Cannabis can lower the IQ of young teenagers and may cause permanent mental impairment, research has shown.
The most persistent users suffer an average eight-point decline in IQ between adolescence and adulthood, according to the study of more than 1,000 participants.
Scientists believe smoking cannabis from the age of puberty may disrupt developing and vulnerable brain circuits.
Users experienced significantly more attention and memory problems than non-users, the study revealed. This was the case even after taking account of different educational backgrounds and use of alcohol and other drugs.
Quitting or cutting down on cannabis later in life did not fully reverse the impact on those who started taking the drug in their early teens.
But the study found no evidence of similar problems affecting people who only took up cannabis as adults.
The international team, led by US psychologist Dr Madeline Meier, from Duke University in Durham, Carolina, wrote in the journal Proceedings of the National Academy of Sciences: "Persistent cannabis use was associated with neuropsychological decline broadly across domains of functioning even after controlling for years of education.
"Impairment was concentrated among adolescent-onset cannabis users, with more persistent use associated with greater decline. Further, cessation of cannabis use did not fully restore neuropsychological functioning among adolescent-onset cannabis users."
The researchers analysed data on 1,037 individuals enrolled into the Dunedin Study, a large lifestyle and health investigation based in New Zealand.
Scientists followed the progress of participants from birth to the age of 38, carrying out neuropsychological tests at age 13 and again at the end of the study.
Cannabis use was recorded at five intervals from the age of 18 onwards.
The tests showed wide-ranging mental declines among men and women who began taking cannabis at a young age and continued using the drug regularly for more than 20 years.
"The most persistent adolescent-onset cannabis users evidenced an average eight-point IQ decline from childhood to adulthood," said the researchers.
Persistent cannabis use appeared to affect everyday mental functioning. Users experienced significant attention and memory problems, according to friends and relatives questioned by the researchers.
While quitting may prevent further impairment, it did not appear to restore normal mental functioning for those whose cannabis habit began in adolescence, said the scientists.
Puberty was a period of "critical brain development" when neural circuits were still forming, they pointed out. At this stage in life the brain was vulnerable to "toxic insult".
Animals studies had shown that exposure to cannabis can alter brain structure, especially in the hippocampal region which is vital to memory.
The researchers concluded: "Prevention and policy efforts should focus on delivering to the public the message that cannabis use during adolescence can have harmful effects on neuropsychological functioning, delaying the onset of cannabis use at least until adulthood, and encouraging cessation of cannabis use particularly for those who began using cannabis in adolescence."
Professor Terrie Moffitt, from the Institute of Psychiatry at King's College London, who took part in the study, said: "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's such a special study that I'm fairly confident that cannabis is safe for over-18 brains, but risky for under-18 brains."
Colleague Professor Robin Murray, also from the Institute of Psychiatry, called the research "very impressive".
He added: "The Dunedin sample is probably the most intensively studied cohort in the world and therefore the data are very good. The researchers, who I know well, are among the best epidemiologists in the world. Therefore, although one should never be convinced by a single study, I take the findings very seriously.
"We have known for some time that heavy use of cannabis increases risk of schizophrenia-like psychoses but this remains a relatively rare outcome so it's not so important from a public health point of view.
"There are far fewer studies on its effect on minor psychiatric illness or on everyday life. However, 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 callers 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."