Coffee, tea and cola (ie caffeinated drinks) are pretty much forbidden for me. I might get away with an occasional cup of coffee (like once a week), but any more than that is liable to trigger diarrhea which can take several days to resolve.
Caffeine is an irritant to the intestine. Red Bull, from what I understand, has quite a bit of caffeine. If you're tolerating it, that's great, but I can't, and other Crohnies I've talked to are similarly affected. Diarrhea is a symptom - what's really happening is inflammation, and that's bad.
I love my caffeine, but I just can't take it. A pity, but I guess it's not the worst sacrifice one can make.
Crohn's since 1988