My daughter recently reacted badly to a certain antibiotic, she just couldn't stomach it. This left us with a brand new, nearly full bottle of medication. I dug around and found that there's an organization, The Health Equity Project, that accepts these and other prescription meds and sends them to developing countries where medication is scarce.
You can find the Health Equity Project HERE. The mailing address and list of accepted items are on that page.
In my searching, many links directed me to the Starfish Project, which is now apparently defunct. It took some hunting to find this, so I thought I'd share.
Have a great day!