By FIONA MACRAE and EMILY ANDREWS

Last updated at 23:57 26 July 2007

A single joint of cannabis raises the risk of schizophrenia by more than 40 per cent, a disturbing study warns.

The Government-commissioned report has also found that taking the drug regularly more than doubles the risk of serious mental illness.

Overall, cannabis could be to blame for one in seven cases of schizophrenia and other life-shattering mental illness, the Lancet reports.

The grim statistics - the latest to link teenage cannabis use with mental illness in later life - come only days after Gordon Brown ordered a review of the decision to downgrade cannabis to class C, the least serious category.

The Prime Minister is said to have a 'personal instinct' that the change should be reversed, with more arrests and stiffer penalties for users.

Cannabis has been implicated in a string of vicious killings, including the recent stabbing of fashion designer Lucy Braham.

The authors of the latest study, the most comprehensive of its kind and commissioned by the Department of Health, said: 'Policymakers need to provide the public with advice about this widely-used drug.

'We believe there is now enough evidence to inform people that using cannabis could increase their risk of developing a psychotic illness later in life.'

The analysis does not look at the age at which schizophrenia is likely to develop. However, previous studies have shown that smoking the drug as a teenager raises the risk of developing schizophrenia in one's twenties or thirties.

The researchers, from four British universities, analysed the results of 35 studies into cannabis use from around the world. This suggested that trying cannabis only once was enough to raise the risk of schizophrenia by 41 per cent.

At greatest risk, however, were heavy users, with those who took cannabis over 100 times having more than double the risk of those who never touched the drug.

With up to 40 per cent of teenagers and young adults in the UK believed to have tried cannabis, the researchers estimate that the drug could be behind 14 per cent of cases of schizophrenia and other psychotic illnesses.

'Although individual lifetime risk of chronic psychotic disorders such as schizophrenia, even in people who use cannabis regularly, is likely to be low - less than three per cent - cannabis use can be expected to have a substantial effect on psychotic disorders at a population level because exposure to this drug is so common.'

Cardiff University researcher Dr Stanley Zammit added: 'Even if cannabis does cause an increased risk of developing psychosis, most people who use cannabis will not develop such an illness.

'Nevertheless, we would still advise people to avoid or limit their use of this drug, especially if they start to develop any mental health symptoms, or if they have relatives with psychotic illnesses.'

In an accompanying editorial in the Lancet, Dutch psychiatrists said the focus on heroin, cocaine and other Class A drugs meant the dangers of cannabis had been overlooked.

'In the public debate, cannabis has been considered a more or less harmless drug compared with alcohol, central stimulants and opioids.

'However, the potential long-term hazardous effects of cannabis with regard to psychosis seem to have been overlooked, and there is a need to warn the public of these dangers, as well as to establish a treatment to help young frequent cannabis users.'

Previous studies have shown a clear link between cannabis use in the teenage years and mental illness in later life.

Research completed by leading psychiatrist Professor Robin Murray in 2005 showed that those who smoked the drug regularly at 18 were 1.6 times more likely to suffer serious psychiatric problems, including schizophrenia, by their mid-20s.

For those who were regular users at 15, the stakes were even higher, with their risk of mental illness by the age of 26 being 4.5 times greater than normal.

It is thought that, used during teenage years, the drug can cause permanent damage to the developing brain.

Professor Robin Murray, of the Institute of Psychiatry in London, warned yesterday that the risks were likely to be heightened by the increasing use of powerful skunk cannabis.

'My own experience suggest to me that the risk with skunk is higher. Therefore their estimate that 14 per cent of cases of schizophrenia in the UK are due to cannabis is now probably an understatement.'

Marjorie Wallace, chief executive of the mental health charity SANE, said: 'This analysis should act as a serious warning of the dangers of regular or heavy cannabis use, doubling the risk of developing schizophrenia - a condition in which a person may hear voices and experience strange thoughts and paranoid delusions.

'The debate about classification should not founder on statistics but take into account the potential damage to hundreds of people who without cannabis would not develop mental illness.

'While the majority can take the drug with no mind-altering effects, it is estimated that 10 per cent are at risk.

'You only need to see one person whose mind has been altered and life irreparably damaged, or talk to their family, to realise that the headlines are not scaremongering but reflect a daily, and preventable, tragedy.'

However, others questioned the link, pointing out there has been little change in rates of schizophrenia in recent years despite the rise in cannabis use and the increasing strength of the drug.

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Three heavy drug users and their horrific killings:

William Jaggs, a 23-year-old Oxford University student and prolific cannabis user, stabbed fashion designer Lucy Braham 66 times at her home near Harrow, the public school in North-West London.

The paranoid schizophrenic was found covered in blood beside Lucy's body, having plunged the knife into his own chest last September.

The former Harrow pupil, whose father is a teacher at the school, started using drugs when he was 14, moving on from cannabis to cocaine.

He was sent to Broadmoor secure hospital this month for an unlimited period after admitting manslaughter on the grounds of diminished responsibility.

Drug-crazed drifter Richard Cazaly is believed to have stabbed pregnant Abigail Witchalls in Surrey, in April 2005.

Cazaly, 23, who killed himself five days after the stabbing, had a history of heavy drug use dating back at least four years.

His girlfriend, Vanessa MacKenzie, told police both she and Cazaly were regular cannabis users, smoking 'a couple of joints a day'.

Surrey police said Cazaly became psychotic and violent as a result of long-term abuse of drugs and the alcohol he had consumed on the day of the random stabbing.

Miraculously, Mrs Witchalls and her unborn baby survived the attack. Her young son - who she was pushing in a pram when she was set upon by Cazaly - was unhurt.

Son of a nurse at Broadmoor Thomas Palmer butchered two of his friends during a woodland walk after his mind was warped by smoking skunk - a particularly potent form of cannabis.

Then aged 18, he virtually beheaded 16-year-old Steven Bayliss and repeatedly stabbed Nuttawut Nadauld, 14, near their homes in Wokingham, Berkshire in September 2005.

Palmer had started using the drug at 14. He told doctors he had not been smoking on the day of the killings but admitted to using skunk regularly in the weeks before the brutal attack.

In March this year, he was given a minimum 20 years in prison for the double murder.