people who otherwise would have been very unlikely to developed psychosis will, as a result of their early cannabis use, be affected by a life-long and severely disabling mental illness that will markedly narrow their life choices and quality of life," said Professor Drummond.
More than 22,000 people needed treatment for cannabis use in Britain last year. It was after publishing these figures that this newspaper revised its stance, abandoning all previous calls for legalisation of the drug.
The decision has been praised by many, including Professor Hamid Ghodse, the director of the International Centre for Drug Policy, who said: "The risks of cannabis have been overlooked for many years no. I'm glad your paper is making the public aware of the dangers. Cannabis is not the harmless drug which many people may have believed."
The main problem, according to medical authorities, is that it is impossible to predict with certainty people who might be vulnerable to psychosis and schizophrenia, aside from those with a family history of such problems.
Professor Hurd said: "Cannabis is a dangerous drug, in particular for the developing brain and for individuals with underlying psychiatric disorders."
Dr Mike McPhillips, the consultant psychiatrist runs the addiction unit at The Priory. He says that they are now seeing new patients on a monthly basis whose psychosis has been triggered by cannabis. "Ten years ago we'd hardly ever get a patient coming in for cannabis addiction but it is not uncommon now. …