Probably a good decision to switch to NSAIDs (at least when possible) please don't flush meds again though.
Curious why you say that? I was always told that this is what you were supposed to do
However, presuming you're not on a septic system, they end up in the river, just like any other form of pollution.
Lots of police departments are taking them no questions asked. The city where I work's police/public affairs building has a big metal box with a one-way lid for unused drugs.
Where I live? eh... it's not common, but it definitely happens. And it's probably not just for opiates, but anything will probably do. I do get that putting them into the river/ocean is undesirable, though.