Down eight glasses of water a day and chances are, many Americans are also "self- prescribing" minute amounts of various prescription drugs, including those used to treat infection, epilepsy, mental illness, heart problems -- even those used to prevent pregnancy. A recent investigative report from the Associated Press (AP) revealed that public drinking water in at least 24 major metropolitan areas is tainted with multiple drug residues.

Of those, Philadelphia's water was most contaminated, harboring a mix of 56 different medications. Twenty-eight of 35 tested watersheds also carried pharmaceuticals.

The federal government doesn't require testing for drugs in drinking water, and sets no limits for safe levels. And while some water suppliers screen for drugs, they rarely tell customers what they find.

How do we end up drinking each other's drugs? As a nation, we pop pills for every imaginable malady and our medicine cabinets overflow with over-the-counter remedies. These drugs are not fully metabolized and some residues are excreted and flushed down the toilet. Another source is unused or expired prescriptions dumped down the drain.

Animals also contribute. Factory farms feed livestock hormones, steroids, and about 40 percent of all antibiotics manufactured in the U.S. Their medicine-laden manure, spread onto crop fields, runs off into waterways.

Our wastewater treatment systems are not designed to deal with modern pollutants, including drugs.

What does this mean for the environment, and for us? Disturbing abnormalities in aquatic animals have been widely documented. When male fish swim in waters containing traces of estrogen from birth control pills they morph into females and stop reproducing. Tadpoles exposed to Prozac don't grow to full size, making them vulnerable to predators.

Among scientists, these findings have heightened worries about human health risks from decades of constant, low-level exposure to random combinations of drugs. The consequences could include reproductive problems, early onset of puberty, and antibiotic resistance -- but much remains unknown.

Recent studies have found disturbing effects on human cells. In the lab, small quantities of estrogen triggered rapid growth in breast cancer cells. Drug residues also caused blood cells to kick into the overdrive mode associated with inflammation, and slowed the growth of developing kidney cells by one-third. This suggests that small drug exposures might exert powerful effects, especially during periods of crucial development -- say if you're mixing infant formula with water that carries traces of hormone-bending medications. These findings are preliminary, and more research is needed to determine real risks. But they raise serious questions about the safety of our country's water system -- and about U.S. Environmental Protection Agency (EPA) oversight.

The pressing question is: Why hasn't the EPA addressed this issue? The AP report shocked readers across the nation, but pharmaceuticals have been showing up in our drinking water for 20 years.

Since 1996, the only contaminants the EPA has added to the Safe Water Drinking Act were either ordered by Congress or mandated by lawsuits. Benjamin H. Grumbles, the EPA's Assistant Administrator for Water, said his agency has analyzed 287 pharmaceuticals for possible regulation under the Act, but only one, nitroglycerin, was listed: the substance is used for heart problems, but is also used to make explosives.

We need to quickly evaluate the extent of the problem and make that information publicly available. But we can't wait for data on health effects to take action: Evaluating the effects of the more than 100 medicines found in our water could take decades.

In April, Sens. Barbara Boxer (D-CA) and Frank Lautenberg, (D-NJ) began hearings on the issue. Also, Rep. Allyson Schwartz, (D-PA), has demanded a national task force to make recommendations to Congress, which should include funding: According to a 2003 EPA estimate of the cost of protecting US drinking water, we are underfunding wastewater treatment by $271 billion.

We also need a cooperative federal and pharmaceutical company program to collect and properly dispose of unused and out-of-date drugs from consumers. Factory farms also should be regulated to keep drug-laden manure from entering our water supply.

It is time we upgraded municipal water systems to protect us from drugs we may be unwittingly ingesting daily. Congress, the EPA, pharmaceutical companies and the public must all cooperate and do their part.

Sharon Guynup's first book is titled, State of the Wild 2006: A Global Portrait of Wildlife, Wildlands, and Oceans. This column was distributed by Blue Ridge Press.