Vitamin B12 (cyanocobalamin) is a water-soluble vitamin primarily from animal sources that is an integral cofactor for enzymes involved in fatty acids, myelin, and deoxyribonucleic acid (DNA) synthesis. There are rare genetic disorders that can result in deficiency despite adequate dietary intake. Its deficiency can result in hematologic and neurologic abnormalities. Oral or parenteral supplementation of vitamin B12 compounds can correct the deficiency; however, severe neurological manifestations are irreversible if diagnosis is delayed.