Best fully active B12 provides methyl-cobalamin, the most biochemically active form of vitamin B12. Various inherited and lifestyle factors contribute to vitamin B12 deficiencies that impair such fundamental life processes as recycling of homocysteine, a potentially toxic metabolite, nerve cell signaling, mood and memory management, and healthy gene regulation (epigenetics), from pregnancy through old age. Best fully active B12 is vitamin B12 at its best.