I really never gave this much thought until the past few years when I read articles like this one that discuss why throwing your old medications down the toilet or your kitchen sink is not a good idea.
It makes sense when you think about. These medications are not impacted by the treatment facilities. Sure, some may get broken down further, or become less active but when you consider how many people may be doing this with pet and human medications and how long it has been going on, you begin to see that it's very possible, we all get exposed to low levels of all sorts of drugs.
As a veterinarian, I can tell you that many practices will either give you a credit or at least take them for stray or charity cases. The article points out we cannot re-sell the medications but we can use them to treat animals that wouldn't otherwise get treatment. I have done this many times and am grateful to have that "stash" available to me.
There's also advice for those with human medications you need to dispose of as well.