Sleeping pills may be disturbing river ecosystems by turning fish into greedy, risk-taking loners, say researchers.
Scientists studied the behaviour of perch exposed to a sedative which is carried into waterways through sewage.
The drug, a benzodiazepine used to treat anxiety and insomnia called oxazepam, made the fish bolder and less social. They left the safety of their schools to forage alone - making them vulnerable to predators - and also ate more quickly than normal.
"Normally perch are shy and hunt in schools," said study leader Dr Tomas Brodin, from Umea University in Sweden. "This is a known strategy for survival and growth. But those who swim in oxazepam became considerably bolder.
"We're now going to examine what consequences this might have."
Fish consuming food at an accelerated rate could have unexpected effects on the aquatic ecosystem, he said. For instance, by altering the balance of species it may increase the risk of algal blooming.
Perch prey on small fish and other creatures that normally keep the algae in check.
The scientists conducted tests using drug concentrations matching those found in waters in densely populated areas of Sweden.
Their findings, published in the journal Science, were presented today at the American Association for the Advancement of Science's annual meeting in Boston, US. Many similar drugs are found downstream from sewage treatment plants, said the researchers.
Environmental chemist Dr Jerker Fick, also from Umea University, said: "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."