Smoking cannabis can lower the IQ of teenagers, new study reveals

Published on 28/08/2012 01:01

CANNABIS can lower the IQ of young teenagers and may cause permanent mental impairment, new research has revealed.

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 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.

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 Durham, North Carolina, wrote in the journal Proceedings of the National Academy of Sciences: “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 an early 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 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”.

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 per cent 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.”