Does marijuana make teens stupid? Study says no

New research released Monday challenges the idea that smoking marijuana during adolescence can lead to declines in intelligence. (Ted S. Warren, Associated Press)

By Jackie Borchardt,

COLUMBUS, Ohio -- A new study questions the long-held belief that smoking marijuana makes you dumber, especially if you get high in your teen years.
That belief has been bolstered by studies suggesting marijuana use can harm teen brain growth and activity. But a new analysis released this week is getting attention for its scope and conclusions.

Where did the study come from?
The study was conducted by researchers at the University of Southern California in Los Angeles and the University of Minnesota, and will be published in the Proceedings of the National Academy of Sciences journal.

What did they study?
Scientists there tapped data collected from two large studies of twins, selecting 3,066 participants who had been given a variety of intelligence tests over a period of 10 years. The twins' intelligence was tested once between ages 9 and 12 (before any marijuana use) and again between ages 17 and 20. Participants self reported marijuana use at each assessment and at various points in between.

What were the researchers' conclusions?
Marijuana users had lower intelligence test scores compared to nonusers and showed a significant decline in their ability to use learned knowledge. But in cases where one twin used marijuana and the other didn't, the decline was about the same, leading researchers to conclude the lower IQ test scores were not a direct result of marijuana exposure.

Why does it matter?
Although it's widely believed marijuana use can harm young, developing brains, the researchers noted that existing research supporting that belief is sparse and conflicting.

An often-cited 2012 Duke University study concluded heavy marijuana use can lower teens' IQ by as much as 8 points. But that study and others that reached similar conclusions have been criticized for their small sample sizes and for excluding environment as a contributing factor in drug use. The USC/Minnesota study is the largest thus far. Also, by studying identical twins who share the same DNA and environments, the authors were able to control for those variables.

The study did not consider other harmful effects of marijuana use, and researchers did not identify why some participants chose to use marijuana.

Dr. Nora Volkow, director of the National Institute on Drug Abuse, told the Associated Press that the study has some limitations but is important. Volkow said it deserves to be followed up by more research and a government-sponsored study examining marijuana and drug use among 10,000 children over time is underway.