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
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.
‘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.’