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Study Finds Teen Cannabis Use Does Not Lower IQ

The study is the latest in a growing body of evidence indicating habitual use of cannabis by teens is not independently linked with IQ declines.

Adolescent cannabis use is not independently linked with negative changes in intelligence quotient (IQ) or executive functioning, according to the findings in a new study published online ahead of print in the peer-reviewed journal Addiction.

A team of researchers from the United States and United Kingdom used a longitudinal co-twin control study to investigate whether marijuana use by teens is linked to neuropsychological performance changes over time. The study’s subjects included 1,989 twins from the Environmental Risk (E-Risk) Longitudinal Twin Study, a nationally representative birth cohort of twins born in England and Wales from 1994-1995. The investigators assessed the frequency of their cannabis use at age 18, evaluated executive functions at age 18, and obtained IQ scores at ages 5, 12, and 18.

They found that adverse cognitive performances in teens were linked to family background and not the use of cannabis independently.

“[W]e found that youth who used cannabis … had lower IQ at age 18, but there was little evidence that cannabis use was associated with IQ decline from age 12 to 18,” the researchers concluded. “Moreover, although cannabis use was associated with lower IQ and poorer executive functions at age 18, these associations were generally not apparent within pairs of twins from the same family, suggesting that family background factors explain why adolescents who use cannabis perform worse on IQ and executive function tests.”

“Short-term cannabis use in adolescence does not appear to cause IQ decline or impair executive functions, even when cannabis use reaches the level of dependence,” they added.

The findings are consistent with those from previous studies, including a 2016 article that found no link to cannabis use and declines in IQ in 3,066 twin teens over an eight-year period. Another study from last year discovered that cumulative cannabis use at the age of 15 had no effect on the IQ or educational performance of 16 year olds.

A common argument among opponents to cannabis legalization is concern that it will lead to an increase in adolescent cannabis use and the associated potential long-term cognitive impact. Studies have indicated, however, that the loosening of cannabis laws does not encourage teens to use marijuana nor does it cause them to start using cannabis earlier in their lives. Multiple studies have confirmed that adolescents are not adversely affected by passing of cannabis laws.

You can access the entire study, “Associations between Adolescent Cannabis Use and Neuropsychological Decline: A Longitudinal Co-Twin Control Study,” by visiting Wiley Online Library.

Twenty-nine states have legalized medical marijuana and eight have passed laws permitting the recreational use of cannabis. As legalization is expected to continue to expand to more states, it’s likely that opponents will the argument of its potential impact on use by teens. However, research has indicated that these concerns are unwarranted. As legalization has expanded throughout the U.S., cannabis use by young people has decreased.

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