Nearly half of patients who go to urgent care clinics seeking treatment for a flu, cold or other conditions that don’t require antibiotics received a prescription for one anyway. That’s three times as often as antibiotics are prescribed to patients with the same illnesses in traditional doctors’ offices, according to a study published Monday.

Patients who get unnecessary antibiotics are at risk for severe side effects, even with just one dose of the medicine, doctors say. Inappropriate use of these lifesaving drugs also puts everyone else at risk because overuse accelerates the emergence of resistant bacteria, or “superbugs,” that can’t be stopped with drugs.

An analysis published in the journal JAMA Internal Medicine is the first study of antibiotic prescribing in the growing number of urgent care centers and retail health clinics, which together serve millions of patients at thousands of locations across the United States each year. Retail clinics are embedded in grocery stores, big-box stores and pharmacy chains. Urgent care clinics typically treat more pressing injuries or illnesses that don’t require an emergency room visit.

Antibiotic overuse is an enormous and growing problem around the world. If left unchecked, a United Kingdom report has forecast, antibiotic-resistant bacteria could result in 10 million deaths each year by 2050 – more than the number of people killed by cancer – at a cost of $100 trillion to the global economy.

In the United States, nearly one-third of antibiotics – or about 47 million prescriptions dispensed every year – in doctor’s offices, emergency rooms and hospital-based clinics are not needed and not effective, according to a 2016 study by the Centers for Disease Control and Prevention and the Pew Charitable Trust, the first to quantify the depth of the U.S. problem.

That 2016 study didn’t have information about a key health sector: urgent care centers and retail clinics, where a growing number of patients get their medical care.

CDC and Pew researchers tried to provide that information with the current study. They analyzed insurance claims from a 2014 database of more than 156 million patient visits to urgent care centers, retail clinics, hospital-based emergency departments and medical offices. The database only included patients with employer-sponsored insurance.

The data show that urgent care and retail clinics are “an underrecognized source of inappropriate antibiotic prescribing,” according to an accompanying commentary titled “Overprescribing in Urgent Care Clinics – The Fast and the Spurious.” The commentary was written by physicians Michael Incze, Rita Redberg and Mitchell Katz. Incze and Redberg are doctors at the University of California, San Francisco; Katz is chief executive of the New York City’s public health system. None were involved in the study.

Researchers focused on respiratory conditions that don’t respond to antibiotics, such as colds, bronchitis, asthma, allergies, influenza and viral pneumonia. Urgent care centers prescribed antibiotics in nearly 46 percent of visits for these conditions. That rate was nearly three times higher than the 17 percent prescribed for antibiotic-inappropriate diagnoses at traditional medical offices, and almost twice as high as the rate at emergency departments, the study found.

One surprise: retail clinics had the lowest rate for these antibiotic-inappropriate diagnoses, 14 percent. Researchers said the proper use of antibiotics has been a focus of large retail clinic chains and could account for the lower percentage.

The new information suggests that unnecessary antibiotic prescribing in the United States is greater than what researchers estimated two years ago.

“This will help us with an antibiotic stewardship strategy that’s not just focused on doctor’s offices and emergency departments,” said David Hyun, a senior officer with Pew’s antibiotic resistance project and one of the authors of the study.

The study was limited by its reliance on diagnosis codes and commercial claims data instead of actual medical records. It also does not include Medicare, Medicaid or uninsured patients, so it’s not considered a nationally representative sample.

Still, “it’s a real-world snapshot that tells us we have much work to do,” said Helen Boucher, an infectious disease physician at Tufts Medical Center in Boston who was not involved in the study. “Urgent care centers are growing by leaps and bounds in this era of consumerism and people wanting everything now.”

Antibiotic-resistant bacteria have evolved the ability to withstand drugs that ought to stop them. It’s part of the broader problem of antimicrobial resistance, which includes antibiotics as well as drugs to fight fungal, viral or parasitic infections. If these lifesaving medicines are rendered ineffective by multidrug-resistant superbugs, even the most minor infections would be untreatable, bringing back a level of danger not seen since the 19th century.

The rise of urgent care centers, an $18 billion industry with more than 8,000 locations, has been fueled by the convenience of evening and weekend hours, proximity and lower out-of-pocket costs than a visit to the emergency department. But the commentary authors said that convenience may be prompting “frequent visits for mild self-resolving illnesses that would be better treated with rest and symptom management at home.”

Antibiotic treatment rates may be higher for patients seen at urgent care clinics because unfamiliar clinicians (instead of a primary care physician) may be less able to convince patients that taking unnecessary antibiotics is a bad idea, the commentary authors wrote.

Some urgent care doctors say patients seek out urgent care clinics with the explicit expectation and sometime flat-out demand to be prescribed antibiotics.

Patients filed complaints if they didn’t get a prescription for a “Z-pack,” or azithromycin, a widely used antibiotic, said Glenn Harnett, a physician and former chief medical officer for a group of urgent care clinics that operates in 23 states.

Urgent care clinicians and owners worry about what they call the “Yelp Effect,” he said. If patients’ expectations aren’t met, they could post lower patient satisfaction scores and negative reviews on social media, which affects the clinics’ bottom line.

Katherine Fleming-Dutra, CDC’s deputy director of the antibiotic stewardship office and another study author, said patients can be satisfied if doctors communicate effectively why antibiotics aren’t needed and what patients can do to feel better. Like any medication, antibiotics can have serious side effects, killing good bacteria and allowing bad bacteria to grow, she said.

The Urgent Care Association is working with others in the industry, the CDC and George Washington University’s Antibiotic Resistance Action Center on ways to ensure proper antibiotic prescribing. The groups hosted a two-day summit that ended Monday in Atlanta.

A major challenge is managing patient expectation. If a patient was able to walk into a clinic and get antibiotics last year, “and now you’re telling them ‘no,’ it really creates a disconnect in the patient’s brain,” said Laura Rogers, deputy director of GW’s center.

Laurel Stoimenoff, chief executive of the Urgent Care Association, said the industry is committed to ensuring the appropriate prescribing of antibiotics and educating patients on the issue.

“Urgent care providers are very focused on antibiotic stewardship,” she said, because urgent care centers tend to treat a higher volume of patients for conditions that do result in antibiotics, such as acute sinus infections.