Just 1 or 2 experiences with marijuana may alter teen brain

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With an estimated 35 percent of U.S. teens using cannabis,the new findings are concerning, the researchers noted.

Orr and her colleagues saw widespread increases in thevolume of grey matter in brain regions that are rich withcannabinoid receptors. Grey matter, which is made up of nervecell bodies, is involved in sensory perception and musclecontrol.

To take a closer look at the impact of mild marijuana use indeveloping brains, Orr's team analyzed brain scans gathered aspart of the larger IMAGEN study, which was designed to look intoadolescent brain development.

The researchers analyzed images from 46 14-year-olds whosaid they had used marijuana once or twice, as well as imagesfrom 46 non-cannabis using teens matched "on age, sex,handedness, pubertal status, IQ, socioeconomic status, and useof alcohol and tobacco," Orr said.

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To try to address this question, the researchers analyzedscans from a third group of teens who had not tried marijuanabefore they had their brain scans at age 14. By age 16, 69 ofthese kids said they had used marijuana at least 10 times. Buttheir brain scans at age 14 looked no different than brain scansof other kids who had not taken up cannabis by age 16 whichmeant there wasn't any inborn brain difference that would havepredicted who would later become a pot user.

There may be serious implications to the brain changes notedby the researchers. "In our sample of cannabis users, thegreater volumes in the affected parts of the brain wereassociated with reductions in psychomotor speed and perceptualreasoning and with increased levels of anxiety two years later,"Orr said.

The reason for the higher volume of grey matter incannabinoid-rich regions of the brain may be related to a normalprocess called "pruning" which may go awry when kids usemarijuana, Orr said. As young brains develop, unnecessary ordefective neurons are pruned away, she explained. When thesystem doesn't work correctly, those cells remain in place.

The new findings are a step toward understanding the impactof cannabis on young brains, said Dr. Michael Lynch, atoxicologist and emergency medicine physician and director ofthe Pittsburgh Poison Center at the University of PittsburghMedical Center. "It's important that there was a change," Lynchsaid. "Adolescent brains are going to be more vulnerable toanything drug or environmentally related."

If pruning isn't working right, "the brain may not work asefficiently as it should," Lynch said. "But I don't think we canmake a final determination on that from this study."