"Back in the day, we just flushed them away. Then we got the bad news—widespread contamination of streams by pharmaceuticals, according to a 1999-2000 study by the U.S. Geological Survey. Next, we learned from a 2008 investigation by the Associated Press that there were drugs in the drinking water supplies of 41 million Americans." ~ NRDC.org
"Topics on this Page Overview Medicines play an important role in treating many conditions and diseases, but when they are no longer needed it's important to dispose of them properly to avoid harm to others. Below, we list some disposal options and some special disposal instructions for you to consider when throwing out expired, unwanted, or unused medicines..."
Useful information here fda.gov