Science & tech

Cannabis May Not Lower Your IQ, After All

Published on October 24, 2014 · Last updated July 28, 2020

Recently I wrote about cannabis use among teenagers and a study that found an IQ drop among underaged heavy cannabis users, which raised concerns about the negative impact cannabis use can have on still-developing brains. Two days ago, The Washington Post published a piece called "No, Marijuana Use Doesn't Lower Your IQ" that challenges the study. 

In the Post article, the Duke University study is criticized for using only 38 users as its sample size and for failing to account for several factors. Additionally, the article highlights a new study from the University College of London that further refutes Duke's findings. This study used a sample size of 2,612 adolescents and tested their IQ scores at age 8 and age 15. The result? After taking several factors into account, researchers found "no relationship between cannabis use and lower IQ at age 15," while alcohol use was strongly associated with a decline in IQ. 

"Slightly impaired educational abilities" were evidenced among the heaviest cannabis users, as they scored about 3% lower on school tests taken when they were 16 years old. However, it's not as drastic as the Duke University study citing a decline of eight IQ points. 

When it comes to cannabis studies, we need as many as we can get, but we also need studies that are conducted as accurately as possible and properly exclude factors or variables that can severely skew results. TheWashington Post sums it all up nicely:

"Many skeptics of legalization in the United States focus on the potential harms of marijuana use alone. But marijuana use is just one of many behaviors that can possibly affect life outcomes. In many cases these other behaviors are likely to play a much larger role in determining a person's trajectory through life […] this study is the latest in a growing, robust body of evidence suggesting it makes little sense to focus on marijuana use to the exclusion of all other factors."

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Rebecca Kelley
Rebecca Kelley
Rebecca is the Content Director at Leafly, where she oversees Leafly News production and other content projects.
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