I took medication that was still usable, not to mention expensive, that I would not be taking, into my neurologist. This was some very heavy duty pain meds given to me from after my cancer surgery. I told him I preferred to stick with my vicodin, which I thought was more useful than the oxy-ones. I dump nothing into my septic system because it goes right into our ground water. He asked if it was okay for him to pass them onto his patients who had trouble affording their meds. I told him it was fine.
Other drugs, including some older ones for the dogs, also gave to the doc so that he could safely dispose of. No antibiotics, because you are supposed to FINISH all of those. After having a CVS pharmacist steal drugs from me, I wouldn't turn in out of date asprin to them.
Just a word of warning, tho...if you are going to give old meds to your doc to dispose of, make sure they are going to do it responsibly. I know of one who took the meds and told his receptionist to throw them in the trash. Uh...no.