Marijuana reported to dumb down the teen brain

Robin Wulffson MD's picture
marijuana, cannabis, IQ, brain damage, adolescents, cognitive impairment
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According to a new international study, the regular, dependent use of marijuana before age 18 can cause lasting harm to an individual’s intelligence, attention, and memory. Researchers from Duke University (Durham, North Carolina), King’s College London (London, England), and the University of Otago (Dunedin, New Zealand) published their findings in the Proceedings of the National Academy of Sciences, Online Early Edition on August 27.

The researchers noted that many adolescents do not believe that regular cannabis use is harmful to health. In addition, adolescents are initiating cannabis use at younger ages, and more adolescents are using cannabis on a daily basis. Therefore, the researchers designed a study to test the association between persistent cannabis use and neuropsychological decline; in addition, they set out to determine whether decline is concentrated among adolescent-onset cannabis users.

The study group as comprised of members of the Dunedin Study, a prospective study of 1,037 individuals followed from birth (1972/1973) to age 38 years. Cannabis use was ascertained in interviews at ages 18, 21, 26, 32, and 38 years of age. Neuropsychological testing was conducted at age 13, before initiation of cannabis use, and again at age 38, after a pattern of persistent cannabis use had developed. Approximately 5% of the study group were considered marijuana-dependent, or were using more than once a week before age 18. The researchers defined a dependent user is an individual who keeps using a drug despite significant health, social, or family problems.

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The researchers found that persistent cannabis use was associated with neuropsychological decline broadly across domains of functioning, even after controlling for years of education. Furthermore, the study participants who regularly used marijuana were found to have more cognitive problems. Individuals who started using cannabis in adolescence and used it for years afterward showed an average decline in IQ of 8 points when their age 13 and age 38 IQ tests were compared.

Impairment was concentrated among adolescent-onset cannabis users, with more persistent use associated with greater decline. In addition, cessation of cannabis use did not fully restore neuropsychological functioning among adolescent-onset cannabis users. Study subjects who did not start using marijuana pot until they were adults with fully-formed brains did not show similar mental declines. The researchers noted that before age 18, the brain is still being organized and remodeled to become more efficient; therefore, it may be more vulnerable to damage from drugs.

The researchers noted that their findings are suggestive of a neurotoxic effect of cannabis on the adolescent brain and highlight the importance of prevention and policy efforts targeting adolescents.

Reference: Proceedings of the National Academy of Sciences

See also: Today's Spice marijuana is more harmful to pregnancy reports new study

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