I'm no expert on this, but I think the differences between all the 5asa drugs has something to do with where in the digestive tract they are optimally released. Mezavant is meant to have it's best effect in the colon. Probably more suited to UC than Crohn's. Again, I'm no expert, perhaps some others can give some insight.
My daughter was diagnosed Feb. 19/07, (13 yrs. old at time of diagnosis), with Crohn's of the Terminal Illium. Initially, 9 weeks of Prednisone, currently taking 2000mgs of Pentasa.