Marijuana IQ Study Successfully Defended by Scientists

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CADCA Jan 24, 2013

A highly-publicized study finding that marijuana use is linked to a severe drop in IQ has been successfully defended by the scientific community overseas and in the United States, including the National Institute on Drug Abuse Director Dr. Nora Volkow.

The original study, published last August in the Proceedings of the National Academy of Sciences by Dr. Madeline Meier of Duke University, and colleagues, was the strongest evidence yet that teen use of cannabis could cause a drop in IQ. Opponents of the study claimed that socio-economic factors are to blame.

Around 1000 people all born in the same year in the New Zealand city of Dunedin were interviewed at ages 18, 21, 26, 32 and 38 about their marijuana use. The participants were also tested for their cognitive abilities at age 13 before starting to use cannabis, and at age 38. The study found persistent cannabis use during teenage years was associated with a drop in IQ of seven or eight points by the age of 38.

A new paper contesting the interpretation of the large-scale marijuana study was published in theProceedings of the National Academy of Sciences by Norwegian Dr. Ole Rogeberg of the Ragnar Frisch Centre for Economic Research in criticizes Meier for failing to control their study for socio-economic status: Poorer kids were getting an initial boost in IQ when they first went to school but that this declined once they left school.

“Indeed, when discussing traits like IQ, it would be surprising for one factor to be 100 percent causal. The strengths of the Meier et al study are that it is longitudinal in nature and that it controlled for a number of factors including years of education, schizophrenia, and other substance abuse. That said, observational studies in humans cannot account for all potentially confounding variables. In contrast, animal studies–though limited in their application to the complex human brain–can more definitively assess the relationship between drug exposure and various outcomes. They have shown that exposure to cannabinoids during adolescent development can cause long-lasting changes in the brain’s reward system as well as the hippocampus, a brain area critical for learning and memory,” Dr. Volkow wrote on NIDA’s website.

“The message inherent in these and in multiple supporting studies is clear. Regular marijuana use in adolescence is known to be part of a cluster of behaviours that can produce enduring detrimental effects and alter the trajectory of a young person’s life–thwarting his or her potential. Beyond potentially lowering IQ, teen marijuana use is linked to school dropout, other drug use, mental health problems, etc. Given the current number of regular marijuana users (about 1 in 15 high school seniors) and the possibility of this number increasing with marijuana legalization, we cannot afford to divert our focus from the central point: regular marijuana use stands to jeopardize a young person’s chances of success–in school and in life,” she concluded.


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