The principal toxic effects on the bone marrow and hematological toxicity are likely to be more profound with chronic overdosage than with a single ingestion of Mercaptopurine. The risk of overdosage is also increased when allopurinol is being given concomitantly with Mercaptopurine. As there is no known antidote the blood picture should be closely monitored and general supportive measures, together with appropriate blood transfusion, instituted if necessary.