Visitors to Colorado are turning up at the emergency room with marijuana-related issues in higher rates than people who live here, according to a study by the University of Colorado School of Medicine.

The study, which appears in the New England Journal of Medicine on Feb. 25, found that the number of marijuana-related emergency room visits to the University of Colorado Hospital doubled among those from out-of-state from 2013 to 2014, while remaining steady for residents.

The study can’t positively peg marijuana use as the cause of the visit, said Andrew Monte, assistant professor of emergency medicine and toxicology at the CU School of Medicine.

“Realistically, these visits could have marijuana mentioned at one point if they came and had a heart attack and said they did smoke a week ago, that would be reflected,” Monte said.

Visitors to University of Colorado Hospital from outside the state with marijuana complaints climbed from 85 per 10,000 visits in 2013, to 168 per 10,000 in 2014, the first year of retail marijuana sales in the state, the study found.

During the same period, visits by state residents didn’t change significantly, going from 106 per 10,000 visits in 2013, to 112 the following year.

The most common complaints bringing users to the emergency department were gastrointestinal, followed by psychiatric, and then cardio pulmonary problems. Monte said marijuana use can exacerbate an existing medical condition, and that explains many of the cases.

Mason Tvert, communications director for the Marijuana Policy Project, said there are questions that remain unanswered, one of which speaks to a trait unique to in-state users — altitude acclimation.

“The number one difference between someone visiting and using marijuana and someone who lives here and using marijuana is that the person visiting has just gone to a much higher altitude and we know that’s attributed to a bunch of symptoms like passing out and nausea.”

While some of those who needed emergency care after taking edibles or smoking weed came to Colorado specifically to get high legally, others came for business or other reasons.

“A lot of people do different things when they’re home versus when they’re on vacation,” said Mike Van Dyke with the Colorado Department of Public Health and Environment. . “Some people tend to do it overboard more when they’re on vacation, and that could also be playing a part.”

The study also analyzed numbers from the Colorado Hospital Association for visits to emergency rooms statewide. Those numbers showed a similar growth rate among people not from Colorado. Out-of-staters with marijuana complaints went from 78 per 10,000 visits in 2012 to 112 per 10,000 visits in 2013 to 163 per 10,000 visits in 2014.

Colorado residents had numbers of 70 to 86 to 101, respectively.

“We are pleased that these data have proven useful in identifying the need for further education around the effects of marijuana use,” Steven Summer, president and CEO of the hospital association, said in a statemet.

Van Dyke also said from a statewide perspective, it’s important to look at the size of the out-of-state pool compared to the in-state.

“When you’re looking at these numbers you see a rate — an increased rate in out-of-state people compared to an increased rate in in-state people,” Van Dyke said. “The out-of-state denominator is fairly small, so a small increase in the numerator is going to impact that rate a lot more, and that’s part of what we’re seeing.”

The study underscores the importance of providing good information to those buying pot at dispensaries, Monte said.

Users should be told that marijuana in the state has higher concentrations of THC “than many people are used to,” Monte said.

The warning is particularly critical with edibles, which can take two to three hours to produce noticeable effects, and can stay in the system longer than marijuana that is smoked.

People have to “understand, respect and acknowledge that this drug can exacerbate medical conditions,” Monte said.

Tom McGhee: 303-954-1671, tmcghee@denverpost.com or @dpmcghee

Updated Feb. 25 at 10:00 a.m. This article has been revised to reflect the following correction. Originally, due to a reporter’s error, Mason Tvert’s association was incorrect. He is communications director for the Marijuana Policy Project.