Azathioprine is considered first line immunomodulatory therapy for patients with ulcerative colitis. Up to 50% are treatment failures or experience adverse events leading to treatment withdrawal. Recent evidence suggests that the combination of allopurinol and low dose azathioprine increases the proportion of treatment responders and reduce the risk of adverse events.

Objectives: To evaluate the beneficial and harmful effects of low dose azathioprine and allopurinol versus standard azathioprine monotherapy in patients with ulcerative colitis.

Investigator initiated, multicentre, parallel arm, open, randomised controlled trial with blinded assessment

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