As for this invention, being something which offers the method of prescribing the folic acid metabolism competition medicine to the mammal where there is a necessity, as for the said method it includes the fact that it prescribes the said folic acid metabolism competition medicine of the effective quantity combining with the methyl malonic acid decrease medicine.本発明は、必要のある哺乳動物に葉酸代謝拮抗薬を投与する方法を提供するものであって、該方法は有効な量の該葉酸代謝拮抗薬をメチルマロン酸低下薬と組み合わせて投与することを含む。