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Pot Use in Teen Years May Lower IQ, Study Shows

Early, Long-Term Marijuana Use Linked to Drop in IQ
WebMD Health News
Reviewed by Louise Chang, MD

Aug. 27, 2012 -- Heavy, long-term marijuana use beginning in the teens can possibly lead to lower IQ, a provocative new study shows.

Frequent, continued use of marijuana starting before age 18 was associated with an eight-point decline in IQ in the study. The decline was not seen in users who started smoking pot in early adulthood or later in life.

The findings suggest that long-term marijuana use can cause long-term harm to some thinking abilities, such as intelligence, memory, and attention span, and that teens are uniquely vulnerable, researchers say.

“Many people today, especially young people, believe that marijuana is risk free, but this research tells us that this is not the case,” says Temple University professor of psychology Laurence Steinberg, PhD, who was not involved with the study.

Pot Users, Others Followed From Birth

The research included slightly more than 1,000 adult participants in a New Zealand health study who were followed from birth.

As part of the study, IQ tests were performed at age 7, 9, 11, 13, and 38.

For the latest analysis, Duke University postdoctoral researcher Madeline H. Meier, PhD, and colleagues compared IQ scores at age 13 -- before most of the participants had used marijuana -- to those at age 38, when many had experience with the drug.

In addition to the IQ tests, family members or close friends completed questionnaires when the participants were 38 years old designed to determine if the study enrollees had trouble with attention, memory, or social functioning.

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