Marijuana is sending visitors to Colorado to hospital emergency rooms at an increasing rate, according to a new study. The same isn’t true of Colorado residents, however. The results suggest that marijuana dispensaries need to do a better job of educating people buying their product, said the study’s lead author.

“The thing we’d like to emphasize is that the Colorado Department of Public Health and Environment has done a good job of educating residents about the health effects of marijuana legalization and marijuana use,” said Dr. Howard Kim, of the Northwestern University Feinberg School of Medicine in Chicago. “I think reaching visitors is a much harder task,” said Dr. Kim, who worked on the study while at the University of Colorado School of Medicine in Aurora.

Alaska, Colorado, Oregon, Washington, and the District of Columbia have all legalized the recreational use of small amounts of marijuana, according to the National Conference of State Legislatures. Colorado legalized medical marijuana in 2000. The state then voted in 2012 to legalize recreational marijuana starting in 2014.

“We noticed an increasing number of patients coming to the ED who seemed to be from out of town and we decided to look at the data to see if that was the case,” said Dr. Kim.

For the study, they compared data on visits to their ED for causes related to use of various drugs by Colorado residents and people from out of state. Symptoms involved in the visits included cardiac, respiratory and gastric problems as well as “altered mental status.” They found that ED visits related to marijuana by people from out of state nearly doubled, from 85 per 100,000 visits in 2013 to 168 per 100,000 visits in 2014, which is the first year recreational marijuana was sold legally.

Meanwhile, ED visits for marijuana use remained stable for Colorado residents between 2013 and 2014 at about 110 per 100,000 visits, the researchers reported in The New England Journal of Medicine. Rates of marijuana-related ED visits were also consistent for the two groups between 2012 and 2013, which was before legalized recreational marijuana.

To confirm their findings on a larger scale, the researchers then looked at the same data from more than 100 hospitals across Colorado and got similar results. For example, ED visits related to marijuana use by people from out of state increased about 46 percent between 2013 and 2014. That compared to about a 17 percent increase among Colorado residents.