You can ask your regular pharmacy; our local pharmacy has a program for taking in old and expired meds, so they are disposed of safely. I can't see that animal meds would be any different.
Meds should never be flushed or put down drain, as they enter the water system and that can be dangerous. I don't put them in bin either, since you don't know if bag will break open somewhere along the way and the meds can possibly enter the water table.