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More than 4 out of 10 senior citizens take five or more prescription medications — a phenomenon known as polypharmacy, research shows.

This practice increases the risk of addiction and medication errors, like taking the wrong dosage or skipping doses altogether. Potential consequences include hospitalization, mental impairment and painful falls.

A new report warns of another possible result — bacterial infections that are more difficult to treat.

A team from the University of South Australia tested nine medications common in nursing homes, finding that two common painkillers help bacteria become more resistant to antibiotics — especially when taken together.

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

“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.”

Antibiotic-resistant infections contribute to an estimated 4.95 million deaths worldwide each year.

Venter’s team assessed the over-the-counter painkillers ibuprofen and acetaminophen, the anti-inflammatory diclofenac, the diuretic furosemide, the diabetes drug metformin, the statin atorvastatin, the opioid tramadol, the sedative temazepam and the decongestant pseudoephedrine.

The researchers explored what happens when bacteria are exposed to these non-antibiotics and the antibiotic ciprofloxacin — a powerful drug used to treat bacterial infections in the respiratory, urinary and gastrointestinal tracts as well as other areas of the body.

They found that the E. coli developed more genetic mutations that promote faster growth and antibiotic resistance when exposed to ciprofloxacin, ibuprofen and acetaminophen than when exposed to just ciprofloxacin.

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

“We also uncovered the genetic mechanisms behind this resistance,” she added, “with ibuprofen and [acetaminophen] both activating the bacteria’s defenses to expel antibiotics and render them less effective.”

Ibuprofen (brand names: Advil and Motrin) and acetaminophen (Tylenol) are extremely popular because they are designed to reduce fever, aches and minor pains.

Venter is not discouraging their use but is recommending consumers “be more mindful about how they interact with antibiotics.”

“Antibiotic resistance isn’t just about antibiotics anymore,” she said.

“This study is a clear reminder that we need to carefully consider the risks of using multiple medications — particularly in aged care, where residents are often prescribed a mix of long-term treatments.”

The findings were published this week in the journal Antimicrobials and Resistance.

The researchers are calling for additional studies on how drug interactions can reduce antibiotic effectiveness.