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16th January 2019
05:25pm GMT

Grey matter usually decreases in teenage brains, study author Dr Catherine Orr explained.
"During adolescence we lose grey matter in the brain.
"Parts of the brain that are not being used are pruned away and connections between parts of the brain become stronger.
"We think neural pruning may have been disrupted or delayed (in the teens who had smoked cannabis)."
Some people may be more vulnerable to this disruption, she said.
Lead scientist Professor Hugh Garavan added that while they couldn't be certain, the results implied that even light cannabis use can alter the brain.
“Most people would likely assume that one or two joints would have no impact on the brain," he said.
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