In recent years, marijuana plants have also been bred to have higher levels of tetrahydrocannabinol (THC), the psychoactive component. The higher potency may be even riskier to the teen brain, he added.
Meier noted in a statement that last year was the first time that smoking pot eclipsed smoking tobacco among U.S. teens.
While the researchers found an association between heavy, long-term marijuana smoking and declines in IQ, they did not prove that the marijuana caused the declines.
The U.S. National Institute on Drug Abuse has more on marijuana.
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