The research, conducted over 20 years by Professor Wayne Hall, an adviser to the World Health Organisation, links use of cannabis to a wide range of harmful side-effects, from mental illness to lower academic attainment to impaired driving ability.
Smoking the class-‘B’ drug while pregnant is linked with reduced birth weights, while long-term use can cause cancer, bronchitis and heart attacks, according to the paper.
Prof Hall, a leading expert in addiction at King’s College, London, also found that:
:: One in six teenagers who regularly smoke cannabis become dependent on it, as are one in ten regular adult users;
:: Cannabis doubles the risk of psychosis and schizophrenia, with withdrawal symptoms including anxiety, insomnia, loss of appetite and depression;
:: Driving after smoking cannabis doubles the risk of a car crash, with the risk heightened yet further if you have had a drink;
:: As many teenagers now smoke cannabis as cigarettes.
The Daily Mail quoted Prof Hall as saying: “If cannabis is not addictive, then neither is heroin or alcohol.
“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.”
Less than half of users stay off the drug for six months or more following treatment, Prof Hall found.
Despite the fact that no cannabis user had died from an overdose, long-term use could be seriously damaging to mental health.
“The important point I am trying to make,” Prof Hall writes, “is that people can get into difficulties with cannabis use, particularly if they get into daily use over a long period.
“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.”
Drugs campaigners said the study showed that heavy cannabis use by teenagers amounted to them playing “Russian roulette” with their mental health.
Mark Winstanley, of the Rethink Mental Illness charity, also called for the Government to focus on educating young people about the dangers, rather than classifying and then reclassifying the drug, as the last Government did.
Mr Winstanley told the Mail: “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 reclassifying, 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.”
Tony Blair’s government relaxed the law on cannabis, reclassifying it from class ‘B’ to ‘C’ in 2004. This was reversed after Gordon Brown entered Downing Street in 2007.