Cannabis can lower the IQ of young teenagers and may cause permanent mental impairment, research has shown.
Persistent users suffer an average eight-point decline in IQ between adolescence and adulthood, according to a study of more than 1000 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 into account different educational backgrounds and the 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 it in their early teens.
However, 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 North 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.
"Impairment was concentrated among adolescent-onset cannabis users, with more persistent use associated with greater decline."
Researchers analysed data on 1037 individuals enrolled into the Dunedin Study, a large lifestyle and health investigation based in New Zealand.
They followed the progress of participants from birth to the age of 38, carrying out tests at the age of 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 took cannabis at a young age and continued using it regularly for more than 20 years.
The study said: "The most persistent adolescent-onset cannabis users evidenced an average eight-point IQ decline from childhood to adulthood."
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.
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