It was about recently-fallen and depressed(?) Castiel who broke his foot, and so he got prescription pain meds for it. Dean, worrying about what he saw in 2014, puts his doses on a very strict schedule.
It might have been Dean/Castiel, but if not, then it would be completely Gen.
For more specific details:
Dean never tells Sam about why he's so worried about the drugs, so at some point Sam gives in while Dean is away and gives Castiel extra meds. And at some other point Dean and Castiel get into a fight so Dean storms out, and while Castiel is alone he manages to get to the pill bottle and ends up ODing (Dean comes back in time to save him, though).
Thanks for any help!