That Tamiflu you’re taking to ward off H1N1? After it runs through you, it could wind up in lakes and rivers. reported that in Japan, concentrations of Tamiflu’s “active form” are showing up in rivers downstream of wastewater treatment plants. The source: human urine.

The active form of Tamiflu is the result of the body converting the drug into oseltamivir carboxylate, or OC. The drug undergoes similar changes at wastewater treatment plants.

The magazine also notes that chemists worry about birds, which carry strains of the flu virus, being exposed to Tamiflu residue. Ducks, in particular, seem to like loitering in water downstream from treatment facilities.

Birds could then build a resistance to the drug and spread resistant strains of seasonal and avian flu, scientists warn, although they don’t appear to carry H1N1.

It’s not surprising that Tamiflu is being detected in waterways. A 2008 Associated Press investigation found prescription drugs have been detected in the drinking water supplies of 41 million Americans living in 24 major metropolitan areas. These drugs include birth control pills, antibiotics and psychotropic medications. Treatment systems can remove some, but not all, of the drugs; in fact, chlorine used to disinfect water can make some drugs more toxic, the AP reported. There are no federal safety limits for medications in water.