Young people who smoke cannabis for years run the risk of a significant and irreversible reduction in their IQ, research suggests.
The findings come from a study of around 1,000 people in New Zealand.
An international team found those who started using cannabis below the age of 18 - while their brains were still developing - suffered a drop in IQ.
A UK expert said the research might explain why people who use the drug often seem to under-achieve.
For more than 20 years researchers have followed the lives of a group of people from Dunedin in New Zealand.
They assessed them as children - before any of them had started using cannabis - and then re-interviewed them repeatedly, up to the age of 38.
Having taken into account other factors such as alcohol or tobacco dependency or other drug use, as well the number of years spent in education, they found that those who persistently used cannabis - smoking it at least four times a week year after year through their teens, 20s and, in some cases, their 30s - suffered a decline in their IQ.
The effect was most marked in those who started smoking cannabis as adolescents.
For example, researchers found that individuals who started using cannabis in adolescence and then carried on using it for years showed an average eight-point IQ decline.
Stopping or reducing cannabis use failed to fully restore the lost IQ.
The researchers, writing in the US journal Proceedings of the National Academy of Sciences, found that: "Persistent cannabis use over 20 years was associated with neuropsychological decline, and greater decline was evident for more persistent users."
"Collectively, these findings are consistent with speculation that cannabis use in adolescence, when the brain is undergoing critical development, may have neurotoxic effects."
One member of the team, Prof Terrie Moffitt of King's College London's Institute of Psychiatry, said this study could have a significant impact on our understanding of the dangers posed by 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.
"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 but not involved in the study, said this was 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.
"It is of course part of folk-lore among young people that some heavy users of cannabis - my daughter calls them stoners - 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.
"I suspect that the findings are true. If and when they are replicated then it will be very important and public education campaigns should be initiated to let people know the risks."
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 also cautioned that there may be another explanation, such as depression, which could result in lower IQ and cannabis use.