I've been on mercaptopurine for about a year. I've had some pretty severe back pain since October. My PCP ordered an MRI about 2 months ago, showing arthritic changes in my lumber spine. I don't know if it is due to the medication, the crohn's or just getting older, but it is seriously affecting my life. And due to the Crohn's, I can't take Advil or other NSAIDs.