Marijuana legalization supporters say new data confirm their claim that legal adult access won't increase teen use, a major concern among opponents of relaxed state laws.

Teen marijuana use has been stable or declining since 2012, when Colorado and Washington became the first states to legalize recreational pot use for adults 21 and older, according to data released Thursday by the National Institute on Drug Abuse.

The results of the national Monitoring the Future survey show eighth-, 10th-, and 12th-grade students had no statistically significant annual change in past-year or daily marijuana use rates from 2016 to 2017. Past-month use increased for 10th grade, as did past-year use when data from all three grades was combined.

Longer-term trends between 2012 to 2017 are either stable or declining. Tenth-grade students had a significant decline in past-year and lifetime use during that period and eighth-grade students reported a significant drop in daily use.

“Fears of increased teen use are no longer a defensible justification for maintaining prohibition,” said Mason Tvert, a public relations consultant who co-directed Colorado’s legalization campaign.

Andrew Freedman, Colorado’s first director of marijuana coordination, said “these survey results are an encouraging sign that a tax and regulate system can effectively prevent youth use if implemented correctly."

Freedman credits state policies restricting ads and discouraging underage use.

The steady rates of teen use come amid another significant annual drop in the percent of teens who believe they risk harming themselves by using pot — breaking a historical link between risk perception and use.

“They are telling us it’s not dangerous, but there’s been no particular increase in their use of marijuana,” said NIDA deputy director Wilson Compton.

The promising federal data come after release earlier this week of state-specific 2016 results from another federal drug-use survey, the National Survey on Drug Use and Health, that showed declining pot use in Colorado and Washington state among 12-17-year-olds.

The NSDUH results showed 16.2 percent of young Coloradans and 13.5 percent of young Washingtonians used marijuana in the past month in 2015-2016, compared to 18.35 percent and 15.6 percent, respectively, the previous year. The two states began retail sales in 2014.

Opponents of marijuana legalization say, however, it’s possible the statistics are misleading.

Kevin Sabet, a former presidential drug policy adviser who leads a large anti-legalization advocacy group, Smart Approaches to Marijuana, said it’s likely that use would be even lower if states had not relaxed their laws.

Sabet emphasized the statistically significant annual change in past-year marijuana use when all three grades were combined.

“At a time when the use of opioids and virtually every other drug, including alcohol and cigarettes, are steeply declining, annual marijuana use among 8th, 10th and 12th graders has now increased for the first time since 2013,” he said. “This is very concerning, and shows that legalization makes marijuana seem safe for teens.”

So far, eight states and the nation’s capital have passed laws allowing legal recreational use of marijuana. More than half of states allow access to marijuana to treat health conditions.

Recreational legalization bills are widely expected to become law in Vermont and New Jersey early next year, with Vermont Gov. Phil Scott, a Republican, and New Jersey Gov.-elect Phil Murphy, a Democrat, indicating support. Canada, meanwhile, is preparing to legalize recreational marijuana use for the entire country.

Marijuana possession for any reason outside limited research remains a federal crime, but the Trump administration has taken no action to shut down state-regulated markets that were allowed to grow into a multibillion-dollar industry under President Obama.

An October poll by Gallup showed that 64 percent of people believe marijuana should be legal for recreational use, including more than half of Republicans, the most opposed major political grouping.

Ethan Nadelmann, a legalization advocate who founded and formerly led the Drug Policy Alliance, said stable or declining teen pot use “clearly does not help the legalization opponents.”

But Nadelmann said he doesn’t believe drug use is closely tied to the harshness of drug laws, and said teen use may change independent of whether or not pot is legal.

“Marijuana use going up or down among adolescents has a lot more to do with fads and fashions, and could have to do with other diversions,” he said. “Some people explain reductions in drug use among young people by the amount of time they’re spending on their smartphones.”