Thank you for replying, any light you can shed on the matter is really appreciated.
He says that he feels like codeine's his 'happy place'. He was prescribed dihydrocodeine after shearing his kneecap off in a sporting accident and, as I say, after the surgery he quickly got addicted. Then his marriage started to breakdown with her infidelity, which hit him hard, and he turned more and more to the tablets to feel better.
We've been together a couple of years now, and I can tell when he's on the codeine. The sweating, the tiny pupils, behaving distant... it's all a dead giveaway. I think he's mostly off the now, but every now and then I'll find a receipt for over the counter cocodamol, solpadein or anything carrying codeine that he can get his hands on. He always starts by denying it, which is by far the worst part for me. I've thrown him out three times over it, including him stealing tablets from me and then gaslighting me, saying I took them.
Otherwise he's a great man. But his addiction turns him on to a total bellend. I just need him to stop.