I was put on Imuran this past summer in conjunction with Remicade in attempt to get an extremely bad flare under control. I am sorry to report that I was one of the few cases where Imuran didn't work, and I ended up with pancreastitis and in the hospital for two weeks! Like you said though the chance of that is very low (I was told about
a 3% chance), and I just seem to have an extremly stubborn body when it comes to medication!
Best of luck!