The great cannabis debate: 50 top experts confirm mental health risk

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.

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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. The age at which people start taking the drug is a real concern. This drug is everywhere and very young children are experimenting with this as their first drug. We are already seeing the consequences of this and among those who are subject to mental illness anyway it is catastrophic."

Dr Linda Harris, clinical director of the substance misuse unit at the Royal College of General Practitioners, said: "From a mental health perspective we do need to look at the reasons behind a society that's drifting towards depressing and anxiety – cannabis could be a factor in this."

The medical profession is not alone in worrying about the possible long-term damage to the country's estimated 2 million cannabis users. Teachers are concerned that the casual acceptance of cannabis will result in an epidemic of children having problems at school. Anthony Seldon, the head of Wellington College, says the drug is "wrecking lives" and describes it as "pernicious". He is among those that think the decision to reclassify cannabis from a Class B to a Class C drug was a mistake and should be reviewed. "The message must be total prohibition," he said.

Keith Hellawell, the Government's former drugs czar who resigned when the Government moved to change the law in 2004, strongly agreed, although last week he worried that the apparent U-turn has come too late. "Reclassifying cannabis now is too late as a generation of young people have been led to believe it is a harmless substance. This move is no more than window dressing to ease the conscience of weak people who lacked the courage to prevent one of their own causing the damage in the first place. Shame on them."

The national cannabis debate encompassed the highest levels of government earlier this month when Ms Smith admitted that she had broken the law by smoking the drug in her youth. Her statement prompted similar confessions from nine fellow cabinet members. Ms Smith said that clearer evidence about the dangers of cannabis use since she was at university in the early 1980s was compelling and young people would be obliged to listen when she urged them not to try the drug, rather than follow her example.

Although the prevalence of skunk, which is routinely offered by dealers instead of milder forms, means smoking can do users more harm than a decade ago, government figures show that the cannabis use is falling among young people. The number of 16- to 24-year-olds who smoked cannabis in 2006 has fallen by a quarter since 1998 – the last time the Government published its drug strategy. And among 11- to 15-year-olds cannabis use is also down: 10 per cent of pupils had smoked cannabis last year, down from 13 per cent in 2003, 2002 and 2001.

Despite this, the number of people needing NHS treatment for cannabis-related mental and behavioural disorders has risen sharply in the last five years from 581 in 2001 to almost 1,000 last year, lending weight to the argument that skunk is harming more users and with greater severity.

Paul Corry, the director of public affairs at the mental health charity Rethink, believes the debate should not focus on reclassification, but instead urges the Government to "crack on with the more important job of informing the public about the health implications".

Nevertheless, despite the mounting evidence that cannabis use causes mental health problems – including The Lancet's publication last week – not everyone believes skunk poses long term health risks.

Professor Tim Kirkham, a psychologist at Liverpool University, argued: "Cannabis has been used safely for many thousands of years," and says there have been "concerted efforts to demonise the drug's use." Dr Trevor Turner, former vice president of the Royal College of Psychiatrists, says: "I don't think it causes mental illness. I have never seen a case of so-called cannabis psychosis."

Dame Ruth Runciman, the chair of UK Drug Policy Centre who set in motion the downgrading of cannabis, disputes that the drug of today is any different to the weed that Ms Smith would have toked back in early 1980s.

"How do you know it's stronger?" she said, adding: "There is indubitably some skunk that is stronger about the place, but the evidence has been hugely exaggerated and does not support such an alarmist view... Cannabis as Class C is exactly where it should be."

Skunk: the drug at the centre of the controversy

Cannabis is more potent than ever, with Britain producing increasing quantities of home-grown, hydroponic marijuana or skunk. This very strong, force-grown form of cannabis is known for its powerful smell – hence its name – and its enhanced effects on the mind of the user. It is often grown in nutrient-rich water under strong lights to produce far more tetrahydrocannabidinol (THC) – the mind-altering compound that gets users high – than regular cannabis, making it many times stronger than conventional "grass" or resin.

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