Obama said earlier this year cannabis is not as dangerous as alcohol

A further 21 states have allowed it to be used for medicinal purposes

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.

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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)

Professor Hall found:

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

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)

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.

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.

Support: Thousands of supporters of decriminalized marijuana smoke at the Civic Center Park in Denver, Colorado in 2012, ahead of the vote to legalize the drug. The bill was passed

Since being legalized in Colorado six months ago, the cannabis industry has boomed. Tax revenue is up $11million, crime is down... but politicians warn the long-term effect have yet to be felt (file picture)

'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 has been legalized in two states - Washington and Colorado both passed the laws in 2012 - while a further 21 states have regulated so it can be used medicinally.

Marijuana and cannabis-infused products are displayed for sale at a marijuana dispensary in Denver, Colorado

Since January 1, 2014, more than 200 licensed marijuana ‘dispensaries’ in Denver, Colorado’s biggest city — along with 100 more in other parts of the state — have been allowed to sell the drug to anyone aged 21 and over for purely ‘recreational’ rather than medicinal use.

Yesterday Colorado's Democrat Governor John Hickenlooper admitted state voters had been 'reckless' when they legalized recreational marijuana two years ago.

Speaking during a debate he said: 'I think for us to do that without having all the data, there is not enough data, and to a certain extent you could say it was reckless.'



A number of other states, including Oregon and Alaska, will vote this year on whether to take the same step, and polls from the Pew Research Center and Gallup have shown a majority of Americans support legalizing the drug.

Use in the U.S. has been on the rise since 2007, possibly due in part to a perception of diminishing risks. In 2008, research found that 42 per cent of people surveyed in the U.S. had tried marijuana at least once.

In his paper, published in the journal Addiction, Professor Hall 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 should be legalised... and the president who said it was safer than alcohol

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 and have suggested taking away the prohibition would help the economy - providing more tax dollars and spending less on prisoners.

Prominent supporters of decriminalisation in the United States have included actor Woody Harrelson, rapper Snoop Dogg and singer-songwriter Willie Nelson.

Robert Downey Jr. participated in an AARP 'Smoke-In' to support marijuana legalization in 2005.

Policy: President Obama notoriously admitted to smoking cannabis as a youngster and told New Yorker magazine: 'I don't think it is more dangerous than alcohol'

Advocates: Prominent supporters of decriminalization in the United States have included actor Woody Harrelson while Robert Downey Jr attended an AARP 'Smoke In' in 2005

Jack Nicholson told the Daily Mail in 2011 that he still occasionally smokes marijuana, adding, 'I don't tend to say this publicly, but we can see it's a curative thing.

'The narcotics industry is also enormous. It funds terrorism and — this is a huge problem in America — fuels the foreign gangs.

Jack Nicholson told the Daily Mail in 2011 that he still occasionally smokes marijuana, and if the government 'were really serious about the economy, there would be a sensible discussion about legalisation'

'More than 85 percent of men incarcerated in America are on drug-related offences. It costs $40,000 a year for every prisoner. If they were really serious about the economy, there would be a sensible discussion about legalisation.'

President Obama notoriously admitted to smoking cannabis as a youngster.

He also told the New Yorker that marijuana was no more harmful than alcohol and was concerned about the effect of drug laws on the poor and ethnic minorities.

He told the magazine: 'As has been well documented, I smoked pot as a kid, and I view it as a bad habit and a vice, not very different from the cigarettes that I smoked as a young person up through a big chunk of my adult life. I don’t think it is more dangerous than alcohol.'

When asked, Bill Clinton said: 'I experimented with marijuana a time or two.'

Actor Danny Glover told the Huffington Post in 2010: 'The draconian laws … impact African-Americans and Hispanics inordinately.'

While promoting 'Savages' in 2012, director Oliver Stone told the Associated Press, '[Legalisation] can be done legally, safely, healthy, and it can be taxed and the government can pay for education and stuff like that. Also, you can save a fortune by not putting kids in jail.'

Jack Black is a panellist at the Marijuana Policy Project, which aims to reform drug laws.

In July, The New York Time's editorial board on Saturday endorsed a repeal of the federal ban on marijuana, becoming the largest paper in the nation to back the idea as it compared the U.S. government's stance on America's most widely used illicit drug to the prohibition on alcohol from 1920 to 1933.