LINDA WERTHEIMER, HOST:

When researchers look for more evidence on how marijuana affects our overall health, they're also learning quite a bit about the teenage brain.

NPR's Patti Neighmond reports that there's mounting evidence that marijuana can harm a developing young brain.

PATTI NEIGHMOND, BYLINE: You could think of the teen years as the last golden opportunity to make the brain as healthy and smart as possible, says researcher Krista Lisdahl, which is why she says when it comes to trying pot....

KRISTA LISDAHL: It's the absolute worst time.

NEIGHMOND: That's because the teen brain is actively building and strengthening its pathways and connections. It's moving away from the brain of a child and developing into the brain of an adult.

LISDAHL: Actually in childhood, our brain is larger, it has more volume and then during the teenage years, it's getting rid of those connections that weren't really used and that actually makes the brain faster and more efficient.

NEIGHMOND: A sort of streamlining that ultimately helps the brain make judgments, think critically and remember what it's learned. Lisdahl says a mind altering drug like marijuana can disrupt that process.

As a researcher with the University of Wisconsin, Milwaukee - Lisdahl points to a growing number of studies which show regular marijuana use - that means at least once a week or more, actually changes the structure of the teen brain - specifically in areas dealing with memory and problem solving. That can affect cognition, she says - and academic performance.

LISDAHL: And indeed, we actually see if we look at actual grades that the chronic marijuana using teens do have on average one grade point lower than their matched peers that don't smoke pot.

NEIGHMOND: In one study, researchers analyzed data from New Zealand. They looked at marijuana use and compared IQs in childhood through adulthood. Researcher Madeline Meier is a psychologist at Arizona State University.

MADELINE MEIER: We found that people who began using marijuana as adolescents - so in their teenage years - and then continued to use marijuana for many years - they lost about eight IQ points from childhood to adulthood, whereas those who never used marijuana did not lose any IQ points.

NEIGHMOND: The amount people smoked made a difference. Those who smoked the most - at least every day, for example - saw the greatest drop in IQ by age 38. And the younger they were when they started using pot, the greater the IQ drop. And it wasn't just IQ. Adults who smoked marijuana as teenagers did worse in tests of memory and decision making than adults who hadn't.

Now there's an important caveat here. In the study, those who used the most marijuana had lower IQs to begin with. Gregory Tau is a child psychiatrist and brain researcher at New York's Columbia University.

GREGORY TAU: It's very possible that the group of adolescents that tend to really get into trouble with marijuana or use heavily, there's something very different about them to begin with.

NEIGHMOND: It could be lower IQs, it could be a sense of insecurity, or emotional problems, such as anxiety or depression.

Longer and larger studies are needed to figure all this out. But in the meantime, neuropsychologist Krista Lisdahl says concerns about marijuana safety seem to be lost among teenagers themselves. Take this recent federal survey of teen opinions about pot.

LISDAHL: Sixty percent of teenagers think that marijuana is safe and we're seeing this directly translated into increased use. So we have 23 percent of high school seniors using pot in the last month and I really have to emphasize that is more than cigarettes.

NEIGHMOND: About 6 percent of high-schoolers say they use pot every day. That's nearly three times more than it was a decade ago. And the marijuana they smoke is a lot more potent than it was in the '70s - with far higher levels of the active ingredient, THC.

LISDAHL: The higher THC levels, the more brain changes there are and the more there is the risk for addiction.

NEIGHMOND: Lisdahl says more teens and young adults are smoking marijuana in states that have made the drug available for medical use. She says that's worrisome because it might be a harbinger of things to come if pot is fully legalized.

Patti Neighmond, NPR News.

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