Recreational marijuana legalization had no impact on how many Colorado teens use pot or on whether they think it is dangerous, but that could be because years of medical marijuana sales already had brought about changes in those measures, according to a new study.

The study, posted on the website of the journal JAMA Pediatrics on Tuesday, looked at national survey data and concluded that the percent of teens from Colorado who said they had used marijuana in the past month was statistically unchanged between the pre-legalization years of 2010 to 2012 and the post-legalization years of 2013 to 2015.

Similarly, the study found that a shift in Colorado teens’ attitudes toward marijuana’s risks — kids are less likely today to say they think using marijuana can be harmful to health — was not statistically different from the national trend.

That contrasts with Washington — which, along with Colorado, legalized recreational marijuana use and sales in 2012. The study found kids in eighth and 10th grades in that state are more likely to use marijuana since legalization and have shifted even more than the national trend toward thinking marijuana use doesn’t pose a great or moderate health risk.

“Our study didn’t particularly tell us why” the two states differed, said Magdalena Cerdá, a researcher at the University of California-Davis Violence Prevention Research Program and the lead author of the study.

But she and her co-authors have a theory. When Colorado voters passed recreational legalization, the state already had a large medical marijuana industry with numerous dispensaries. In Washington, the industry was less developed — meaning its teens weren’t as exposed as those in Colorado to marijuana ads and other commercialization.

Cerdá’s study is the latest to try to make sense of legalization’s impacts in Colorado.

To conduct their research, Cerdá and her colleagues dug into raw data from the University of Michigan’s national Monitoring the Future survey.

A different national survey, by the federal government, this year estimated that the percent of kids ages 12 to 17 in Colorado who used marijuana in the past month was the highest in the country. But that same survey concluded the rate was flat since recreational marijuana stores opened — something echoed this year by the state Health Department’s Healthy Kids Colorado Survey. The percentage of Colorado kids who reported using marijuana in the previous year actually declined in the federal government’s survey this year.

The state’s long-term trends, though, point toward higher marijuana use and lower perceptions of risk for Colorado’s youth. A decade ago, the federal government estimated that about 7.6 percent of kids ages 12 to 17 in Colorado used marijuana in the prior month. In the survey results released this year, that rate had grown to about 11.1 percent.

Cerdá said it will take more study to determine whether changes in the state’s marijuana laws are the cause of that trend.