Beneficial and Harmful Effects of Azathioprine and Allopurinol Versus Standard Azathioprine Therapy for Patients With Ulcerative Colitis

Inclusion Criteria: * Willingness to comply with all trial procedures and being available for the duration of the trial. * Clinically and histologically verified ulcerative colitis eligible for treatment with thiopurines due to steroid dependence (failure to taper steroid or starting a second course of systemic steroids within 1 year) or patients with the need for rescue therapy with anti-TumorNecrosisFactorα (anti-TNFα) * A sigmoidoscopy or colonoscopy showing active inflammation during the present disease flare * Negative stool test for pathogen bacteria incl. Clostridium difficile * Informed consent. * Normal TPMT genotype (homozygous wild-type). * Oral 5-Asa dose stable for 2 weeks Exclusion Criteria: * Kidney disease with a GlomerularFiltration Rate (GFR) \< 50 ml/min. * Persistent alanine aminotransferase U/L (ALT) twice above upper limit of the normal range. * Participation in other interventional clinical trials. * Pregnancy or breastfeeding. * Previous thiopurin treatment. * Previous or current treatment with other biologics than anti-TNFα * Not being able to comply with the study, assessed by investigator