Before antibiotics became widely available in the first half of the 20th century, we humans were vulnerable to all kinds of ailments.

An infected tooth could take you out. So could minor problems during childbirth. Even a bad case of diarrhea could be fatal, especially in children.

But as the 1980s power-ballad classic goes, “Every rose has its thorn.”

So it is with antibiotics. The miracle medicine that extended human lifespans by decades now often pollutes our waterways.

A new study from Canada’s McGill University finds that more than 9,300 tons of antibiotics — almost one-third of the antibiotics we consume — end up in rivers around the world, even after going through wastewater treatment systems.

Amoxycillin [uh-mock-suh-sill-in], the world’s most-used antibiotic, is the likeliest to be found at higher levels. That’s especially true in Southeast Asia, thanks to increasing use and less-efficient wastewater systems.

The study looked at 40 of the most used antibiotics. It involved sampling of some 900 waterways and a contaminant modeling system.

The study only evaluated human antibiotic use and did not include antibiotics used for lifestock or pharmaceutical factories, both of which are big contributors to environmental contamination.

The researchers said their findings should not be taken to mean that antibiotics are bad. Global health depends on them.

But strategies are needed to avoid unintended, detrimental effects on aquatic life, especially in those areas most at risk.

Living longer is a blessing, but we’ll need the kind of brainpower and innovation it took to make penicillin viable if we hope to keep our water sources clean.