A newly published study tracks what it calls a "significant decrease" in youth marijuana use from 2011 to 2021 -- a period in which more than a dozen states legalized marijuana for adults -- detailing lower rates of both lifetime and past-month use by high-school students nationwide.

The findings run contrary to claims from legalization opponents who said the policy change -- first enacted in Colorado and Washington State in 2012 -- would lead to skyrocketing cannabis consumption by teens.

In 2021, says the study, published this month in the journal Pediatric Reports, 39.9 percent of adolescents reported ever having tried marijuana. As of 2021, that number had fallen to 27.8 percent. The share who said they consumed cannabis at least once in the past month, meanwhile, fell from 23.1 percent in 2011 to 15.8 percent in 2021.

Drawing on data from the Centers for Disease Control and Prevention's (CDC) Youth Risk Behavior Survey -- which polls ninth through 12th graders on various health-related behaviors -- the new analysis also found that proportion of young people who reported trying cannabis before age 13 also fell markedly during the study period, dropping from 8.1 percent in 2011 to 4.9 percent in 2021.

"The significant decreases observed in both the 'ever used marijuana' and 'currently use marijuana' categories highlight a promising reduction in adolescent marijuana use, with usage dropping to approximately 70% of the levels recorded in 2011," the report says. "Similarly, the percentage of adolescents who tried marijuana before age 13 decreased to about 60% of the 2011 levels."

Despite the long-term declines, the decade saw at least some ups and downs in teen use rates, including a slight rise in 2019 followed by a rapid slowdown in following years, noted the report's authors, from Florida Atlantic University medical school.

"Overall, from 2011 to 2013, we observed an increase in use for all these behaviors," they wrote, "a decline from 2015 to 2017 with a slight peak in 2019 (except for tried marijuana before age 13 which continued to decline), and a rapid decrease in 2021."

The declines came as marijuana use by adults climbed to "historic highs," according to a federally funded report published last year. That report found that teen use rates, however, were for the most part stable.

Despite the declines across races, genders and grade levels, authors highlighted trends they said raise concerns, such as the fact that more girls than boys reported consuming marijuana in 2021, reversing a longtime trend of boys reporting higher use rates.

"One of the most significant findings of this study is the shift in marijuana use trends by gender, with girls surpassing boys in reported marijuana use by 2021," the paper says, suggesting higher use among girls "could be attributed to evolving social dynamics, including more integrated friend groups where girls may experience greater exposure to marijuana offers from male peers."

But while girls' use rates were higher than boys' in 2021, that year also saw the lowest use rates among both genders of any year of the study period.

One persistent disparity the report highlights is consistently different use rates by students' race. While all groups saw declines from 2011 to 2021, racial differences were evident throughout.

"Concerning race/ethnicity, a decline in marijuana use was observed across all racial/ethnic groups between 2011 and 2021," authors observed. "In 2021, however, the proportion of Black adolescents reporting marijuana use was higher (20.5%) when compared to White (14.8%), Hispanic (16.7%), or Asian (5.1%) adolescents. This was also true for all of the other years except for 2019."

Perhaps unsurprisingly, the report also shows that marijuana use rates increase with grade level, suggesting more students are trying and using cannabis as they get older.

"While we found a net decline in the percentage of marijuana use among students between 2011 and 2021 for all grade levels," authors wrote, "there was consistently higher usage for older grades throughout all years, especially among 12th graders. The consistent observation that 12th graders have the highest rates of marijuana use across all survey years suggests that older adolescents may have greater access to marijuana, possibly due to more developed peer networks and increased independence."

Authors of the report were cautious to cheer the survey results, emphasizing that further public health measures are needed to mitigate the risks of underage drug use.

"While the overall decrease in US high school adolescent's marijuana usage from 2011 to 2021 is encouraging, it is crucial to sustain and build on these gains through continued public health efforts," the paper says. "Behavioral interventions that promote positive connections within school and family environments are essential in mitigating the risk of marijuana use."

Further, despite the paper's study period overlapping a time of numerous U.S. states legalizing marijuana for adults, authors warn that the end of prohibition could potentially increase teen use.

"As more states continue to legalize recreational marijuana, the accessibility and perceived normalcy of the drug may increase, particularly for adolescents who may view its legal status as an indication of safety or acceptability," authors wrote. "Marijuana legalization can influence adolescent behavior through reduced risk perception and increased availability, which may counteract efforts to reduce use in this vulnerable population."

That's counter to data from at least some jurisdictions showing that perceived ease of access has actually fallen among teens since legalization.

In Washington State, for example -- the second state in the country to launch an adult-use marijuana market -- data from a recent survey of adolescent and teenage students found that perceived ease of access to cannabis among underage students has generally dropped since the state enacted legalization for adults in 2012. That survey also showed overall declines in both lifetime and past-30-day marijuana use since legalization, with striking drops in recent years that held steady through 2023.

A separate 2018 survey, however, did suggest that fewer teens perceived occasional or frequent cannabis use to be harmful, even though underage use rates had not increased.

Data from CDC's 2023 Youth Risk Behavior Survey, meanwhile, was released earlier this year. The updated numbers show a continued decline in the proportion of high-school students reporting past-month marijuana use over the past decade.

Another federal report published this summer concluded that cannabis consumption among minors -- defined as people 12 to 20 years of age -- fell slightly between 2022 and 2023.

And despite methodological changes in recent years that make comparisons difficult over time, that report similarly suggested that youth use had fallen significantly in the past decade, as dozens of states have legalized marijuana for adult or medical use. The percentage of young people aged 12 to 17 who've ever tried marijuana dropped 18 percent from 2014, when the first legal recreational cannabis sales in the U.S. launched, to 2023. Past-year and past-month rates among young people also declined during that time period.

Separately, a research letter published by the Journal of the American Medical Association (JAMA) in April said there's no evidence that states' adoption of laws to legalize and regulate marijuana for adults have led to an increase in youth use of cannabis.

Another JAMA-published study earlier that month that similarly found that neither legalization nor the opening of retail stores led to increases in youth cannabis use.

A separate study late last year also found that Canadian high-school students reported it was more difficult to access marijuana since the government legalized the drug nationwide in 2019. The prevalence of current cannabis use also fell during the study period, from 12.7 percent in 2018-19 to 7.5 percent in 2020-21, even as retail sales of marijuana expanded across the country.

In December, meanwhile, a U.S. health official said that teen marijuana use has not increased "even as state legalization has proliferated across the country."

"There have been no substantial increases at all," said Marsha Lopez, chief of the National Institute on Drug Abuse's (NIDA) epidemiological research branch. "In fact, they have not reported an increase in perceived availability either, which is kind of interesting."

Another earlier analysis from CDC found that rates of current and lifetime cannabis use among high school students have continued to drop amid the legalization movement.

A study of high school students in Massachusetts that was published last November found that youth in that state were no more likely to use marijuana after legalization, though more students perceived their parents as cannabis consumers after the policy change.

A separate NIDA-funded study published in the American Journal of Preventive Medicine in 2022 also found that state-level cannabis legalization was not associated with increased youth use. The study demonstrated that "youth who spent more of their adolescence under legalization were no more or less likely to have used cannabis at age 15 years than adolescents who spent little or no time under legalization."

Yet another 2022 study from Michigan State University researchers, published in the journal PLOS One, found that "cannabis retail sales might be followed by the increased occurrence of cannabis onsets for older adults" in legal states, "but not for underage persons who cannot buy cannabis products in a retail outlet."

The trends were observed despite adult use of marijuana and certain psychedelics reaching "historic highs" in 2022, according to separate data released last year.