Rates of marijuana use among Colorado’s teenagers are essentially unchanged in the years since the state’s voters legalized marijuana in 2012, new data from the Colorado Department of Public Health and Environment shows.

In 2015, 21 percent of Colorado youths had used marijuana in the past 30 days. That rate is slightly lower than the national average and down slightly from the 25 percent who used marijuana in 2009, before legalization. The survey was based on a random sample of 17,000 middle and high school students in Colorado.

“The survey shows marijuana use has not increased since legalization, with four of five high school students continuing to say they don’t use marijuana, even occasionally,” the Colorado health department said in a news release.

The numbers out of Colorado are being closely monitored by policymakers and advocates on both sides of the marijuana legalization divide. Researchers generally agree that marijuana use during adolescence should be strongly discouraged – younger users are more likely to become dependent on the drug, and teens who use marijuana heavily are at higher risk of a number of mental and physical health problems later.

Opponents have often claimed that marijuana legalization would lead to more kids smoking pot, with all the negative health consequences that would entail. But the scant data available until now hasn’t borne this out.

National surveys have shown that teen marijuana use rates are falling across the country. But there haven’t been many numbers available specifically for states such as Colorado and Washington where it is legal. Federal data released late last year showed that teen use rates in Colorado and Washington were essentially flat, but they covered only 2014, the first year commercial marijuana was available in those states.