Leucovorin injection is used to prevent harmful effects of methotrexate (Rheumatrex, Trexall; cancer chemotherapy medication) when methotrexate is used to to treat certain types of cancer. Leucovorin injection is used to treat people who have accidentally received an overdose of methotrexate or similar medications. Leucovorin injection is also used to treat anemia (low level of red blood cells) caused by low levels of folic acid in the body. Leucovorin injection is also used with 5-fluorouracil (a chemotherapy medication) to treat colorectal cancer (cancer that begins in the large intestine). Leucovorin injection is in a class of medications called folic acid analogs. It treats people who are receiving methotrexate by protecting healthy cells from the effects of methotrexate. It treats anemia by supplying folic acid that is needed for red blood cell formation. It treats colorectal cancer by increasing the effects of 5-fluorouracil.