azathioprine, immunosuppressive drug that is used to treat rheumatoid arthritis and to suppress the body’s rejection of transplanted organs. By inhibiting several enzymatic pathways required for the synthesis of deoxyribonucleic acid (DNA), azathioprine decreases the number of lymphocytes (a type of white blood cell) that can migrate to inflammatory sites. Leukopenia (an abnormally low number of white blood cells in the circulation) is the major undesirable side effect of the drug, although anemia and hemorrhage may also occur. When used with allopurinol, the pharmacological effect of azathioprine is increased.