http://abcnews.go.com/Health/WellnessNews/politicians-battle-unused-medicine/story?id=9915299
This was an issue when my mother died. She bought all kinds of vitamins, kept unused portions of medications she was prescribed (mostly antibiotics) and had years' worth expired medications from herself, her children, her grandchildren, and even from children she used to babysit.
My sister thought we should just flush them. I insisted we take them to the hospital to be properly disposed of - they have the facilities as they are always having to get rid of medications. An alternative would be to take them to a hazardous waste disposal facility, but not every city has one of those, and even in the cities that do, most people don't know where it is or what its hours of operation are. It's often hard to find them. But hospitals are easy to find and they'll often take the unused and expired medications for disposal.
I disagree with disposing of unused medication in the trash, even surrounded by kitty litter or coffee grounds and sealed into it's container. It can still leach into the ground and ground water - as water tests are now showing.
A better alternative would be to donate the unused and unexpired medications to charities, but there are currently laws against that in many places. Obviously, certain types couldn't be donated because of contamination concerns, such as liquid medications, opened containers of loose pills, and medicine that may not yet be expired but is past a certain use-by date. But medication in sealed bubble packaging surely has a better use going to charity than it does to a landfill.
Hospitals generally either incinerate their leftover medications or send them back to the pharmaceutical companies for disposal - and they incinerate the meds.
None of these options are actually environmentally good ones, but incineration and passing the unused and uncontaminated medication along to those who can't afford it are currently the best alternatives.
I would highly recommend that you have a plan for your unused medications in your end-of-life plans. Mine is simple - I don't plan to be taking any medications at the end of my life so there shouldn't be any to dispose of, but should there be some, I will have the addresses and contact information of facilities that can properly dispose of them in as environmentally safe a way as possible - whatever way that might be by the time I die.