Teenagers who regularly use cannabis could be permanently impairing their mental development and lowering their IQ, according to a major new study.
Researchers found that heavy users who started regularly using the drug before the age of 18 suffered an average eight-point decline in IQ between adolescence and adulthood.
Users were also found to have more problems with their attention span and memory than non-users.
Dr Madeline Meier, who led the study, said the message was clear that the drug “is not harmless, particularly for adolescents”.
She added: “Somebody who loses eight IQ points as an adolescent may be disadvantaged compared to their same-age peers for years to come.”
The study was based on more than 1,000 people in New Zealand.
The study revealed: “Persistent cannabis use was associated with neuropsychological decline broadly across domains of functioning, even after controlling for years of education.
“Informants also reported noticing more cognitive problems for persistent cannabis users.
“Impairment was concentrated among adolescent-onset cannabis users, with more persistent use associated with greater decline.”
The researchers also found “cessation of cannabis use” did not appear to fully restore intellectual functioning “among adolescent-onset cannabis users”.
Professor Robin Murray, a consultant psychiatrist, said: “It is, of course, part of folklore among young people that some heavy users of cannabis seem to gradually lose their abilities and end up achieving much less than one would have anticipated.
“This study provides one explanation as to why this might be the case.”
The study also indicated that those who started smoking cannabis as adults only suffered small falls in IQ.
In June experts warned that most people underestimate the health risks of smoking cannabis.
The British Lung Foundation (BLF) carried out a survey of 1,000 adults, and a third wrongly believed cannabis did not harm health.
The BLF report warned that smoking one cannabis cigarette every day for a year increases the chances of developing lung cancer by a similar amount as smoking 20 tobacco cigarettes each day for one year.