 Patients will undergo a physical examination, baseline blood tests including measurement of lipase andor amylase, liver biochemistry and fasting lipid profile. If not already tested, the patients TPMT activity will be tested. Participants will also have a baseline abdominal ultrasound to confirm normal anatomy and absence of cholelithiasis At week 0, if there are no clinical, biochemicals or ultrasound suggestions of pancreatitis, the participant will be commenced on an alternative thiopurine from what was used in the past. That is, if in the past they were treated with AZA, they will now be commenced on 6-MP, and if in the past they were treated with 6MP, they will be commenced on AZA. The medications will be commenced at standard dose ie AZA 2.5mgkgday, 6MP 1.5mgkgday. Depending on the specific situation, and assuming all relevant information is available, the screening visit may be merged with the week 0 visit. 