Popular urgent care centers may be the biggest—and most overlooked—culprits in the dangerous overuse of antibiotics in clinics, according to a new analysis in JAMA Internal Medicine.

Based on insurance claims from patients with employee-sponsored coverage, researchers estimated that about 46 percent of patients who visited urgent care centers in 2014 for conditions that cannot be treated with antibiotics—such as a common cold that’s caused by a virus—left with useless antibiotic prescriptions that target bacterial infections. That rate of inappropriate antibiotic use is almost double the rate the researchers saw in emergency departments (25 percent) and almost triple the rate seen in traditional medical offices (17 percent).

The authors of the analysis—a team of researchers from the Centers for Disease Control and Prevention, the University of Utah, and the Pew Charitable Trusts—concluded that interventions for urgent care centers are “urgently needed.”

The data is concerning, the team notes, because such misuse of antibiotics can fuel the development and spread of drug-resistant bacteria, which can go on to become resistant to multiple types of antibiotic drugs and cause intractable, sometimes deadly, infections.

As antibiotics have become less effective at knocking back infections in recent decades, public health experts worldwide have amped up calls for more judicious use of the critical drugs. In 2016, researchers analyzing data from 2010-2011 estimated that 30 percent of antibiotics prescriptions given in US emergency rooms and traditional medical offices were inappropriate. But that estimate didn’t account for prescriptions doled out at urgent care and retail clinics, which are growing in popularity and now prescribe about 40 percent of all antibiotics used outside of hospitals. In other words, the true rate of antibiotic overuse may be even worse than experts thought.

Moreover, some doctors fear that overuse in urgent care centers will be tricky to fix. In an accompanying commentary, doctors and editorial staff from JAMA Internal Medicine, argue that judicious use of antibiotics doesn’t seem to fit into the business model of urgent care centers. These clinics thrive on providing same-day appointments in convenient locations with low out-of-pocket costs for ailments that might not warrant a trip to the emergency room or standard doctor’s office. This model may select for patients seeking an easy answer—like an antibiotic prescription—for a relatively mild illness that would otherwise resolve on its own, the doctors suggest. And doctors at urgent care centers may not have the necessary relationships with patients to talk them out of wanting an antibiotic. But they do have a financial incentive to keep them happy and returning.

This scenario is particularly concerning given the boom in urgent care centers in recent years. They now make up a $15 billion sector with more than 10,000 high-volume clinics around the country.

JAMA Internal Medicine, 2018. DOI: 10.1001/jamainternmed.2018.1632 (About DOIs).