Allopurinol interferes with degradation of azathioprine to cause a 3- to 4-fold increase in its cytotoxic capacity. This can result in severe and potentially fatal pancytopenia. A patient is reported who was receiving azathioprine and had self-administered allopurinol for an attack of gout. Multinucleated red cell precursors with grossly abnormal features relative to normal erythroblasts provided evidence of the resulting bone marrow toxicity.

Patients taking azathioprine must be alerted to the danger of co-medication with allopurinol.

Reference: Australian Prescriber 22 (1): 19, 1999.