Fewer Colorado teenagers report using marijuana after the state legalized its adult-use commercial cannabis industry compared with the percentage of teens who reported marijuana use before the state opened retail pot shops, new federal data finds.

According to a new report from the National Survey on Drug Use and Health, which broke down marijuana use in the past month per age group within each U.S. state, 18.35 percent of Colorado residents from 12 to 17 years old report that they used marijuana in 2015, down from the 20.81 percent who reported marijuana use in 2014.

The 12 percent drop in teen use year-over-year in Colorado mirrors a similar downward trend for teen pot use in other states, including Washington, which also opened recreational marijuana markets in 2014, reports Christopher Ingraham of the Washington Post. This is the first definitive study that shows teen pot use decreased after legalization. The overall rate of teen marijuana use is higher in Colorado than in any other state, but as the Washington Post found, the trend began in 2005, before legal markets opened in the state.

The data showed that adult use in Colorado has increased since legalization.

One of the main arguments from anti-legalization groups was that legalization would lead to more teenagers using marijuana. Tom Angell, the founder of pro-legalization organization Marijuana Majority, says legalization is about regulating cannabis, closing the black market, and ensuring only adults are able to obtain marijuana.

"We've always argued that taking marijuana out of the unregulated criminal market and putting sales into the hands of responsible retailers would actually make it harder for young people to get," says Angell.