Cannabis permanently lowers IQ of teenagers

Cannabis can lower the IQ of 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 a 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, 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 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 associ-ated 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 recor-ded 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.

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”.

The scientists 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.”