Too Much Prozac Turns Minnows Into Killers

A research group studied how exposure to anti-depressants altered fish behavior.

June 13, 2013, 4:05 PM

June 14, 2013 — -- Fish swimming in water with a trace of the anti-depressant Prozac did not adopt a cheery disposition. Instead, they became edgy, aggressive and some even killed their mates.

The fish were subjected to traces of the drug by a research group at the University of Wisconsin-Milwaukee that examined how environmental exposure to the medication altered the behavior of fathead minnows, a common fish found throughout the Midwest.

Lead researcher Rebecca Klapper says that this experimental setup could actually be a reflection of the fishes' reality.

The human body does not absorb medications 100 percent, so a trace amount is excreted in urine. Water treatment centers are unable to completely filter out all of those contaminant and can trickle down and affect the wildlife. Klapper sees the minnows as a way to gauge the long-term effects of Prozac in humans.

"It's not just an environmental question but a human question as well," she tells ABC News.

Changes in the minnows' reproductive behavior were seen in as low concentrations as 1 microgram per liter, equal to a single dose of Prozac dissolved in over 5,000 gallons of water.

"They spent more time in their nest than they did interacting with the females," Klapper says. At higher concentrations, that behavior switched from disinterest to outright aggression with the male fish started to attack and sometimes killing their mates.

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Similar effects have been seen in humans who take the antidepressant. Common Prozac side effects include a decrease in sex drive, impotence, or difficulty having an orgasm, according to WebMD and

Fathead minnows are small fish, the biggest ones measuring only a couple of inches long. Their small size could be a factor in how they react to low levels of Prozac. However, Klapper believes that the concentrations used in this experiment, as well as the levels seen in the environment, could also impact larger species of fish.

"Fish don't clear the medication as quickly as other animals," she says, which can lead to a possible build up of Prozac ultimately changing the fish's behavior.

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