American teenagers are not using marijuana in greater numbers even as states have legalized the drug for adult recreational use, a third national survey has found.

In fact, past-month marijuana use is on a two-decade slide among high school students, according to a statistical analysis of results from the Centers for Disease Control and Prevention's 2015 Youth Risk Behavior Survey, published Thursday.

The survey is conducted every other year among a representative sample of U.S. high school students. The 2015 results indicate a statistically significant downward trend in past-month marijuana use since 1995 and a downward trend in lifetime use since 1997.

Despite increasingly liberal state laws and public attitudes, students' reported lifetime pot use fell more than 2 percentage points to 38.6 percent in 2015. Past-month use slipped more slightly to 21.7 percent, though neither change is itself statistically significant.

The results mirror the 2015 Monitoring the Future survey, whose eighth-, 10th- and 12th-grade respondents reported a collective non-statistically significant drop in past-month use, from 14.4 percent in 2014 to 14 percent, even as they viewed pot use as less risky.

A third federally funded survey, the National Survey on Drug Use and Health, found in 2014 that reported past-month pot use among people ages 12 to 17 ticked up from 7.1 percent in 2013 to 7.4 percent, a non-statistically significant increase and still below the 2011 rate of 7.9 percent. The 2015 results of that survey have not been released.

The latest results from the CDC survey also come after a study published in The Lancet Psychiatry, a medical journal, reviewed 24 years of data and concluded last year states legalizing marijuana for medical purposes had not led to increased teen use of the drug.

The findings are good news for legalization advocates as five states prepare for November votes on legalizing recreational marijuana, a policy adopted already by four states and the nation's capital. And at the polls, Florida residents may enact a relaxed medical marijuana law, making their state the 26th, along with the District of Columbia, to adopt such a policy.

Pot possession for any reason outside limited research remains a federal crime, but the Obama administration has allowed states broad leeway to establish regulated markets. With majority support in polls, reformers aim ultimately to repeal federal prohibition.

National teen marijuana use has not increased as states legalize pot for recreational use. Ethan Rosenberg for USN&WR

The static or downward-trending marijuana use figures among teens come after Colorado and Washington voters in 2012 legalized marijuana under local law, followed by the opening of shops in 2014. Residents of Oregon, Alaska and the nation's capital also voted to legalize marijuana in 2014, though only Oregon had regulated sales last year.

Dr. Stephanie Zaza, director of the CDC's Division of Adolescent and School Health, says she's relieved that teen marijuana use has not increased, but notes that aside from alcohol, cannabis remains the most common drug used illegally by high school students.

"We're always happy to see it coming down but while the prevalence rate is this high we can't get complacent," Zaza says. "Even if the [state-level legalization] policies are for adult use, we know there will be changes in access and use for young people."

Dr. Laura Kann, chief of the CDC's School-Based Surveillance Branch, says trends seen in the Youth Risk Behavior Survey generally are seen in the Monitoring the Future and National Survey on Drug Use and Health results too, though she notes that as with any poll, "these are estimates, they are not absolute truths."

Marijuana Policy Project spokesman Mason Tvert, a leader of Colorado's legalization campaign, says the Youth Risk Behavior Survey results "once again dispel the myth that more teens will use marijuana if we roll back our failed marijuana prohibition policies."

"From Alaska to Illinois to Maryland, states are rethinking and reforming their marijuana laws [and] in every one of those states, we've heard opponents argue that it will result in increased teen use," he says. "Voters and legislators who are considering proposals to repeal marijuana prohibition should be wary of opponents who claim it will result in more use by teens. The evidence suggests otherwise."

The CDC report has frustrating limitations for those who would like to analyze state-specifc effects of legalization, however.

Three states, including Oregon and Washington, don't participate in a separate state-level survey included in the Youth Risk Behavior Survey report that asks about drug use. And Colorado was among a handful of states that in both 2013 and 2015 had a response rate too low to be considered representative by the CDC.

Kevin Sabet, a former presidential drug policy adviser who now leads the anti-marijuana legalization group Smart Approaches to Marijuana, says "the real finding from this survey, in terms of marijuana legalization, is that there is no finding at all."

"It is not surprising that marijuana use is flat among a national sample, since 46 out of the 50 states have kept marijuana illegal," Sabet says.

Some state-specific data does exist for the pioneering states, but the information is getting stale.

The 2015 YRBS report featured a national survey and supplemental surveys conducted in many states. Ethan Rosenberg for USN&WR; Source: CDC Youth Risk Behavior Survey

In Colorado, a state government survey found that from 2011 to 2013 – a period including when penalties were dropped but before stores opened – pot use among high school students dropped by a non-statistically significant amount, continuing a slide from 2009.