Mixtures of commonly used medications that end up in waterways and the natural environment could be promoting the development of antibiotic-resistant bacteria, according to a new study.

As much as 90 per cent of the medications humans and animals use end up passing through the body and into the natural environment via wastewater or runoff from fields, researchers behind the study say.

“We’ve found that the kind of common medications that many people might routinely take can form a pharmaceutical cocktail in the environment and our waterways that promotes antibiotic resistance,” lead author and University of Exeter microbiologist April Hayes said in a news release.

“This poses a potential threat to human health, because if we then ingest these bacteria and are infected, we may not be able to easily treat them, as antibiotics are more likely to fail.”

The study was published Tuesday in the International Society for Microbial Ecology’s journal ISME Communications.

While the buildup of antibiotics in the environment has already been shown to increase drug resistance in bacteria, researchers wanted to see what role other medications can play. By combining drugs used for pain relief, diabetes treatment and hormone replacement with the common antibiotic ciprofloxacin, lab tests on wastewater bacteria found that the mixture increased the development of antibiotic resistance genes more than just exposure to ciprofloxacin alone.

“Antimicrobial resistance is a growing global problem,” Hayes added. “And these mixtures might form an important contribution to that problem.”

The World Health Organization recently warned that in 2023, one in six bacterial infections worldwide showed resistance to antibiotics – a 40 per cent increase over 2018. Such resistance is compromising the effectiveness of common antibiotic treatments and transforming typically minor infections into potentially deadly illnesses.

“These findings are deeply concerning,” Yvan J-F. Hutin, who leads the WHO’s antimicrobial resistance department, told reporters on Monday. “As antibiotic resistance continues to rise, we’re running out of treatment options and we’re putting lives at risk.”