If you throw away or flush old medicine, it can pollute the water and soil. "The medicine can leak out of landfills and get into animals, who then release it into water and soil," says Dr. Alan Greene, author of Raising Baby Green. When you flush it, the medicine gets into the water supply."
Healthier option: If you're disposing of medicine, put it in a sealable bag full of coffee grounds (which makes them less appealing to animals). When in doubt, check out the U.S. Food and Drug Administration's information page or ask your local pharmacist.