A definitive 20-year study into the effects of long-term cannabis use has demolished the argument that the drug is safe.

Cannabis is highly addictive, causes mental health problems and opens the door to hard drugs, the study found.

The paper by Professor Wayne Hall, a drugs advisor to the World Health Organisation, builds a compelling case against those who deny the devastation cannabis wreaks on the brain. Professor Hall found:

Scroll down for video

Lasting effects: One in six teenagers who regularly smoke the drug become dependent on it and cannabis users do worse at school. Heavy use in adolescence appears to impair intellectual development. (File image)

One in six teenagers who regularly smoke the drug become dependent on it,

Cannabis doubles the risk of developing psychotic disorders, including schizophrenia,

Cannabis users do worse at school. Heavy use in adolescence appears to impair intellectual development

One in ten adults who regularly smoke the drug become dependent on it and those who use it are more likely to go on to use harder drugs,

Driving after smoking cannabis doubles the risk of a car crash, a risk which increases substantially if the driver has also had a drink,

Smoking it while pregnant reduces the baby’s birth weight.

Last night Professor Hall, a professor of addiction policy at King’s College London, dismissed the views of those who say that cannabis is harmless.

‘If cannabis is not addictive then neither is heroin or alcohol,’ he said.

‘It is often harder to get people who are dependent on cannabis through withdrawal than for heroin – we just don’t know how to do it.’

Those who try to stop taking cannabis often suffer anxiety, insomnia, appetite disturbance and depression, he found. Even after treatment, less than half can stay off the drug for six months.

The paper states that teenagers and young adults are now as likely to take cannabis as they are to smoke cigarettes.

Addiction: Those who try to stop taking cannabis often suffer anxiety, insomnia, appetite disturbance and depression, the study found. (File image)

Professor Hall writes that it is impossible to take a fatal overdose of cannabis, making it less dangerous at first glance than heroin or cocaine. He also states that taking the drug while pregnant can reduce the weight of a baby, and long-term use raises the risk of cancer, bronchitis and heart attack.

But his main finding is that regular use, especially among teenagers, leads to long-term mental health problems and addiction.

‘The important point I am trying to make is that people can get into difficulties with cannabis use, particularly if they get into daily use over a longer period,’ he said. ‘There is no doubt that heavy users experience a withdrawal syndrome as with alcohol and heroin.

‘Rates of recovery from cannabis dependence among those seeking treatment are similar to those for alcohol.’

Mark Winstanley, of the charity Rethink Mental Illness, said: ‘Too often cannabis is wrongly seen as a safe drug, but as this review shows, there is a clear link with psychosis and schizophrenia, especially for teenagers.

‘The common view that smoking cannabis is nothing to get worked up about needs to be challenged more effectively. Instead of classifying and re-classifying, government time and money would be much better spent on educating young people about how smoking cannabis is essentially playing a very real game of Russian roulette with your mental health.’

Cannabis was given a Class B rating when the classification system for illegal drugs was set up in 1971, putting it below Class A substances heroin and cocaine in seriousness but above Class C drugs such as steroids.

The Labour government downgraded the drug to Class C in 2004 – meaning officers did not normally arrest those caught with it – but reversed its decision within five years. Other failed attempts to liberalise the approach to cannabis include that of former Metropolitan Police chief Brian Paddick, who spearheaded a ‘softly, softly’ scheme while borough commander in Lambeth in 2001.

His party leader, Nick Clegg, has previously backed moves to partially decriminalise the sale of cannabis. At the Liberal Democrat conference yesterday, he called for people to be spared jail if they are caught with small amounts of drugs.

Widespread: Teenagers and young adults are now as likely to take cannabis as they are to smoke cigarettes. Regular use, especially among teens, leads to long-term mental health problems and addiction. (File image)

In 2005, David Cameron, when he ran for the Tory leadership, said it would be ‘disappointing’ if radical options on the law on cannabis were not looked at. He said he favoured ‘fresh thinking and a new approach’ towards drugs policy.

Mr Cameron also voted, when he was a member of the Home Affairs Select Committee, for the UN body on drugs policy to look at whether to legalise and regulate the drugs trade. Today, he no longer supports decriminalisation.

Professor Hall last night declined to comment on the decriminalisation debate.

But in his paper, published in the journal Addiction, he wrote that the rise of medical treatment for cannabis ‘dependence syndrome’ had not been stopped by legalisation. The number of cannabis users seeking help to quit or control their cannabis use has increased during the past two decades in the United States, Europe and Australia,’ he wrote. ‘The same increase has occurred in the Netherlands, where cannabis use was decriminalised more than 40 years ago.’

David Raynes, of the National Drug Prevention Alliance, added: ‘There is no case for legalisation and we hope that this puts an end to the matter. The two main parties agree that cannabis needs to remain illegal – we hope the Liberal Democrats see this research and re-examine their policies.’

The celebrities and campaigners who claimed cannabis was safe

For years, activists and celebrities trying to decriminalise cannabis have campaigned on the claim that the real health damage to users is done by the legal ban on drugs. They have dismissed the growing evidence that smoking cannabis is a serious risk to mental health.

Prominent supporters of decriminalisation have included comedian Russell Brand, singer Sting, writer Will Self and left-wing barrister Michael Mansfield.

A key figure has been David Nutt, who was chairman of the Home Office Advisory Council on the Misuse of Drugs, until sacked for his campaigning five years ago. The professor said the risk of lung cancer from smoking was vastly greater than the risk of psychosis from cannabis.

Personal: Sir Richard Branson (left) has a long-running campaign against the legal ban on drugs and comedian Russell Brand, pictured right last night, has been a prominent supporter of decriminalisation

He gave a lecture in 2009 in which he said: ‘The analysis we came up with was that smokers of cannabis are about 2.6 times more likely to have a psychotic-like experience than non-smokers. To put that figure in proportion, you are 20 times more likely to get lung cancer if you smoke tobacco than if you don’t.

‘The other paradox is that schizophrenia seems to be disappearing from the general population, even though cannabis use has increased markedly in the last 30 years.

‘So, even though skunk has been around now for ten years, there has been no upswing in schizophrenia. Where people have looked, they haven’t found any evidence linking cannabis use in a population and schizophrenia.’

The claim that cannabis is harmless is repeated in a documentary shortly to be released in Britain called The Culture High, which features interviews with Sir Richard Branson and Mike Trace, Britain’s deputy drugs czar under Tony Blair. He was sacked after the Mail revealed he was planning to launch a decriminalisation pressure group.

The film contains an interview with an academic who states that ‘marijuana is the most non-toxic medicine I have ever come across’ and maintains, according to reports, that ‘scientific evidence overwhelmingly shows it has medical benefits’.