High school students did not use marijuana at higher rates in Colorado as the state's pioneering market for recreational marijuana opened for adults.

The school survey results, published Monday, may dampen concern about the possible effects of marijuana legalization, and show that reported use among Colorado high school students remains slightly below the national average.

The Healthy Kids Colorado Survey -- conducted every two years -- found 21.2 percent of high school students said they had used pot in the past month, a non-statistically significant increase from 2013. Reported lifetime use was 38 percent.

In 2013, the first full year state law allowed adults 21 and older to use pot recreationally, survey data found non-statistically significant drops in high school use. The 2015 data reflects teen use after retail stores opened in 2014.

Despite the opening of stores, the reported use rates are below 2009 and 2011 rates, and below the national averages in the 2015 Youth Risk Behavior Survey conducted by the Centers for Disease Control and Prevention.

Colorado High School Pot Use (HKCS) 2009 2011 2013 2015 Past Month 24.8 percent 22 percent 19.7 percent 21.2 percent Ever 42.6 percent 39.5 percent 36.9 percent 38 percent

National High School Pot Use (YRBS) 2009 2011 2013 2015 Past Month 20.8 percent 23.1 percent 23.4 percent 21.7 percent Ever 36.8 percent 39.9 percent 40.7 percent 38.6 percent

The YRBS and two other federally funded surveys have found that U.S. teen marijuana use is static amid a surge in support for legalization and increasingly liberal state laws allowing for medical marijuana or adult use of recreational marijuana.

The unchanged teen use in Colorado -- often discussed as a pot policy guinea pig -- may affect politicians and voters mulling reform elsewhere. Voters in five states may legalize marijuana in November, and Florida voters will consider a lax medical pot measure.

"One of the concerns many people have about making marijuana legal for adults is it could fall into the hands of teens, and these statistics show teens will not necessarily use more marijuana just because adults are no longer being punished for it," says Mason Tvert, a spokesman for the Marijuana Policy Project.

Tvert points out that the HKCS found drops in use among middle school students too, with reported lifetime use dropping from 8.8 percent to 7.6 percent.

But legalization opponents aren't ready to throw in the towel.

Smart Approaches to Marijuana, a group that opposes legal access to recreational marijuana, pointed in a press release to increases in reported pot use among high school juniors and seniors, under the subject line "New Data Shows Colorado Youth Marijuana Use on the Rise Since Legalization."

The release said: "This rise is a result of particularly pronounced increases among juniors and seniors, whose last-month pot use rose from 22.1 to 26.3 percent (juniors) and from 24.3 to 27.8 percent (seniors)."

But Leo Kattari, the survey's coordinator at the Colorado Department of Public Health and Environment, says some changes appear significant when they are not.

The survey is a poll of students, and therefore cannot capture the precise use rates. So researchers establish margins of error within which changes are not considered statistically significant.

Kattari says changes in past-month and lifetime pot usage was within the margin of error for middle school students and for each high school class, with one exception.

The past-month use rate for 11th grade was .2 percentage points above the confidence interval, Kattari says. Lifetime use was within the margin of error, he says, and he resists a conclusive statistical conclusion.

"Further analysis would be needed to see if it's actually statistically significant or not," he says about the 11th grade change.

Kattari says one of the few changes that clearly was statistically significant was a "quite large" dip in the perception of marijuana's potential harm, despite the statistically static use rates -- something also seen in recent national survey data, perhaps indicating a break in the historical relationship between youth pot use and perception of harm.

The state HKCS provides the closest tracking of Colorado high school pot use, as Colorado in 2015 and 2013 had an insufficient response rate for a state-level YRBS survey. (Oregon and Washington, the only other states with regulated recreational pot sales last year did not participate in that survey.)

Of states that did participate in the YRBS supplemental survey, nine had higher past-month high school use rates than Colorado -- Arizona, California, Delaware, Massachusetts, New Hampshire, New Mexico, North Carolina, Rhode Island and Vermont.

More data from the 2015 Healthy Kids Colorado Survey:



Colorado Department of Public Health and Environment