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Marijuana has been widely reported to cause a decline in teenager’s brain function. But a new meta-study published in JAMA Psychiatry shows that the negative mental effects of heavy marijuana use are short-lived: after 72 pot-free hours, users’ memory and cognitive performance was indistinguishable from non-users. Perhaps the most heartening finding was that teens did not have a higher risk of cognitive problems than adults.

The "Duh" Results: It is pretty clear that marijuana causes cognitive impairment. Memory, coordination, and cognitive ability all decline under the influence. But some studies have suggested that heavy use could cause long-lasting damage to mental performance, especially to the teenage brain that is (as we are so often reminded) "still developing".

This research looked at nearly 70 previous studies of younger marijuana users, and it found cognitive differences between heavy pot smokers and non-users—but they were a lot smaller than anyone expected. The lead researcher concluded, “The clinical significance of that is kind of questionable. It does raise a question of how big are these differences in a practical sense, and what those differences mean in someone's life.”

The Big Conclusion: The evidence showed that by four days after heavy use, the small difference in mental performance between smokers and non-users was completely gone. There was no difference between adolescents and adults. Researchers couldn’t say whether “decades” of heavy use might cause persistent declines in brain performance, or whether years of heavy use by very young people could have long-run impacts. But with so many states legalizing cannabis, it’s encouraging to know that short-term use only has short-term effects on mental health.

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