I haven't had a problem with Zithromax.
However, Omnicef gave me diarrhea and, I think, triggered my UC. Just one person's experience.
Diagnosed with UC in transverse and descending colon in Feb. 2007
Doing well so far on Asacol 800 mg. 2x a day
Also take 25 mg. Paxil, 100 mg. Wellbutrin, 1 probiotic pearl per day
Currently off other meds because I'm newly PG, due next July