PHILADELPHIA (CBS) – Parents can rest easy: Legalizing marijuana does not result in an increase in adolescent use of the drug, a new study shows.

The research, which analyzed data from states with and without medical marijuana laws from the Youth Risk Behavioral Surveillance Survey between 1991-2011, found no significant increase in marijuana use related to changes in policy.

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Researchers say they looked at a sample of nearly 12 million students from across the country and found that almost 21% of them had used marijuana in the past month. However, scientists say they found no real difference in pre- and post-policy change use.

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The study is published in the May 2014 issue of the Journal of Adolescent Health.

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