Indication for the treatment of osteosarcoma (after high dose methotrexate therapy). used to diminish the toxicity and counteract the effects of impaired methotrexate elimination and of inadvertent overdosages of folic acid antagonists, and to treat megaloblastic anemias due to folic acid deficiency. also used in combination with 5-fluorouracil to prolong survival in the palliative treatment of patients with advanced colorectal cancer.mechanism of action as leucovorin is a derivative of folic acid, it can be used to increase levels of folic acid under conditions favoring folic acid inhibition (following treatment of folic acid antagonists such as methotrexate). leucovorin enhances the activity of fluorouracil by stabilizing the bond of the active metabolite (5-fdump) to the enzyme thymidylate synthetase.