Imuran is metabolized to 6-mercaptopurine [6-MP] in the body. I wrote the following sometime ago in response to a similiar question:
Azathioprine [Imuran, Azacan, etc] is a folic acid antagonist. Folic acid [folate; B9] is found in highest concentrations of foods that people with CD often avoid, fresh, uncooked fruits and vegetables. While your doctor may have measured your folate level in your blood and pronounced it "normal" it still could be too low. A folate level on the lower end of the normal range in a person with CD can lead to deficiency. Since azathioprine interferes with folate, and your diet might be insufficient in folate, deficiency can occur. My doctor prescribed Folic Acid 1 mg daily. Within two months of adding this, my excessive hair loss had stopped and I was regrowing hair.
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CD, Ankylosing Spondylitis, lupus, small fiber peripheral neuropathy, avascular necrosis, peripheral artery disease, degenerative disc disease, asthma, severe allergy and a host of other medical problems.