Scientists at Umeå University in northern Sweden examining perch exposed to the sedative Oxazepam, which like many drugs passes through the human body, found that it made them more likely to leave their school and strike out on their own.
"Normally, perch are shy and hunt in schools. This is a known strategy for survival and growth," said ecologist Tomas Brodin, lead author of the article, which will be published in Friday's edition of Science.
"But those who swim in Oxazepam became considerably bolder," he said.
Going rogue means the fish were at greater risk of being eaten by predators, he explained.
Brodin and other researchers tested the fish by exposing them to drug concentrations corresponding to those found in wastewater in densely populated areas of Sweden.
In addition to growing bolder, the fish also ate more quickly, which the researchers fear could disrupt the ecological balance.
"In waters where fish begin to eat more efficiently, this can affect the composition of species, for example, and ultimately lead to unexpected effects, such as increased risk of algal blooming," Brodin said.
With the use of such drugs on the rise, in Sweden and elsewhere, the researchers said the changes in the fish could be a global phenomenon, adding that more research is necessary before broad-based conclusions can be drawn.
"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," said environmental chemist Jerker Fick, in a statement released ahead of the article's publication.
The scientists were to present their findings at the annual American Association for the Advancement of Science (AAAS) meeting in Boston on Thursday.