It is true that trace amounts of birth control and other medications—as well as household and industrial chemicals of every stripe—are present in many urban and suburban water supplies around the country, but there is considerable debate about whether their levels are high enough to warrant concern.

In 2008 the U.S. Geological Survey (USGS) tested water in nine states across the country and found that 85 man-made chemicals, including some medications, were commonly slipping through municipal treatment systems and ending up in our tap water. Another report by the Associated Press found trace amounts of dozens of pharmaceuticals in the drinking water supplies of some 46 million Americans.

But according to USGS, such chemicals and medications are so diluted—at levels equal to a thimble full of water in an Olympic-sized swimming pool—that they do not pose a health threat. But others aren’t so sure. Researchers have found evidence that even extremely diluted concentrations of drug residues harm fish, frogs and other aquatic species, and have been shown to labs to impair human cell function.

One of the common culprits is estrogen, much of which is inadvertently released into sewers through the urine of women taking birth control. Studies have shown that estrogen can wreak reproductive havoc on some fish, which spawn infertile offspring sporting a mixture of male and female parts. Researchers at the University of Pittsburgh found that human breast cancer cells grew twice as fast when exposed to estrogen taken from catfish caught near untreated sewage overflows. “There is the potential for an increased risk for those people who are prone to estrogenic cancer,” said Conrad Volz, lead researcher on the study.

What may be more troubling is the mixture of contaminants and how they might interact to cause health problems. “The biggest concern is the stew effect,” says Scott Dye of the Sierra Club’s Water Sentinels program. “Trace amounts of this mixed with trace amounts of that can equal what? We don’t know.”

With such contaminants proving elusive to municipal filtration systems, the burden of protection often lies with the end user. But getting traces of birth control and other drugs out of your tap water isn’t so easy. Of the many different kinds of in-home water filtration systems available today, only those employing reverse osmosis have been shown to filter out some drugs. Some makers of activated carbon water filters claim their products catch pharmaceuticals, but independent research has not verified such claims.

“The best choice,” says Cathy Sherman of the natural health website Natural News, “would probably be a combination of a reverse osmosis filter augmented by pre- and post-activated carbon filters.” Installing such a system just for drinking water is sufficient, she says, given that water used for cleaning and plumbing doesn’t typically get ingested. As to prevention, the non-profit public health and safety agency, NSF International, urges individuals to not use their toilets or sinks to dispose of unused medications and to opt for the garbage instead; most modern landfills are lined to keep such contaminants inside.