Scientists have long been worried about the buildup of antibiotics in the environment.

But in a recent study I led, we wanted to know what happens when bacteria are exposed not just to antibiotics, but to antibiotics and another type of medicine – together, at the low concentrations now typically found in nature.

Up to 90% of the medicines we take pass straight through our bodies, and most are not removed by wastewater treatment plants. These drug residues end up in rivers, lakes and other freshwater systems. In fact, traces of medicines have now been detected on every continent, at concentrations that vary from........

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