An investigation by the Associated Press (AP) has revealed that the drinking water of at least 41 million people in the United States is contaminated with pharmaceutical drugs, including medications used for farm animals.

Pharmaceutical products consumed by humans are not fully absorbed into their systems and significant amounts eventually pass out of the body, primarily through urine. Consequently, water treatment facilities are not able to extract all traces of medication before release back into the water supply.

Medications for farmed animals are also contaminating the water supply. One study found that steroids given to cattle pass directly through their bodies while another found that steroid concentrations in the water downstream of a Nebraska feedlot were four times higher than in the water upstream. Fish were also affected in this case, with males having depressed levels of testosterone and smaller heads than normal.

Although the concentrations of drugs in drinking water tends to be low, some pharmaceuticals are potent even at extremely low concentrations. Furthermore, there is evidence that chlorine, which is commonly used to treat drinking water, may make some pharmaceuticals more toxic.

Specifically, the Associated Press surveyed the water providers of the 50 largest cities in the United States and 52 smaller communities. Widespread evidence of contamination with painkillers, hormones, antibiotics, anti-convulsants, anti-depressants, and drugs for cancer or heart disease were found in the water. Of the 28 major cities that tested their water supplies for pharmaceuticals, only two found no contamination.