Symptoms of allergic reaction to Mercaptopurine include rash, itching, swelling, dizziness, trouble breathing.
Mercaptopurine causes myelosuppression, suppressing the production of white blood cells. It may be toxic to bone marrow. Weekly blood counts are recommended for patients on mercaptopurine. The patient should stop taking the medication at least temporarily if there is an unexplained, abnormally large drop in white blood cell count, or any other blood count.
Patients who exhibit myelosuppression or bone marrow toxicity should be tested for Thiopurine methyltransferase (TPMT) enzyme deficiency. Patients with TPMT deficiency are much more likely to develop dangerous myelosuppression. In such patients it may be possible to continue using mercaptopurine, but at a lower dose.