A free download from the EPA: How to Dispose of Medications Safely

Do Not: Flush expired or unwanted prescription and over-the-counter drugs down the toilet or drain unless the label or accompanying patient information specifically instructs you to do so. DO: Return unwanted or expired prescription and over-the-counter drugs to a drug take-back program or follow the steps for household disposal below.
http://water.epa.gov/scitech/swguidance/ppcp/upload/ppcpflyer.pdf