"Scientists in Erie, Pennsylvania, have found that minute concentrations of fluoxetine, the active ingredient in Prozac, are killing off microbial populations in the Great Lakes." 
Killing off bacteria might seem like a good thing. "Your immediate thought is, 'well, that's good, because they're not supposed to be there anyways," said Mercyhurst College microbiologist Steve Mauro, whose team found fluoxetine in low doses in water near Lake Erie's beaches. "But what about all the other bacteria that are supposed to be there and part of that ecosystem?"Prozac Killing E. coli in the Great Lakes (Thanks, Marilyn! via boing)Treating clean lake water with similar strength doses killed off E. coli and enterococcus bacteria, both of which can cause serious infections in humans.
The fluoxetine found in Lake Erie is at very low levels--about one nanogram per liter of water, Mauro said. "It doesn't appear to be at a level that would be harmful to humans," or invertebrates, for that matter, though Mauro suspects that fluoxetine combined with other chemicals could be having a cumulative effect on the lake's ecosystem.