I assume that by Z-pac you mean z-pak which is azithromycin. It s interactions with methadone are minimal. They are significant if you suffer from long QT syndrome. Other wise, it is something quite negligible. I am not sure which steroid you used but they generally have no issues with azithromycin. Azithromycin has a very consistent side effect and that is nausea (feeling sick), diarrhea and abdominal pain (tummy ache). About a quarter of patients put on in azithromycin can get one or more of these problems. It is a good antibiotic though. I suggest talking to your doctor if another alternative can be offered. You can simply say that this medication is not agreeing with you.