Don't Blame Pot for Teens' IQ Drop, Study Says

A marijuana cigarette
(Image credit: Pe3k/

Smoking pot can be a health concern for teens, but it is unlikely to cause a decline in their thinking abilities, a new study finds.

Instead, the results suggest that if teens experience a cognitive decline, other factors, such as genetics or that young person's family environment, are more likely to be responsible for the drop, the researchers said. "It could be that they come from a neighborhood or a home where intellectual development is not highly encouraged," said study author Joshua D. Isen, of the University of Minnesota in Minneapolis.

The implications of the new findings are that "it is unlikely that the exposure to marijuana itself is causing children to show intellectual change," Isen told Live Science.

Previous research on marijuana use during adolescence has yielded mixed results. Some studies have linked use of the drug during adolescence with a decline in cognitive ability. But other studies have suggested that the potential effects of marijuana on cognition were temporary, and that they wore off within several months after young users quit smoking.

In the new study, the researchers looked at the relationship between marijuana use and intelligence based on data from two studies of twins that involved more than 3,000 participants. In the first study, which involved nearly 800 twins, the researchers administered IQ tests to measure the twins' intelligence when they were 9 or 10 years old, and again when they were 19 or 20 years old. The researchers also asked the young people whether they used marijuana at any time throughout their middle school or high school years. [6 Foods That Are Good For Your Brain]

In the second study, which involved nearly 2,300 twins, the researchers also used IQ tests, this time testing the twins at age 11 or 12, and again when they were 17 to 18. Again, the researchers asked the young people whether they used pot while they were in middle school and high school.

In the study of nearly 800 twins, the participants' baseline IQ scores did not show significant differences between the average IQ scores of kids who had smoked marijuana and those who had not used the drug.

However, it may be that smarter kids don't start using marijuana in the first place, the researchers said. For example, among the kids in the other study, the average score on the vocabulary test was 98.8 among the kids who later used marijuana, compared with 100.7 among those who didn'tuse the drug. Similarly, the average score on the general knowledge test was 97.9 among future users, compared with 101.2 among nonusers. 

When the researchers looked at the IQ patterns in both groups, comparing the beginning and the end of the follow-up periods, results showed that the kids' vocabulary IQ scores declined over time among the young people who used marijuana while they were in middle school or high school.

However, when the researchers compared the changes in IQ between individuals within the the same twin pairs, in which one twin had used marijuana and the other had not, they found that there were no significant differences in the extent to which both twins' IQ's might have changed over time. In other words, the twins who used marijuana did not develop greater cognitive deficits over time, compared to their twin siblings who didn't use the drug.

Though the new findings may cast doubt on the idea that smoking marijuana makes kids less intelligent, "it doesn't mean that marijuana use itself is harmless," Isen told Live Science.

Dr. Scott Krakower, an assistant unit chief of psychiatry at Zucker Hillside Hospital in Glen Oaks, New York, who was not involved in the study, agreed, saying that, despite the new findings on marijuana and overall IQ, there may be "other variables, other consequences that can come from using cannabis." For example, the marijuana users in the study used more drugs and alcohol, compared with marijuana nonusers, he noted. Moreover, the users did have some reduction in their vocabulary scores, he added.

The new study was published Monday (Jan. 18) in the journal Proceedings of the National Academy of Sciences.

Follow Agata Blaszczak-Boxe on Twitter. Follow Live Science @livescience, Facebook & Google+. Originally published on Live Science.