A new Study published online in the Journal of Adolescent Health, the study compared 20 years of data from the Centers for Disease Control and Prevention’s Youth Risk Behavioral Surveillance Survey in states that have legalized medical marijuana with data from neighboring states that continue to ban the plant. It found that legalization of marijuana for medical purposes does not result in greater illicit use of the substance by high school students as fear by most parents and guardians.

Looking at high school students across the nation, the report found that nearly 21 percent of those surveyed had used marijuana in the past month. But according to the study’s abstract, there were “no statistically significant differences in marijuana use before and after policy change for any state pairing.”

Dr. Esther Choo, lead author on the report, said, the new study adds to a “growing body of data” demonstrating little effect from legal medical marijuana on adolescent drug use. A 2013 report from Monitoring the Future, an ongoing study of the behavior of American youth, found nearly zero year-to-year changes in marijuana use among teens. The behavior of teens in states with legal medical marijuana was also examined in a 2012 study published by the IZA Journal, which also found that legalization does not lead to increased underage use.

“My other research involves designing programs to reduce drug use among high risk populations, and I’m also a parent of three young children,” Choo said. “So I don’t reflexively advocate for increasing the availability of recreational drugs. But as a physician, I see a lot of patients with chronic conditions who do not respond well to existing therapies. This study arose both from my concern for potential negative consequences of medical marijuana and my desire to not automatically write off such policies because of my fears.”

Currently, 21 states and the District of Columbia have legalized marijuana for medical use. Colorado and Washington are the only two states that have also lifted the ban on recreational marijuana among adults. About a dozen other states are considering legalization of cannabis in some form in the coming years.