And it's not just marijuana: Rates of teen alcohol, tobacco and heroin use are down sharply in the state, as well.

Colorado, which was the first in the nation to open recreational marijuana markets in 2014, is viewed as a bellwether by both opponents and supporters of legalization.

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For state-level data, the survey uses pooled two-year periods to increase sample sizes and statistical accuracy. Last year the survey showed that Colorado was ranked No. 1 in the nation on adolescent marijuana use, a fact seized by marijuana opponents to argue that legalization was failing to protect children from drug use.

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With the sharp drop in this year's data, Colorado has fallen to No. 7 in the national ranking of teen marijuana use, behind Alaska, Maine, New Mexico, Oregon, Rhode Island and Vermont. A separate survey administered by officials in Colorado has found that teens in the state are in the middle of the pack on marijuana use.

“Teen use appears to be dropping now that state and local authorities are overseeing the production and sale of marijuana,” said Brian Vicente of Vicente Sederberg LLC, one of the drafters of Colorado's marijuana ballot measure, in a statement. “There are serious penalties for selling to minors, and regulated cannabis businesses are being vigilant in checking IDs.”

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Attorney General Jeff Sessions is an outspoken critic of marijuana legalization, but thus far he has hewed to the prior administration's policy of noninterference with state-level legalization efforts. While marijuana remains illegal at the federal level, voters in eight states plus the District have legalized the recreational use of the drug. Lawmakers in Vermont have signaled they will legalize the recreational use of marijuana early next year.

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The new federal data shows that adolescent marijuana use fell nationwide in 2016. In no states did the share of teens using pot increase by a significant amount, and in a number, including California, Colorado, Maryland, New Jersey and Texas, rates of teen marijuana use fell considerably.

Use is up, however, among young adults age 18 to 25 and adults age 26 and up. Alcohol use, meanwhile, is falling across the board, according to the federal survey data.

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In Colorado, for instance, the number of 18-to-25-year-olds using alcohol on a monthly basis fell by four percentage points between 2014-2015 and 2015-2016. That's the group with the highest propensity to use marijuana, suggesting that a number of young adults are opting to smoke weed instead of get drunk now that the option is available to them.