Drug mixes threaten aquatic ecosystems. (Killer Cocktails).

Combinations of drugs that have been detected in trace amounts in waterways from Nevada to Switzerland could deform and kill native microscopic organisms, a new study finds.

Since the 1970s, scientists have discovered more and more rivers, lakes, and streams tainted with tiny amounts of common pharmaceuticals, including antibiotics, antidepressants, hormones, chemotherapy drugs, cholesterol-lowering medications, and over-the-counter pain killers such as ibuprofen (SN: 3/21/98, p. 187; 6/17/00, p. 338). Researchers have estimated that up to 80 percent of drugs taken by people and livestock emerge from the body intact, so mixtures of medications may pollute aquatic ecosystems.

"Nobody really knows whether these ambient concentrations are affecting the ... environment," says Colleen Flaherty, a zoologist at University of Wisconsin-Madison. Flaherty set out to test some trace pharmaceuticals on Daphnia magna, the tiny freshwater crustacean that is food for many fish.

In laboratory tests, Flaherty exposed D. magna, more commonly called daphnia, to low concentrations of either of two drugs: clofibric acid, used to lower people's blood concentrations of cholesterol, and fluoxetine, an antidepressant. These drug concentrations--36 parts per billion of clofibric acid and 100 parts per billion of fluoxetine--are similar to those detected in aquatic environments, Flaherty says.

Over periods up to 6 days, neither drug by itself "had any apparent effects," says Flaherty.

On a whim, she says, she exposed the daphnia to a cocktail of both drugs at the concentrations she had just tested. To her surprise, most of the tiny crustaceans--up to 90 percent in some trials--died. Mixtures with less clofibric acid resulted in less mortality but cause some daphnia offspring to have malformed carapaces and appendages.

In further tests, she exposed daphnia to five common antibiotics at 10 parts per billion. Though they, too, had no detrimental effect individually, a combination of three--triclosan, erythromycin, and trimethoprin--yielded generations of daphnia with abnormally large proportions of males.

Such a change in sex ratio can indicate that an organism is under environmental stress, says Flaherty, who presented her results last week in Tucson at the annual meeting of the Ecological Society of America.

Both experiments show the importance of testing combinations to get an accurate picture of drugs' effects, says Flaherty. She notes that most research into pollutants has only examined the effects of single chemicals. "In reality, these critters are not being exposed to just one drug, one heavy metal, or one pesticide," she says.

Though studies have shown drugs widespread in waterways, scientists know "next to nothing about their ... potential for effects," says Christian Daughton of the Environmental Protection Agency's National Exposure Research Laboratory in Las Vegas. Although he commends Flaherty's research, Daughton argues that clofibric acid and fluoxetine haven't been found in the environment in concentrations quite as high as she used.

He notes, "The big question is, Is there a reason for concern?"