While ibuprofen and acetaminophen remain among the most commonly used drugs in the United States, new research published on Aug. 25 suggests that these over-the-counter medications may be fueling antibiotic resistance.

Published in the journal Nature, researchers from the University of South Australia investigated whether commonly used non-antibiotic medications, including ibuprofen, diclofenac, acetaminophen, furosemide, metformin, atorvastatin, tramadol, temazepam, and pseudoephedrine, could lead to antibiotic resistance in people.

The researchers studied what happens when bacteria are exposed to those non-antibiotic drugs in combination with ciprofloxacin, a powerful antibiotic used to treat a range of bacterial infections.

Findings showed that of those nine drugs, “ibuprofen and acetaminophen significantly increased mutation frequency and conferred high-level ciprofloxacin resistance,” the researchers said.

They noted that the resistance to antibiotics was more acute in individuals who combined ibuprofen, the active ingredient in Advil, and acetaminophen, commonly sold as Tylenol in the United States and known as paracetamol in other countries.

Researchers found that E. coli developed more mutations promoting antibiotic resistance—and did so more quickly—when exposed to ciprofloxacin, ibuprofen, and acetaminophen together, compared with exposure to ciprofloxacin alone, according to a university statement.

“Antibiotics have long been vital in treating infectious diseases, but their widespread overuse and misuse have driven a global rise in antibiotic-resistant bacteria,” Rietie Venter, an associate professor and lead researcher in the study, said in the statement.

“This is especially prevalent in residential aged care facilities, where older people are more likely to be prescribed multiple medications—not just antibiotics, but also drugs for pain, sleep, or blood pressure—making it an ideal breeding ground for gut bacteria to become resistant to antibiotics.”

Venter added that bacteria “were not only resistant to the antibiotic ciprofloxacin, but increased resistance was also observed to multiple other antibiotics from different classes.”

The study, he said, shows that officials need to consider the risks of using multiple medications simultaneously, particularly among older adults receiving long-term treatments.

The researchers also called for more studies into antibiotic resistance and non-antibiotic medications.

Antimicrobial resistance was directly responsible for 1.27 million deaths and contributed to 4.95 million deaths worldwide in 2019, according to the World Health Organization.

The U.N. health body said that the overuse of antibiotics in both animals and people has driven the development of bacterial and fungal strains that are resistant to the drugs.

Inside the United States, more than 2.8 million antimicrobial-resistant infections occur each year on average, according to the Centers for Disease Control and Prevention (CDC). More than 35,000 people with those infections die annually.

“Antimicrobial-resistant infections that require the use of second- and third-line treatments can harm patients by causing serious side effects, such as organ failure and [can] prolong care and recovery, sometimes for months,” the CDC states on its website.

CDC officials also say that to curb the proliferation of antimicrobial-resistant strains, people are advised to take antibiotics only when they are needed.