I'm not entirely sure what exactly other meds you're describing, but when I was given a Fentanyl patch, it was near the end of my treatment and I couldn't bear the pain any more. I wasn't on much of anything else FOR the pain at that point, and I think I would remember, because I was strictly feeding tube by that point, and it would have had to be liquid or a pill ground up (which we did with some of my regular meds). I had the mouthwash stuff, but nothing else (and no brains to ask). Since then, I've gotten other meds for migraines that I believe are opioid derivatives (maybe?) if that helps. I'm careful about dosing and it's never been a problem. (I know some folks have strong feelings about those kinds of meds. But they do have their place.)

When I broke down at radiation and begged for mercy (pretty literally) they gave me something in a shot right then and there, and a prescription for the Fentanyl patch that my mom helped me get filled on our way home. (She was my driver, since my husband was working.)

I don't recall any bad side effects ... honestly, the radiation burns were so bad, I'm not sure I would have noticed anything short of a severe allergic reaction. (I do get allergic reactions to a lot of meds, so I watch for those.) I do think I should have asked for pain relief a lot sooner ... and I think my doctors should have let me know it was available, a lot sooner.

At that point in my life I didn't have severe breathing allergies, though I do now. You mentioned something about respiratory issues ... is this something you have a concern about? You'd want to talk to your doctor directly about that, for sure. I've got an inhaler and an Epi-pen these days (gotta love compromised immune systems), so I figure I'm covered, but then I didn't. Didn't have a problem, but didn't expect one either. If you have reason to worry ... talk to your medical professional!

For what it's worth ... as a patient, not a medical professional ... I read all the medical literature very carefully, but I read it for MY keywords. I know my allergies, my usual side effects, the things that stack from one med to another ... and I fret about those, and not the rest of it. Otherwise I'd go crazy. (Crazier.) Talk to your doctor and maybe your pharmacist for advice about med interactions, if that worries you. And then ... do what seems right to you, what will get you through treatment. It'll be different for everyone; nobody can make that call for you. But you're the one who has to get through this, and it's okay to use medical help to do it. ((hugs))