Parents and physicians concerned about an increase in adolescents' marijuana use following the legalization of medical marijuana can breathe a sigh of relief. According to a new study at Rhode Island Hospital which compared 20 years worth of data from states with and without medical marijuana laws, legalizing the drug did not lead to increased use among adolescents. The study is published online in advance of print in the Journal of Adolescent Health.

"Any time a state considers legalizing medical marijuana, there are concerns from the public about an increase in drug use among teens," said principal investigator Esther Choo, M.D., an attending physician in the department of emergency medicine at Rhode Island Hospital. "In this study, we examined 20 years worth of data, comparing trends in self-reported adolescent marijuana use between states with medical marijuana laws and neighboring states without the laws, and found no increase in marijuana use that could be attributed to the law."

Choo continued, "This adds to a growing body of literature published over the past three years that is remarkably consistent in demonstrating that state medical marijuana policies do not have a downstream effect on adolescent drug use, as we feared they might."

Currently, medical marijuana is legal in 21 states and the District of Columbia.

The study examined a nationally representative sample of high school students. The data showed that past-month marijuana use was common, at nearly 21 percent of the study population. However, there were no statistically significant differences in marijuana use before and after policy changes in any state pairing.

"Researchers should continue to monitor and measure marijuana use," Choo said. "But we hope that this information will provide some level of reassurance to policymakers, physicians, and parents about medical marijuana laws."

Choo's principal affiliation is Rhode Island Hospital, and she also holds an academic appointment as an assistant professor of emergency medicine at The Warren Alpert Medical School of Brown University.