What happens to fish that swim in waters tainted by traces of drugs that people take? When it s an anti-anxiety drug, they become hyper, anti-social and aggressive, a Swedish study found.

It may sound funny, but it could threaten the fish population and upset the delicate dynamics of the marine environment, scientists say.
The findings, published online Thursday in the journal Science, add to the mounting evidence that minuscule amounts of medicines in rivers and streams can alter the biology and behavior of fish and other marine animals.

"I think people are starting to understand that pharmaceuticals are environmental contaminants," said Dana Kolpin, a researcher for the U.S. Geological Survey who is familiar with the study.Calling their results alarming, the researchers who did the study suspect the little drugged fish could become easier targets for bigger fish because they are more likely to venture alone into unfamiliar places.

"We know that in a predator-prey relation, increased boldness and activity combined with decreased sociality ... means you re going to be somebody s lunch quite soon," said Gregory Moller, a toxicologist at the University of Idaho and Washington State University. "It removes the natural balance."

Researchers around the world have been taking a close look at the effects of pharmaceuticals in extremely low concentrations, measured in parts per billion. Such drugs have turned up in waterways in Europe, the U.S. and elsewhere over the past decade.

They come mostly from humans and farm animals; the drugs pass through their bodies in unmetabolized form. These drug traces are then piped to water treatment plants, which are not designed to remove them from the cleaned water that flows back into streams and rivers.