The paper by Professor Wayne Hall, who is an advisor for the World Health Organisation, says that Cannabis use can have numerous harmful side-effects, including mental illness, impaired driving ability and lower educational attainment.
Professor Hall found that it could double the risk of severe psychiatric disorders, including schizophrenia, that one in six teenagers and one in ten adults who regularly use it can become dependent on it, and that smoking it during pregnancy can reduce the baby’s weight.
The Daily Mail quotes Professor Hall as dismissing those who claim the drug is harmless: “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.”
He added that those who try to stop taking the drug can often suffer anxiety, lower appetite, depression and insomnia, and less than half of people who stop taking it can stay off it for six months.
Professor Hall writes: “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 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.”
Mark Winstanley, of the charity Rethink Mental Illness, 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 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.”