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NIH-funded study links long-term marijuana use, especially when started during adolescence, with decreased IQ and impaired cognitive function

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September 10, 2012

NIH-funded research shows that long-term marijuana is associated with impaired intellectual functioning, especially if usage starts during the teen years. Over 1,000 study participants were given neuropsychological tests in early adolescence, prior to initiation of marijuana use, and then re-tested in mid adulthood. Study members with more persistent marijuana dependence showed greater IQ decline and greater impairment across five different cognitive domains, especially executive function and processing speed. This is the first long-term prospective study to test young people before their first use of marijuana and again after 20+ years of use. The study was thus able to rule out pre-existing differences in IQ between heavy marijuana users and others; it is also significant for including degree of cannabis exposure and age of onset as factors. Those who started use during the teen years showed greater IQ decline than those who began use as adults. These latter results are especially troubling, given recent data showing increased marijuana use among teens over the last five years, along with declines in perceived risk of harm associated with use. The results of this study are consistent with the notion that cannabis may actually cause some of the neuropsychological deficits seen in regular cannabis users.

The study was funded in part by the National Institute on Drug Abuse, the National Institute on Aging and the National Institute of Mental Health. For a copy of the study, go to www.pnas.org/content/early/2012/08/22/1206820109.abstract?sid=aaccd18b-26ef-4497-8da0-fa55c5ad15fe. For a Message from the NIDA Director on marijuana’s lasting effects on the brain, go to /about-nida/directors-page/messages-director/2012/09/marijuana%E2%80%99s-lasting-effects-brain.

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About the National Institute on Drug Abuse (NIDA): The National Institute on Drug Abuse (NIDA) is a component of the National Institutes of Health, U.S. Department of Health and Human Services. NIDA supports most of the world’s research on the health aspects of drug use and addiction. The Institute carries out a large variety of programs to inform policy, improve practice, and advance addiction science. Fact sheets on the health effects of drugs and information on NIDA research and other activities can be found at www.drugabuse.gov, which is now compatible with your smartphone, iPad or tablet. To order publications in English or Spanish, call NIDA’s DrugPubs research dissemination center at 1-877-NIDA-NIH or 240-645-0228 (TDD) or email requests to drugpubs@nida.nih.gov. Online ordering is available at drugpubs.drugabuse.gov. NIDA’s media guide can be found at www.drugabuse.gov/publications/media-guide/dear-journalist, and its easy-to-read website can be found at www.easyread.drugabuse.gov. You can follow NIDA on Twitter and Facebook.

About the National Institutes of Health (NIH): NIH, the nation's medical research agency, includes 27 Institutes and Centers and is a component of the U.S. Department of Health and Human Services. NIH is the primary federal agency conducting and supporting basic, clinical, and translational medical research, and is investigating the causes, treatments, and cures for both common and rare diseases. For more information about NIH and its programs, visit www.nih.gov.

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    NIDA. (2012, September 10). NIH-funded study links long-term marijuana use, especially when started during adolescence, with decreased IQ and impaired cognitive function . Retrieved from https://www.drugabuse.gov/news-events/news-releases/2012/09/nih-funded-study-links-long-term-marijuana-use-especially-when-started-during-adolescence-decreased

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