Anxiety-reducing drug is extending lifespan of certain fish populations, a new study has found. Such drastic changes in fish mortality rates can have a cascading effect on other species.

The drug Oxazepam is used to treat anxiety and insomnia in adults. The medication finds its way into rivers and other water bodies via wastewater discharge. Previous research has shown that the drug makes fish aggressive and even alters their reproduction.

Standard tests that are used to detect water quality check for only toxic chemicals and their potential harms. The current study, which is published in the journal Environmental Research Letters, shows that these tests must also account for the "benefits" of certain chemicals on fish population.

"Ecotoxicological tests were designed with traditional toxic contaminants in mind, such as heavy metals and dioxins, which have historically been the major apparent threat against aquatic organisms in surface waters," said Dr Jonatan Klaminder from the Umeå University in Sweden.

"Pharmaceuticals, which are designed to improve health, are a new group of contaminants that do not necessarily fit into the traditional view," said Klaminder in a news release. "I think there is a 'bandwagon effect' within the research community where the old test and the traditional view of a contaminant is routinely used without reflection about the conceptual flaw implicit in the methods."

The team first checked concentration of the drug in a two-year-old Eurasian perch, caught in a lake in Sweden. The researchers found that the level of the drug was low in this fish population

They also collected fish eggs from a different perch population and exposed them to varying concentration of the chemical. The team found that eggs, exposed to high levels of the drug, had reduced mortality rates than other fish.

The increase in lifespan in certain populations of fish is problematic because any change in mortality rates affects the other species in the food chain.

"A new, conceptual view of ecotoxicological testing should include the possibility that a substance can improve the health of an organism and make individuals affected by contamination more competitive than non-affected individuals," said Tomas Brodin, who was the ecologist in the research team.

According to Brodin, other pharmaceutical agents as well as chemicals might also be influencing fish behavior.