This is an awesome article on ferret G.I. issues by Dr. Mark Burgess. It includes descriptions and treatments (including dosing.) It might be worth printing and taking to your vet, especially if your vet does not treat very many ferrets.
There's info on the use of imuran for IBD as well.
Well, crap! I can't just copy the link and paste it, but you can do this instead:
Google the following:
Gastrointestinal & Hepatic Diseases in the Ferret Burgess
A .doc file will be listed as the first entry in the results list. Double click on this and open it with MS word...or you can save it.
Ferrets: Contessa, Frankenfurter, Butterscotch. Kitties: Watson, Oskar DIP Sinnead, Vincent, Boris, Zeus and Athena, Willow, Mr. Frodo, Indie, Lucrezia, Judge, Odin, Miss Emily, Suki, Cody, Aristotle. RIP Herne, Ligeia, and Mr. Stubbs