New research has cast serious doubts on the argument that legalization of medical marijuana would lead to more widespread use among teenagers.

The study, which was published Tuesday by Columbia University researchers in the The Lancet Psychiatry, examined marijuana use among more than a million teenagers over 24 years.

The researchers found no discernible impact in the 21 states that had legalized the drug for medical use. “The risk of marijuana use in states before passing medical marijuana laws did not differ significantly from the risk after medical marijuana laws were passed,” the co-authors wrote.

While researchers observed no discernible change after the laws were passed, they did find drug use rates were generally higher in states that had legalized marijuana. “State-level risk factors other than medical marijuana laws could contribute to both marijuana use and the passage of medical marijuana laws, and such factors warrant investigation,” the authors concluded.

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