The active ingredient in a drug used to lessen the toxic effects of substances that block the action of folic acid, especially the anticancer drug methotrexate. Leucovorin 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 is a form of folic acid. It is a type of chemoprotective agent and a type of chemosensitizing agent.