How should I dispose of unused or expired medication? Well... some labels will say to flush down the toilet, but that really isn't the best option, especially with strong pain killers. The article How to Get Rid of Anything recommends this.
"Flushing isn’t the best method for most drugs, because they can contaminate the water supply and affect fish and wildlife. For those meds, law-enforcement agencies sponsor “take-back” days or supply drop-off boxes. You can also mix medication with kitty litter or coffee grounds (to prevent animals from eating it), seal it in a plastic bag, and toss it -- but scratch off personal information from the prescription bottles first. If you use needles for medical purposes (insulin or hormone injections, say), see if your pharmacy will take back used ones for safe disposal."