I would also keep it until it expires. The only things you have to worry about putting in the municipal garbage are sharps, or extremely large quantities of a drug. Sharp used needles can harm people who handle the garbage, so put them in a hard-sided box or bottle (like a 2L soda bottle) before throwing them out (in the future). Unused needles, as long as they're still in sterile packaging, should be fine, and most medicines will keep for at least 6 months after they're made. Some medicines can leach into the groundwater from a landfill, but that takes large quantities of them, and most landfills are designed to stop that from happening. The very best place to dispose of medicine is the pharmacy. Flushing it, which I know a lot of people do, is not the best way to go, and in fact I would throw it out in the municipal garbage before I flushed it--it goes more quickly to the water supply when you flush it.