Scientists at the University of Wisconsin-Milwaukee found that minnows administered Prozac became aggressive, anti-social and sometimes homicidal.
But why put a fish on Prozac? It's not a fix for sad fish - rather, human medications are ending up in waterways and creating ecological effects scientists are only just beginning to research.
Anti-depressant drugs are the most commonly prescribed drugs in the U.S. - about 250 million prescriptions are filled every year. They're also the most-documented drugs contaminating waterways.
Traces of the drugs typically get into the water when people excrete them. Sewage treatment plants discharge the filtered effluent, but most aren't equipped to filter out the drugs.
The scientists wanted to study the effects of this drug exposure, and chose the fathead minnow, a fish common fish in Midwest waterways, as their subject.
Rebecca Klaper, an ecologist at the University of Wisconsin-Milwaukee's Great Lakes Water Institute, presented results of the study at the meeting of the North American division of the Society of Environmental Toxicology and Chemistry in Long Beach, California.
Fathead minnows usually display complex mating behavior, with males building the nests where females comes to lay their eggs. After they're laid, the males fertilize them and keep watch, cleaning away fungus and dead eggs.
Klaper said that the fluoxetine, the active ingredient in Prozac, was given in very low concentrations - 1 part per billion - which is the same as that found in waste water discharged into waterways.
The male of the species spent more time hiding alone, hunting and ignoring females.
Female fathead minnows seem to be unaffected by the chemical except for producing fewer eggs.
When the concentration of fluoxetine was increased to the highest levels found in waterways, male minnows started to spend more time building their nests.
Scientists increased the dose tenfold, in an effort to see what might happen in our waterways in the future, and the males 'become obsessive, to the point they're ignoring the females', Klaper said.
When fluoxetine concentrations are increased again, fathead minnows stop reproducing all together and turn violent: 'The males start killing the females,' said Klaper.
Strangely, if the females are introduced a month after the males are exposed to the chemical, the males don't show aggressive behavior towards them - but the females still don’t lay any eggs.
The research has shown that exposure to the drug can alter the genes responsible for building fish brains and controlling their behavior.
The drugs seem to cause these changes in behavior by scrambling how genes in the fishes' brains are turned on and off. The minnows were exposed when they were a couple of months old and still developing.
Klaper said there appeared to be 'architectural' changes to the young minnows’ brains.
'At high doses we expect brain changes,' Klaper told the conference. 'But we saw the gene expression changes and then behavioral changes at doses that we consider environmentally relevant.'
These new findings build on Klaper’s previous research, which tested minnows exposed to the drug to see how they dealt with predators. The fish swam longer distances and made more directional changes, which suggests that the drugs induced anxiety.
It is unclear whether any of these effects are being felt by wild fish populations, but Klaper said that any changes in reproduction, eating and avoiding prey can have devastating impacts for fish populations.
The most vulnerable fish populations are those downstream of sewage treatment plants, where prescription drugs consistently show up in higher levels than in other waterways.
Steve Carr, supervisor of the chemistry research group at the Los Angeles County Sanitation Districts told Environmental Health News that in the past decade technology has allowed plants to test for chemicals in their waste water and in waters downstream - but most don't.
Studies have shown that drugs can build up in some fishes' systems, meaning the drug levels could accumulate in fish the longer they are exposed to even low concentrations.
The U.S. Environmental Protection Agency considers pharmaceuticals an 'emerging concern,' and that chemicals from prescription drugs may pose risks to wildlife and humans, but there are no federal regulations in place as yet.
While traces of prescription drugs in drinking water is 'unlikely to pose risks to human health' according to the World Health Organization, we are discovering that the effects on wildlife could be serious.
'Fish do not metabolize drugs like we do,' Klaper said.
'Even if environmental doses aren’t thought to be much for a human, fish could still have significant accumulation, and, it appears, changes in their brain’s gene expression.'