What begins in the bathroom often ends in the water supply. No, not that, the drugs in your medicine chest—and that, a new study suggests, could have a significant impact on aquatic life.

Toxicologists at Clemson University in South Carolina have found that hybrid striped bass exposed to the antidepressant fluoxetine (the generic name for Eli Lilly's Prozac) were markedly less interested in feeding than other fish. The more fluoxetine ingested, the less the appetite. The fish also did things that could lead to life-shortening events—like failing to take usual precautions around predators and making them easier prey.

Fluoxetine and related drugs block the reuptake of serotonin, a neurotransmitter strongly tied to emotion, by nerve cells, or neurons.  As a result, more serotonin remains in synapses (small gaps between cells over which information flows from one neuron to another), which has the effect of boosting mood. Serotonin is also linked to appetite and aggression.

Study co-authors Stephen Klaine and Kristen Gaworecki exposed hybrid striped bass, the kind stocked for sport fishing throughout the U.S. Southeast, to varying amounts of fluoxetine—zero, 35, 75 and 150 micrograms per liter—over six days, followed by another six-day washout period in clean water.

The concentrations they used were more than 1,000 times greater than those detected in waste water, Klaine says, noting that the researchers wanted to determine if—and then how—the drug might affect fish. "We're in the midst of some longer-term experiments now," he says, "and we're hoping to see responses at concentrations that would be found in the environment."

Bass were offered four live fathead minnows every three days, during which the researchers measured the animals' eating habits; they measured serotonin levels in brains harvested from each bass.

"In general, it took exposed bass longer [than bass not given Prozac] to eat each minnow," he says. "Some bass exposed to the higher levels of fluoxetine pretty much gave up trying to capture prey by the third or fourth minnow. They really didn't have the kind of appetite the controls had."

Bass exposed to  the greatest amounts of fluoxetine acted remarkably un-basslike, staying at the top of the tank with their dorsal fin above the waterline or tilting to a vertical position. Such behavior in the wild could make a bass an easy target for a hungry predator.

Theodore Henry, a fish toxicologist at the University of Tennessee Knoxville, who has studied fluoxetine's effects on the sexual development of mosquito fish, says he is "very interested" in the latest findings published in the journal Aquatic Toxicology. "That is something that we would hypothesize would happen, that the drug would affect the uptake of serotonin and lead to behavioral consequences in the fish."

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