5-MTHF 5 mg5 MTHF, also known as Methylfolate, is the preferred form of folate supplementation because it is the active form of folate used by the bodys tissues.* Many individuals don't get sufficient 5 MTHF (L 5 Methyltetrahydrofolate), the active form of folic acid, because they have intestinal or liver dysfunction, or because they are among the three out of five Americans whose genetic makeup makes it difficult to convert folic acid into active 5 MTHF.