I've been on 6MP, a sister medication to azathioprine, now for 4 years and have experienced zero side effects from it.
We're all a little worried when starting a new class of medications, myself included. What helped me was to look at the actual odds of risks and benefits. There's about
70 percent odds that aza will improve your UC symptoms. The odds of side effects are very small. If you want to take a deeper dive into it, see The Crohn's and Colitis Foundation of America resources:
Moderator Ulcerative Colitis
John, 39, UC Proctosigmoiditis
Rx: Remicade @5mgs/kg/6wks; daily 75mgs 6MP, 4.8g generic-Lialda, and rowasaInvisible Illness: If I looked as bad as I feel then I'd scare your children!