A study of around 1,000 people in New Zealand suggests that young people who smoke cannabis for years risk a significant and irreversible reduction in their IQ.
According to the finding of the international team, those who started using cannabis below the age of 18, whose brains were still developing, suffered a drop in IQ.
The research might explain why people who use the drug often seem to under-achieve, said a UK expert.
Researchers have followed the lives of a group of people from Dunedin in New Zealand for more than 20 years. They were first evaluated when they were children, before any of them had started using cannabis, and were then re-interviewed repeatedly up to the age of 38.
Other factors were taken into account, including alcohol, tobacco dependency or other drug use, as well as the number of years spent in education.
Researchers discovered a decline in the IQ of the people who continually used cannabis, smoking it at least four times a week year after year through their teens, 20s and for some cases their 30s.
The more they smoked, the more they lost in IQ.
However the effect was only observed in people who started smoking cannabis in their teens. There was an average eight-point IQ decline in individuals who started using the drug as adolescents and carried on using it for years.
And stopping or reducing cannabis use did not fully restore the lost IQ.
“Persistent cannabis use over 20 years was associated with neuropsychological decline, and greater decline was evident for more persistent users,” said the researchers in an article in the US journal Proceedings of the National Academy of Sciences.
“Collectively, these findings are consistent with speculation that cannabis use in adolescence, when the brain is undergoing critical development, may have neurotoxic effects.”
According to Prof Terrie Moffitt of King’s College London’s Institute of Psychiatry, who was a member of the team, this study could improve our understanding of the dangers of cannabis use.
“This work took an amazing scientific effort. We followed almost 1,000 participants, we tested their mental abilities as kids before they ever tried cannabis, and we tested them again 25 years later after some participants became chronic users.
“Participants were frank about their substance abuse habits because they trust our confidentiality guarantee, and 96% of the original participants stuck with the study from 1972 to today.
“It is such a special study that I’m fairly confident that cannabis is safe for over-18 brains, but risky for under-18 brains.”
Robin Murray, professor of psychiatric research also at the King’s College London Institute of Psychiatry, was not involved in the study, which he described as an impressive piece of research.
“The Dunedin sample is probably the most intensively studied cohort in the world and therefore the data are very good.
“Although one should never be convinced by a single study, I take the findings very seriously.
“There are a lot of clinical and educational anecdotal reports that cannabis users tend to be less successful in their educational achievement, marriages and occupations.
However Prof Val Curran, from the British Association for Psychopharmacology and University College London, said: “What it shows is if you are a really heavy stoner there are going to be consequences, which I think most people would accept.
“This is not occasional or recreation use.”
She added that depression could be another explanation which could result in lower IQ and cannabis use.
“In a significant minority of people who are vulnerable the drug can act as a trigger to illnesses like schizophrenia which may last a lifetime”, said Marjorie Wallace, chief executive of the mental health charity SANE.
Although since the mid 1990s illicit drug use by young people has been decreasing, official statistics show that the rate of decline in cannabis use has been slow throughout most of the last decade.