I do not think there is an issue using vitamin D and zinc supplements in conjunction with an antibiotic. Actually, my primary care doctor instructed me to take a small dose of vitamin D (1000 IU/day) and he definitely knew I was already taking doxy. For other reasons, I've had blood work done twice since then and the results have always been fine.

I don't know the answer to question about tapering off one antibiotic before switching. My suspicion is you just stop one and start the other, with no taper down. Seems like less fluctuations for your body to handle that way. I'd call the docs office and ask them. That's why they're making the big $$$ )