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:
- 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
'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.
'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.
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.'