This chemically reduced derivative of folic acid is used generally as an antidote to drugs that act as folic acid antagonists. Specifically, it can reverse the toxicity of methotrexate and potentiate and modulate flourouracil. Leucovorin can be administered orally, intramuscularly, or intravenously. Chemically, leucovorin is N[4[[(2amino-5-formyl1,4,5,6,7,8hexahydro4oxo6-pteridinyl)methyl]amino]benzoyl],calcium salt (1:1). Possible side effects can include: increased pernicious anemia symptoms, allergic reactions, nausea, and vomiting.