Teens who use pot just one or two times may end up with changes to their brains, a new study finds.



There were clear differences on brain scans between teens who said they had tried cannabis a couple of times and those who completely eschewed the drug, researchers reported in the Journal of Neuroscience.



There have been hints that even small amounts of pot at a young age might impact the brain, said the study's lead author, Catherine Orr, a lecturer at the Swinburne University of Technology in Melbourne, Australia. "Research using animals to study the effects of cannabis on the brain have shown effects at very low levels, so we had reason to believe that brain changes might occur at even the earliest stages of cannabis use," Orr said in an email.



Still, she said, "I was surprised by the extent of the effects."





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 the volume of grey matter in brain regions that are rich with cannabinoid receptors. Grey matter, which is made up of nerve cell bodies, is involved in sensory perception and muscle control.



To take a closer look at the impact of mild marijuana use in developing brains, Orr's team analyzed brain scans gathered as part of the larger IMAGEN study, which was designed to look into adolescent brain development.



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

Brain volume

The researchers spotted clear differences between the two groups, which they suspect are due to the low-level pot use.

They acknowledge that the study didn't actually prove that marijuana led to the differences seen in the scans. It's possible that those who chose to use weed were different to begin with and that the marijuana hadn't played a role in brain development.





To try to address this question, the researchers analyzed scans from a third group of teens who had not tried marijuana before they had their brain scans at age 14. By age 16, 69 of these kids said they had used marijuana at least 10 times. But their brain scans at age 14 looked no different than brain scans of other kids who had not taken up cannabis by age 16 which meant there wasn't any inborn brain difference that would have predicted who would later become a pot user.



There may be serious implications to the brain changes noted by the researchers. "In our sample of cannabis users, the greater volumes in the affected parts of the brain were associated with reductions in psychomotor speed and perceptual reasoning and with increased levels of anxiety two years later," Orr said.



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



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



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