Azathioprine may cause a rare type of lymphoma (cancer) of the liver, spleen, and bone marrow that can be fatal. This has occurred mainly in teenagers and young men with Crohn's disease or ulcerative colitis.
Before taking this medicine
You should not take  azathioprine if you are allergic to azathioprine.
You should not use azathioprine to treat rheumatoid arthritis if you are pregnant. This medicine can harm an unborn baby. Use effective birth control to prevent pregnancy while using this medicine.
Azathioprine may cause a rare type of lymphoma (cancer) of the liver, spleen, and bone marrow that can be fatal. This has occurred mainly in teenagers and young men with Crohn's disease or ulcerative colitis. However, anyone with an inflammatory autoimmune disorder may have a higher risk of lymphoma. Talk with your doctor about your own risk.
While taking azathioprine, you may have a higher risk of developing skin cancer. Ask your doctor about skin symptoms to watch for.
Tell your doctor if you have ever had:
kidney disease, or a kidney transplant (if you are using azathioprine for rheumatoid arthritis);
any type of viral, bacterial, or fungal infection;
liver disease; or
chemotherapy with medications like cyclophosphamide, chlorambucil, melphalan, busulfan, and others.
You should not breast-feed while you are using azathioprine.
