I think my Master and I can completely relate to this. He has Multiple Sclerosis and Epilepsy. The MS has caused a fair amount of nerve damage and he is very badly affected by it in extreme temps or when he is fatigued. The epilepsy is very poorly controlled and he has frequent seizures (sometimes 5+ in a given week).
There are times with MS related issues where I will have to really get verbally forcefull about telling him not to do certain things that he feels he physically should be up to, but realistically isn't. And when those times occur I will not back down from the fight because I know in my heart that I am taking care of him and being a good slave by arguing with him about it. Usually, he realizes that I am correct and eventually will avoid the activity that would harm him. In the end I can't stop him, but I can sure try like hell!
Regarding the epilepsy- about 3 minutes before a seizure he will begin to lose his ability to understand that he is about to seize. Even though everyone in the room can tell; he can't. He often becomes very silly and laughs when I try to tell him he is about to "go" and refuses to lie down on his side like he needs to for safety. At those times, I have to get very stern and demand that he do it, and if all else fails attempt (not always successful) to wrestle him into a safe place and position. Again, as above... I would not be a good slave by letting him get hurt or worse yet, even die because I don't want to tell him what to do.
Some of the medications that he used to be on for these conditions would very severely affect his thinking on occasion and like in your situation with pain meds, he could be of serious danger to himself and others if I let him go off and do something that he would normally never do... that most people would never do. Thankfully, it's been nearly a year since those meds were stopped!
Unfortunately, the lack of meds does make the epilepsy worse, so I have to help with that more frequently than I used to. It's still a far better trade off though. In conclusion, I guess I never really struggled with feeling unsubbly or unslavish because of these issues, but he used to. Now though, he agrees with me that I am not in any way being dominant or him submissive by me caring for his health above all else.