A vast array of pharmaceuticals — including antibiotics, anti-convulsants, mood stabilizers and sex hormones — has been found in the drinking-water supplies of at least 41 million Americans, an Associated Press investigation shows.

To be sure, the concentrations of these pharmaceuticals are tiny, measured in parts per billion or trillion, far below the levels of a medical dose. Also, utilities insist their water is safe.

But the presence of so many prescription drugs — and over-the- counter medicines like acetaminophen and ibuprofen — in our drinking water is heightening worries among scientists of long-term consequences to human health.

In a five-month inquiry, AP discovered that drugs have been detected

in the drinking-water supplies of 24 major metropolitan areas — from Southern California to northern New Jersey, from Detroit to Louisville, Ky. Denver was among them.

Water providers rarely disclose results of pharmaceutical screenings unless pressed, AP found. For example, the head of a group representing major California suppliers said the public "doesn't know how to interpret the information" and might be unduly alarmed.

How do the drugs get into the water? People take pills. Their bodies absorb some of the medication, but the rest of it passes through and is flushed down the toilet.

The wastewater is treated before it is discharged into reservoirs, rivers and lakes. Then, some of the water is cleansed again at drinking-water treatment plants and piped to consumers. But most treatments do not remove all drug residue.

Researchers do not yet understand the exact risks from decades of persistent exposure to random combinations of low levels of pharmaceuticals. But recent studies have found alarming effects on human cells and wildlife.

"We recognize it is a growing concern, and we're taking it very seriously," said Benjamin Grumbles, assistant administrator for water at the U.S. Environmental Protection Agency.

Tip of the iceberg?

Members of the AP National Investigative Team reviewed hundreds of scientific reports, analyzed federal drinking-water databases, visited environmental-study sites and treatment plants and interviewed more than 230 officials, academics and scientists. They also surveyed the nation's 50 largest cities and a dozen other major water providers, as well as smaller community water providers in all 50 states.

Some key test results obtained by AP:

• Officials in Philadelphia said testing there discovered 56 pharmaceuticals or byproducts in treated drinking water, including medicines for pain, infection, high cholesterol, asthma, epilepsy, mental illness and heart problems. Sixty-three pharmaceuticals or byproducts were found in the city's watersheds.

• Anti-epileptic and anti-anxiety medications were detected in a portion of the treated drinking water for 18.5 million people in Southern California.

• A sex hormone was detected in San Francisco's drinking water.

• Three medications, including an antibiotic, were found in drinking water supplied to Tucson.

• Antibiotics were found in Denver's water supply.

The situation is likely worse than suggested by the positive test results in the major population centers documented by AP.

The federal government doesn't require any testing and hasn't set safety limits for drugs in water. Of the 62 major water providers contacted, the drinking water for only 28 was tested. Colorado Springs was among the cities that do not do such testing.

Some providers screen only for one or two pharmaceuticals, leaving open the possibility that others are present.

Watersheds affected too

AP's investigation also indicates that watersheds, the natural sources of most of the nation's water supply, also are contaminated. Pharmaceuticals were detected in 28 watersheds of 35 surveyed by AP.

In several cases, officials at municipal or regional water providers told AP that pharmaceuticals had not been detected, but AP obtained the results of tests conducted by independent researchers that showed otherwise.

Of the 28 major metro areas where tests were performed on drinking-water supplies, only Albuquerque; Austin, Texas; and Virginia Beach, Va.; said tests were negative. The drinking water in Dallas has been tested, but officials are awaiting results.

AP also contacted 52 small water providers — one in each state, and two each in Missouri and Texas — that serve communities with populations around 25,000. Among them were Grand Junction and Laramie. All but one said their drinking water had not been screened for pharmaceuticals; officials in Emporia, Kan., refused to answer AP's questions, citing post- 9/11 security issues.

Rural consumers who draw water from their own wells aren't in the clear either, experts say. Watershed tests in upstate New York found relatively high levels of caffeine, a common contaminant that scientists often look for as a possible signal for the presence of other pharmaceuticals.

Researcher Anthony Aufdenkampe said he suspects caffeine escapes from failed septic tanks, maybe with other drugs. "Septic systems are essentially small treatment plants that are essentially unmanaged and therefore tend to fail," Aufdenkampe said.

Testing of Denver's watershed also found traces of unspecified antibiotics, according to AP.

Even users of bottled water and home filtration systems don't necessarily avoid exposure. Bottlers, some of which simply repackage tap water, do not typically treat or test for pharmaceuticals, according to the industry's main trade group. The same goes for the makers of home filtration systems.

Pharmaceuticals also permeate aquifers deep underground, the source of 40 percent of the nation's water supply. Federal scientists who drew water in 24 states from aquifers near contaminant sources such as landfills and animal feed lots found minuscule levels of hormones, antibiotics and other drugs.

"Little or no risk"

Ask the pharmaceutical industry whether the contamination of water supplies is a problem, and officials will tell you no.

"Based on what we now know, I would say we find there's little or no risk from pharmaceuticals in the environment to human health," said microbiologist Thomas White, a consultant for the Pharmaceutical Research and Manufacturers of America.

But at a conference last summer, Mary Buzby — director of environmental technology for drugmaker Merck & Co. Inc. — said, "There's no doubt about it, pharmaceuticals are being detected in the environment, and there is genuine concern that these compounds, in the small concentrations that they're at, could be causing impacts to human health or to aquatic organisms."