"Normally, perch are shy and hunt in schools, but those who swim in oxazepam became considerably bolder, " said ecologist Tomas Brodin of Sweden's Umea University.
Marketed under a variety of trade names, oxazepam belongs to a class of compounds widely used since the 1960s to treat panic attacks, anxiety and insomnia.
Laboratory tests with wild perch show that a widely prescribed medication called oxazepam makes the fish more mellow but also more antisocial and vulnerable to predators, the researchers said.