Prozac is making fish… different.

Discover reports on unanticipated consequences of antidepressants getting into the water and changing animal behavior:

[Teresa] Dzieweczynski, a psychologist at the University of New England, looked at just one of the drugs that’s crept into American waterways: fluoxetine, better known as Prozac.

She tested fluoxetine on male Siamese fighting fish, or Betta splendens. No one knows how much Prozac is in the water in Thailand, where these fish live naturally. But they make a good model, Dzieweczynski explains, because scientists already understand a lot about their brains, behavior and hormones.

Dzieweczynski and her coauthors put the fighting fish into three different lab environments. Some of the fish swam in tanks with clean water. Others swam in water with a low concentration of fluoxetine—0.5 micrograms per liter, which is at the higher end of what’s been found in U.S. waterways. The third group of fish swam in a stronger dose of fluoxetine, 5 micrograms per liter.

After getting baseline behavioral scores for all their fish, the researchers started dosing them with fluoxetine. They retested the fish after a day of drug exposure. After a week of drug exposure, they tested them again. Then they put all the fish into clean water for a week and tested them a final time.

Across all the behavioral tests, fish exposed to the antidepressant were less bold. They stayed in one place, explored their environment less, and were more hesitant to approach other fish. Their behavior was also more erratic. A higher dose of the drug caused a more dramatic effect.

“Perhaps most importantly and alarmingly,” the authors write, they could still see these effects after the fluoxetine was gone. Even after a week of swimming in clean water, drugged fish behaved less boldly than normal.