Methyl folate (6S) -5-methyltetrahydrofolic acid or (6S) -5-MTHF) is the most biologically active form of folic acid. Quatrefolic is the fourth generation development in folate administration. With certain hereditary errors in metabolism and physiological stresses, the transformation of folic acid into the active form of folate is inadequate. Unlike folic acid, 5-MTHF can cross the blood-brain barrier.