Frederick,
I used to have aggressive Crohns/Colitis, and used to take Azathioprine and almost every other med for it in days gone by. The CAA were very good about it, and after a 6 weeks of taking it, with steady blood results (organised by the GP weekly) I was allowed back to work - Class 1 unrestricted. Eventually I was 'bagged' and haven't looked back (the real advantage being no drugs anymore!).
However, just be aware that with IBD, (Crohns especially) your blood tends to be naturally thicker and can lead to clotting issues ie. DVT. Guess how I found out?
Make sure you drink plenty, as dehydration is the biggest factor.
Talk it through with your AME as he/she will have to negotiate with the feds to see what they want. Generally if the AME is happy, they will be.
I never had any side effects with Azathioprine. But then again I never read the warning leaflets, as i'd just slit my wrists there and then to save the worry
especially considering how big the leaflets sometimes are.
Also, please bear in mind that Winnerhofers quoted research is now some 10 years old, (not saying its wrong, just very dated considering how fast medicine moves) so I would take the time to explain the situation to your gastroenterologist/specialist and formulate an aviation friendly solution. Mine was excellent. If you are worried you can always print Winnerhofer gubbins off and take it with you for them to look at? Also playing Devils advocate, why are they going to prescribe you something that makes you worse? Just wastes everyones time and costs more money to the NHS.
Best of luck, and PM me if you have any more questions that you think I might be able to answer. Remember to keep smiling as you are on the mend now