For more than a decade, scientists have known that pharmaceutical medications are ending up in lakes and rivers all over the planet. The drugs pass through our bodies and are excreted in our waste. Even after being treated at sewage plants, the drugs can remain in the water that is dumped back into the environment.

A few earlier studies have raised concerns about birth-control hormones disrupting the delicate balance of aquatic wildlife. But little is known about the vast number of other medications that are accumulating in surface water.

Now, researchers in Sweden have demonstrated the effects on fish of another commonly used type of pharmaceutical: psychiatric medications.

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In a laboratory, wild European perch were exposed to Oxazepam at the same level as the anti-anxiety drug exists in streams and rivers around populated areas of Sweden.

The drug dramatically changed the behaviour of the fish, according to the findings presented on Thursday in Boston at the annual meeting of the American Association for the Advancement of Science and also published simultaneously in the journal Science.

"Normally, perch are shy and hunt in schools. This is a known strategy for survival and growth. But those who who swim in Oxazepam become considerably bolder," Tomas Brodin, the lead researcher, said in a statement released along with the study.

The drug essentially made the fish braver and less social – even anti-social. "Perch that were exposed to Oxazepam lost interest in hanging out with the group, and some even strayed as far away from group as possible," Brodin said.

The new-found independence allowed the perch to look for food on their own, a behaviour that can be risky, explained Brodin, an ecologist at Uppsala University. A fish that strays far from the group is more likely to be gobbled up by a predator.

Perch that were exposed to the drug also ate more quickly than the other fish.

"It might have grave consequences on the ecological structure of aquatic communities that the drug enters," Brodin told a media briefing on the Internet.

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The research team noted that it is hard to predict how these various effects will play out in the natural environment where all types of wildlife are swimming in a virtual drug cocktail.

"We expect it to affect all fish … [but] it might not affect all fish exactly the same way," he said. "We think this drug is working through stress release on the fish." For example, it could remove their fear of being eaten. "And if you have a lot of stress, this drug might affect you a lot."

On the other hand, the medication may have less influence on the top food-chain predators that are under considerable less stress than their small prey.

The researchers pointed out that this form of ecological disruption is not unique to Sweden. "It is a global issue," Brodin said. "We find these [drug] concentrations, or close to them, all over the world and it is quite possible, and even probable, that these effects are happening as we speak."

His follow researcher, environmental chemist Jerker Fick, said steps must be taken to keep pharmaceutical contaminants out of natural waterways. "The solution to the problem is not to stop medicating ill people, but to try to develop sewage-treatment plants that can capture environmentally hazardous drugs," he said. Fick told the media conference he is aware of only one treatment facility – a plant in Switzerland – that has this capacity.