B-Complex is a comprehensive formula including Metafolin®, Methylcobalamin Vitamin B12, and Intrinsic Factor in its formulation. Metafolin® contains only the S isomer of 5–MTHF and has been shown to be the only form of folate to be able to cross the blood–brain barrier. Studies indicate that methylcobalamin, a coenzyme form of B12, may be better utilized and better retained in the body. Intrinsic factor is a protein produced by cells in the stomach lining and is needed for the intestines to absorb vitamin B12 efficiently.