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Marijuana preparations are becoming more widely available and more potent and contain lower levels of protective cannabidiol (Biol Psychiatry 2016 Jan 19; [e-pub]). Investigators studied marijuana use and intelligence quotient (IQ) in two samples of twins, comprising more than 3000 children, starting before they had used cannabis (baseline ages, 9 to 12 years). At follow-up (ages, 17–20 years), participants reported their frequency of marijuana use: ever, at least 30 times, or every day for at least 6 months.
Baseline IQ was significantly lower among later users of marijuana than among nonusers. By ages 17 to 20, vocabulary and information scores (IQ measures that depend on learning) had declined significantly more in marijuana users than no…