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The researchers found teen use was higher in the states that adopted medical marijuana laws, but that pattern of higher use existed before the laws came into effect.CHAD HIPOLITO/The Canadian Press

A new study suggests passing a medical marijuana law does not lead to increased use of the drug by teenagers in that jurisdiction.

American researchers looked at self-reported marijuana use among more than one million adolescents from 48 U.S. states between 1991 to 2014.

Twenty-one of the 48 contiguous states have passed medical marijuana laws.

The researchers compared use in states with and without legislation, but also looked closely at usage rates in states with medical marijuana laws in the periods before and after the legislation went into effect.

The researchers found teen use was higher in the states that adopted medical marijuana laws, but that pattern of higher use existed before the laws came into effect.

They say that given studies have shown use of marijuana in early adolescence can have negative effects in adulthood, research should focus on finding and addressing the factors that do lead to early use in teens.

"Our study findings suggest that the debate over the role of medical marijuana laws in adolescent marijuana use should cease, and that resources should be applied to identifying the factors that do affect risk," they wrote.

The study is published in the journal Lancet Psychiatry. The work was led by Prof. Deborah Hasin of the Mailman School of Public Health, at New York City's Columbia University.

A Canadian addictions expert lauded the research.

"This is probably the best and highest-quality study on a question that has been lingering for some time, which is whether the liberalization of marijuana control and more specifically medical marijuana regimes in the U.S. lead to higher levels of cannabis use, especially among young people," said Benedikt Fischer, a senior scientist at the University of Toronto's Centre for Addiction and Mental Health.

"What this study importantly finds is that while states with medical marijuana regimes have somewhat higher levels of marijuana use among young people, the explanation is not that it is the medical marijuana regimes that make marijuana use go up.

"These states were different from the outset — probably had a more accepting or softer disposition towards marijuana use."

A chief concern of opponents of medical marijuana laws is that adopting them will lead to a general increase in use of the drug, especially among young people. Previous studies of the question have produced mixed results.

These researchers set out to resolve the issue, using nearly a quarter-century's worth of data from a nationally administered survey American teenagers complete in Grades 8, 10 and 12. They analyzed information provided by 1,098,270 adolescents.

There was no statistically significant increase in marijuana usage rates among Grade 10 and Grade 12 students after medical marijuana laws came into effect in the states that adopted them. And use of the drug actually dropped among Grade 8 students after legislation was put in place.

"In conclusion, the results of this study showed no evidence for an increase in adolescent marijuana use after passage of state laws permitting use of marijuana for medical purposes," the authors wrote.

A commentary published with the study noted the work underscores how important it is to actually rigorously test assumptions before making public policy.

"Policies might sometimes be shaped by preconceived notions that do not end up being true, and Hasin and colleagues' study is an example of such an occurrence," wrote Dr. Kevin Hill from the Division of Alcohol and Drug Abuse, McLean Hospital, Belmont, Mass.