A 32-year-old male presented with diarrhoea, mucus and bleeding per rectum. On the basis of sigmoidoscopy, rectal mesalazine was commenced uneventfully, and subsequently changed to oral mesalazine due to failure to improve.He re-presented 4 days later with frequent rigors, lethargy, palpitations and was generally unwell. His condition settled with conservative management and IV antibiotics.Oral mesalazine was withheld during the first 24 hours but was recommenced on day 2. After 2 doses he developed rigors, pyrexia, tachycardia and vomiting.Oral mesalazine was discontinued thereafter and his condition progressively improved. Mesalazine has not been re-introduced.