Legal marijuana shops are linked to higher levels of property crime in nearby areas, according to a nearly three-year study in Denver.

Researchers found that crime isn't higher in the area immediately surrounding marijuana outlets. But adjacent areas saw about 84 more property crimes per year than neighborhoods without a nearby marijuana store.

In Denver, no significant increase in violent crime was seen as a result of marijuana sales.

The results show that legal marijuana sales come with a cost, said Bridget Freisthler, lead author of the study and professor of social work at The Ohio State University.

"If you're looking strictly from a public health standpoint, there is reason to be somewhat concerned about having a marijuana outlet near your home," Freisthler said.

Putting this risk in context, marijuana outlets led to similar levels of property crime as bars, liquor stores and restaurants that serve alcohol, data from the study suggests. And businesses that sold alcohol led to much more violent crime than marijuana outlets.

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The study was published online today in the Journal of Primary Prevention.

The researchers examined crime statistics for 481 Census block groups in Denver over 34 months (January 2013 to October 2015). When the study began, marijuana could only be sold for medical purposes. But beginning in January 2014, marijuana outlets were able to sell to the general public, giving the researchers the opportunity to see if recreational sales were tied to increases in crime.

They examined three types of crime, based on data from the Denver Police Department: Violent crime, property crime and marijuana outlet specific crime.

The change in the law allowing recreational sales did not result in an increase in crime, results showed.

"It is the number and density of outlets that is important, not whether they are medical or recreational," Freisthler said.

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But there is a caveat to that finding. After the law was first changed to allow recreational sales, only those dispensaries that already were selling for medical purposes were allowed to apply for a license to sell recreational marijuana in Colorado.

As a result, the number of outlets didn't change much.

This is the second study Freisthler and colleagues have published on crime and marijuana outlets. The earlier study, published last year, was done in Long Beach, Calif. In both studies, property crimes didn't increase right next to the outlet, but in the adjacent neighborhood.

That's probably because the dispensaries often have security guards and cameras keeping an eye out on the immediate area, Freisthler said.

"The areas we examined in our study were relatively small (about a third of a square mile), so a guard could conceivably be keeping criminals away from the neighborhood directly surrounding the outlets," she said.

One way to understand the effect of marijuana stores on crime is to compare them to places that sell alcohol.

Data from the study showed that marijuana outlets contributed to 1,579 property crimes in Denver over 34 months, compared to the combined alcohol outlet contribution of 1,521.

"The levels of property crimes were similar, although marijuana outlets were responsible for slightly more," Freisthler said.

Alcohol outlets, however, were responsible for about four times more violent crimes during the 34 months of the study than those that sold marijuana (372 vs. 93).

But Freisthler cautioned that a direct comparison is difficult because the effects related to marijuana outlets take into account crimes in local and adjacent areas while the data for alcohol outlets only look at adjacent areas.

Still, she said it is concerning that there is this level of crime associated with marijuana sales, despite the fact that the density of marijuana outlets is much lower compared to that of alcohol outlets. "Over time, as marijuana grows in popularity, densities of marijuana outlets may increase, resulting in higher crime," Freisthler said.

While this study did not find a significant increase in violent crime related to marijuana shops, the study in Long Beach did. That suggests it is too early to say that legal marijuana sales don't result in significantly more violent crime, she said.

This new study did find, not surprisingly, that legal sales were linked to an increase in burglaries and other crimes at marijuana outlets themselves.

From the data in the study, the researchers can't tell who is committing the crimes and who the victims are in the nearby neighborhoods. Customers of the marijuana outlets could be the victims or the perpetrators, according to Freisthler.

"That's important to know, because residents may want to mobilize if they are the victims of increased crime. But if it is not the residents being victimized, they may not care as much," she said.

Freisthler said the findings of this, as well as her previous study, suggest there are reasons for citizens to be cautious about legal marijuana sales.

"There are definitely negative public health consequences, including increased crime," she said. "There may be economic benefits in terms of more tax revenue and money spent in neighborhoods. Citizens have to decide how they want to measure the benefits and costs."