Adolescent cannabis use went down dramatically in Colorado in the years 2014 and 2015, after the state legalized recreational use, new federal survey data show.

The state-level data from the National Survey on Drug Use and Health showed that 18.35 percent of Coloradans ages 12 to 17 had used cannabis in the two years in question, down sharply from 20.81 percent in 2013/2014. This is roughly a 12 percent drop in cannabis consumption in a year-to-year basis.

At the same time, adult cannabis use rose significantly in Colorado over the same time period. Among Coloradans ages 26 and older, past-year cannabi use rose from 16.8 percent in 2013/2014 to 19.9 percent in 2014/2015. Annual adult use was up in most states during the same time frame. The legal marijuana markets in Colorado, Washington and elsewhere feature strict age and purchasing limits.

This federal data released this week is the first clear evidence of a drop in teen cannabis use in Colorado after legalization. Legalization supporters have long argued that the best way to prevent underage marijuana use is to legalize and regulate it.

The federal data doesn’t speak to what, exactly, is behind the decrease in teenagers using pot.

Some experts had expected more permissive attitudes toward pot to lead to increased teen use and have subsequently been surprised to find that teen marijuana use has held steady or even fallen nationwide over the past few years.

The federal survey data do that the overall rate of teen cannabis use remains higher in Colorado than in any other state. But that trend began well before legalization, as the chart below of monthly marijuana use in Colorado and the United States shows.

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Other data sources, including the Colorado Department of Health’s own numbers, show that Colorado is essentially middle-of-the-pack among the states on adolescent marijuana use.

In either case, the overall trend – flat or falling teen use – appears to support legalization supporters’ arguments that liberalizing cannabis policies will not pose a serious public health threat to adolescents.