The World Health Organization earlier this year called antimicrobial resistance—pathogens’ ability to evade medical interventions—one of the 10 largest threats to global health. In the U.S. alone, according to a recent Centers for Disease Control and Prevention (CDC) report, 35,000 people die each year due to antibiotic-resistant infections.

A new study published in the BMJ points to one major propagator of the problem: doctors are prescribing antibiotics when they shouldn’t. In fact, up to 43% of U.S. antibiotic prescriptions may be “inappropriate,” according to the research.

Antibiotics either kill bacteria or keep them from multiplying. By definition, they work only against bacterial illnesses—and yet, research shows they’re often needlessly prescribed for viral illnesses like the flu and common colds. That’s not only a waste of resources; it may also contribute to antibiotic resistance, since bacteria get better and better at evading drugs each time they encounter them.

The authors of the BMJ paper analyzed data from the National Ambulatory Medical Care Survey, which asks a representative sample of U.S. physicians to describe patients’ office visits during a specified period of time. The authors examined data from more than 28,000 medical visits that took place in 2015, which can be statistically extrapolated to represent almost 991 million visits nationwide.

Antibiotics were prescribed during 13.2% of these visits. Based on the medical office reports, the researchers concluded that 57% of those prescriptions were appropriately written for treating conditions known to respond well to antibiotics. A quarter were written for explicitly inappropriate reasons, such as for “treating” viral illnesses, with upper respiratory tract infections being among the most common. The final 18% were not documented well enough to assess the prescription’s validity—itself a problem that needs fixing, and one that makes it difficult for researchers to accurately analyze how and how often improper prescriptions occur.

Specialists were more likely than primary care physicians to write an inappropriate prescription, the researchers found.

The health community is already seeing the consequences of these improper prescriptions. In addition to the deaths detailed in the CDC’s recent report, an estimated 2.8 million Americans contract and survive antibiotic-resistant infections each year, the agency says. As illnesses get more and more difficult to treat, patients may suffer longer, and doctors might be forced to turn to increasingly powerful drugs, which can come with harmful side effects.

Doctors all over the world are rushing to keep up with infections that are getting increasingly good at resisting antibiotic treatment, such as gonorrhea, tuberculosis, and bacterial pneumonia.

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Write to Jamie Ducharme at jamie.ducharme@time.com.