The active ingredient in a drug used to lessen the toxic effects of the anticancer drug methotrexate or other substances that block the action of folic acid. It is also used to treat some types of anemia and is used with fluorouracil to treat advanced colorectal cancer. It is also being studied in the treatment of other conditions and types of cancer. Leucovorin is a form of folic acid. It is a type of chemoprotective agent and a type of chemosensitizing agent.