Every year, millions of Americans with common infections leave the doctor’s office with a prescription for antibiotics.

Yet, only half of all patients seeking care for three of the most common medical ailments receive the right kind of drug for their condition, according to a new study from the Centers for Disease Control and Prevention and the Pew Charitable Trusts.

The study showed that an alarming number of doctors around the country are failing to follow national recommendations when prescribing antibiotics for the three ailments: sinus infections, sore throats and ear infections.

The findings come less than six months after research from the CDC and Pew confirmed that one in three antibiotic prescriptions are unnecessary. Taken together, the two studies suggest that doctors nationwide are inadvertently contributing to the rise of potentially deadly drug-resistant “superbugs.” These bacteria now infect roughly 2 million people per year in the United States, and cause an estimated 23,000 deaths.

The challenge posed by many common antibiotics is that the more that modern medicine makes use of them, the easier it is for germs to become resistant to them.

But Monday’s findings from Pew and the CDC highlights how antibiotic resistance is not only fueled by the number of prescriptions being written by doctors, but also what kinds of medications they are prescribing.

According to the study, only 52 percent of patients were prescribed the recommended “first-line” antibiotics that target fewer types of bacteria. Others were given drugs that target a broader spectrum of bacteria and are more likely to lead to drug resistance, according to David Hyun, an infectious diseases doctor from the Pew antibiotic resistance project and a co-author of the report.

Experts say that the proportion of Americans taking “first-line” drugs should be higher, at 80 percent — a figure that takes into account people who are allergic to certain drugs and those who fail to respond to initial treatment.

“We’re going to potentially lose the effectiveness of these life-saving drugs and may face a situation where there’s a patient with an infection caused by a bacteria that is resistant to all the antibiotics that are currently available,” Hyun said.

The study also found that adults were much more likely than children to receive an inappropriate, broad-spectrum antibiotics for a simple infection. More than 60 percent of adults diagnosed with strep throat, for example, were prescribed an antibiotic that is not recommended by medical guidelines. By comparison, 40 percent of children faced the same predicament.

Both CDC studies relied on national data sampled from doctor’s offices, hospital outpatient facilities and emergency departments between 2010-2011.

This study did not examine why doctors are ignoring guidelines set by organizations like the Infectious Diseases Society of America and the American Academy of Pediatrics. However, Hyun said that doctors are more likely to prescribe a broad spectrum antibiotic when they are unsure what illness their patient is facing. An antibiotic that targets more types of bacteria can be perceived to be “the safer, more conservative route,” said Hyun, even though it is at odds with what is best for public health.

With global health experts warning that antibiotic-resistant infections could one day kill more people than cancer, the White House in 2015 set an ambitious goal to reduce the unnecessary use of antibiotics by half. To reach that milestone, the CDC and Pew estimate that outpatient prescriptions would need to come down by about 23 million per year between now and 2020. Prescriptions for sinus infections, ear infections and sore throats, the authors note, account for roughly 44 million prescriptions annually.

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