Serum sickness associated with 6-mercaptopurine in a patient with Crohn's disease

A 43-year-old man with a 19-year history of Crohn's disease experienced serum sickness with acute lobular panniculitis and vasculitis within 2 weeks of initiating 6-mercaptopurine therapy. He had no history of extraintestinal manifestations of Crohn's disease or drug allergy. Symptoms included disabling joint pain, proteinuria, fever, rash, and malaise. To our knowledge, this adverse drug reaction has not been described in association with 6-mercaptopurine.