I have a sulfa allergy and was given Asacol (though was also first diagnosed as UC). I think there is a drug like that which is non-sulfa and absorbs earlier than Asacol, which doesn't break down until the large intestine. I suppose it depends upon where your Crohn's issues are.
And, though it didn't work for me, Asacol helps many, many people! I have a friend with Crohn's who maintains remission using Asacol and the same with my aunt who has UC!
Also, if you are bad enough, I recommend asking your doctor about Remicade. It seems scary to people at first, but it did wonders for me and there are no purse-loads of pills to carry about on a daily basis!
--39 year old female, dx as UC in '04 (1st symptoms in '03), switched to Crohn's in '05, 1 fistula, crohn's colitis, limited to large intestine
--rejected (reaction/didn't work): Asacol, AZA, 6-MP, MTX, Remicade, Humira, prednisone
--stuck on methylprednisolone, tried Prochymal in Phase III study (can't wait til it's approved!)
--started Tysabri 3/21/08
--single mom to 10-year-old girl