Nature and course of pancreatitis caused by 6-mercaptopurine in the treatment of inflammatory bowel disease

6-Mercaptopurine (6-MP) has an important role in the treatment of inflammatory bowel disease. Its most frequent short-term complication has proven to be pancreatitis, which we have now seen in 13 of 400 (3.25%) patients (12 Crohn's disease, 1 ulcerative colitis) and which we here describe. The timing of the pancreatitis was such that it could not be attributed to sulfasalazine, which was also being taken by 9 patients, or corticosteroids, which were being taken by 7 patients. The dosage of 6-MP ranged from 50 to 100 mg daily, and the pancreatitis, which was uncomplicated in all cases, occurred within 8-32 days with one exception (6.5 mo). Symptoms included epigastric pain, back pain, fever, and nausea. The serum amylase was elevated in 12 patients. The average elevation was 5.9 times normal. In all cases, the 6-MP was discontinued and symptoms and signs returned to normal over a period of 1-11 days. No other complications of 6-MP occurred; there was no leukopenia. Of 7 patients rechallenged with 6-MP, all developed recurrent pancreatitis, including 4 in less than 24 h. In 3 patients, desensitization attempted by a gradual increase in dose from 1/8 tablet (approximately 6 mg) daily also led to recurrence. The timing of the initial pancreatitis and the recurrence at rechallenge are best explained by an allergic reaction. 6-Mercaptopurine should not be reinstituted once it has caused pancreatitis.