Calcium leucovorin is the antidote for neutralising the immediate toxic effects of methotrexate on the haemopoietic system. It may be administered orally, intramuscularly or by an intravenous bolus injection or infusion. In cases of accidental overdosage, a dose of calcium leucovorin equal to or higher than the offending dose of methotrexate should be administered within one hour and dosing continued until the serum levels of methotrexate are below 10-7M. Other supporting therapy such as a blood transfusion and renal dialysis may be required.