I'm in Australia, and I'm pretty sure it would be illegal for me to give prescription medication away in *any* circumstance. Even if it were legal to give them to the salvation army or something, it would be Illegal for them to give them to anyone else. This is to ensure that anyone who takes prescription meds has seen a doctor, and in theory has had all the right health checks to make sure it is safe for them to take said medication. The only thing I can do in Australia (and the only thing you're *supposed* to do) is hand them into a pharmacy for them to dispose of properly. Maybe it's different in the states, I don't know.
Sorry if this sounds harsh, I don't mean it to.. You're trying to do something kind, and that's good, but this is the way the law works here.