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    Early Marijuana Use Leads to Later Brain Problems

    Adults Who Started Smoking Marijuana Before Age 16 Performed Worse on Tests Than People Who Started Smoking Later

    Test Results

    The healthy comparison group performed better on several measures, compared to the marijuana smokers, and the early-onset smokers did worse than the late-onset smokers. The early-onset smokers ''kept not getting it," Gruber says. Despite being told their sorting was wrong after the rules change, the early-onset users kept making repeated errors. The early users also had more problems maintaining a set of rules.

    For one category, for instance, early-onset smokers made 14.2 mistakes, compared to 6.4 by the late-onset smokers.

    How might these cognitive problems translate to everyday life? Gruber says the early-onset smokers may have trouble with abstract thinking, for instance, or may say something socially inappropriate.

    Early Marijuana Use: Perspective

    The new research adds to previous studies finding other harmful brain effects, says Frances Jensen, MD, a professor of neurology at Harvard Medical School who moderated the news conference detailing Gruber's findings.

    She speculates that the marijuana exposure ''is somehow modifying the way that part of the brain [the prefrontal cortex, involved in executive function tasks] is developing," she tells WebMD. "That area is one of the last parts of the brain to develop fully, she says, so the teen brain is especially vulnerable.

    ''Currently, it looks irreversible," she says of the damage incurred by early-onset smoking of marijuana.

    This study was presented at a medical conference. The findings should be considered preliminary as they have not yet undergone the "peer review" process, in which outside experts scrutinize the data prior to publication in a medical journal.

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