As marijuana legalization took hold in Colorado, the estimated percentage of regular cannabis users in the state jumped to the second-highest level in the country, according to new federal data.

When asked, roughly one out of every eight Colorado residents over the age of 12 reported using marijuana in the previous month. Only Rhode Island topped Colorado in the percentage of residents who reported using marijuana as frequently.

The results come from the National Survey on Drug Use and Health and represent the average of estimates gathered in 2012 and 2013.

The numbers are among the first measurements of marijuana use in Colorado to be released after it became legal in late 2012 for people over 21 to use and possess marijuana in the state. But because they do not include data from this year, the numbers aren’t able to answer the question experts have watched Colorado closely for: How will widespread commercial sales of marijuana impact use?

“This is not surprising, given what’s going on on the medical side,” Mark Kleiman, a University of California-Los Angeles professor who studies marijuana policy, said of the increase, referring to the uptick in medical marijuana patients in Colorado in the same period. “I don’t think this tells us about the long-term impacts of legalization.”

State-specific data from the survey are averaged over two-year periods to compensate for relatively small sample sizes.

For the 2011-12 period, 10.4 percent of Coloradans 12 and older reported using marijuana in the month prior to being surveyed. That placed Colorado seventh in the country for monthly marijuana use.

Monthly use in Colorado jumped to 12.7 percent — a 22 percent increase — in the 2012-13 data. The result means the survey estimates about 530,000 people in Colorado use marijuana at least once a month.

Nationally, monthly marijuana use by people 12 and older nudged upward by about 4 percent to 7.4 percent. In Washington state — which, like Colorado, in 2012 legalized marijuana use and limited possession for adults — monthly marijuana use rose by about 20 percent to 12.3 percent.

Kleiman said researchers will get a better idea about marijuana use in Colorado once they are able to zoom in on data showing how many people use marijuana daily. A study commissioned by Colorado’s Marijuana Enforcement Division this year found that people who use marijuana almost every day account for about 22 percent of cannabis users in Colorado but consume nearly 67 percent of the marijuana used. Other studies have warned about a possible uptick in heavy marijuana use.

“The fraction of people who are monthly users who are in fact daily users has gone way, way up,” Kleiman said.

Monthly marijuana use increased across all age groups in Colorado, according to the new survey numbers. The number of people who reported using marijuana in the past year also increased in Colorado in the 2012-13 data, but the state ranked only sixth nationally in the measurement. Measurements for alcohol consumption and illicit drug use increased, as well.

This month, a different federally funded survey found that teen marijuana use had not increased nationally, despite marijuana legalization. A Colorado survey released this year found no increase in marijuana use by high schoolers in 2013.

John Ingold: 303-954-1068, jingold@denverpost.com or twitter.com/johningold