When water providers find pharmaceuticals in drinking water, they rarely tell the public. When researchers make the same discoveries, they usually don't identify the cities involved.

There are plenty of reasons offered for the secrecy: concerns about national security, fears of panic, a feeling that the public will not understand - even confidentiality agreements.

"That's a really sensitive subject," said Elaine Archibald, executive director of California Urban Water Agencies, an 11-member organization comprised of the largest water providers in California.

She said many customers "don't know how to interpret the information. They hear something has been detected in source water and drinking water, and that's cause for alarm - just because it's there."

As The Associated Press documented in a five-month investigation, drinking water provided to at least 41 million people living in 24 major metropolitan areas has tested positive for trace amounts of pharmaceuticals.

Most Americans probably think they have a good idea of what's being detected in their water. Federal law requires water providers to distribute annual "consumer confidence reports" that reveal levels of regulated contaminants. Providers are not, however, required to tell people if they find a contaminant that is not on a U.S. Environmental Protection Agency list. And there are no pharmaceuticals on the EPA list.

In Philadelphia, the water department has not informed its 1.5 million users that traces of 56 pharmaceuticals or their byproducts - like the active ingredients in drugs to treat depression, anxiety, high cholesterol, fever and pain - have been detected in the drinking water, and that 63 pharmaceuticals or byproducts had been found in the city's source watersheds.

New York City water officials declined repeated requests for an interview and waited more than three months before participating in an AP survey, supplying information only after being informed that every other major city in the nation had cooperated.

Ron Rhodes, water treatment plant supervisor in Emporia, Kan., explained why he wouldn't disclose whether his community's source water or drinking water had been tested for pharmaceuticals. "Well, it's because of 9/11. We want everybody to guess."

How, Rhodes was asked, could it endanger anyone to know if Emporia's water has been screened for traces of pharmaceutical compounds?

"We're not putting out more information than we have to put out," said Rhodes. "How about that?"

Milwaukee's water department is an anomaly, posting on its Web site a detailed water quality report that includes test results for 450 unregulated contaminants, including pharmaceuticals. While they found minute concentrations of cotinine, a nicotine derivative, they didn't detect hundreds of other contaminants.

When asked what power the EPA had to require public disclosure when pharmaceutical contamination is discovered in a water provider's supplies, Benjamin H. Grumbles, the agency's assistant administrator for water, said, "We work very closely with utilities across the country and we encourage them to share with their community information they find out about their source water."

Grumbles was asked how he thought water providers have been responding to the EPA's "encouragement."

"I think we have more work to do," he said.

It's not just the water departments that have failed to disclose such information.

The AP spoke with many scientists, federally funded researchers, university professors and private drinking water experts who have detected pharmaceuticals in drinking water, but would not say where they had obtained their samples.

Robert Renner, the foundation's executive director, said AWWARF study participants are routinely promised anonymity. Citing confidentiality agreements, he declined to name the 20 different drinking water treatment plants around the U.S. where pharmaceuticals have been detected in water heading to more than 10 million people.

"It's a hard topic to talk about without creating fear in the general public," Renner said.

Some said those fears could lead to much larger problems than the actual contamination.

Doctors "don't want people to be afraid to take their medicine because of environmental concerns," said Virginia Cunningham, an environmental executive for drug maker GlaxoSmithKline PLC.

Utilities also generally only allow scientists to test their water if they ensure confidentiality.

Peter Rogers, Harvard University professor of environmental engineering, said improvements in detection techniques could help fuel fears among the general public.

"We're chasing this down to molecular-sized measurements, so the more you look, the more you find," said Rogers. "I think the government and utilities are quite right to be very skittish about telling people their results. People will claim it is causing all sorts of problems. If I were a water utility, I would stop those measurements right away because if you measure something, it will get out, and people will overreact. I can just imagine a whole slew of lawsuits."