An active metabolite of folic acid (also called folinic acid and citrovorum factor), which does not require metabolism by dihydrofolate reductase, the molecular target of folate antagonist-type chemotherapeutic drugs. Leucovorin calcium counteracts the toxic effects of these medications, 'rescuing' the patient while permitting the antitumor activity of the folate antagonist. This agent also potentiates the effects of fluorouracil and its derivatives by stabilizing the binding of the drug's metabolite to its target enzyme, thus prolonging drug activity. or A drug used to lessen the toxic effects of substances that block the action of folic acid, especially the anticancer drug methotrexate. Leucovorin calcium is used to treat some types of anemia and is also used with fluorouracil to treat colorectal cancer. It is also being studied in the treatment of other types of cancer and other conditions. Leucovorin calcium is a form of folic acid. It is a type of chemoprotective agent and a type of chemosensitizing agent. Also called calcium levoleucovorin, citrovorum factor, and Wellcovorin.