Vitamin B12 is required for proper red blood cell formation as well as neurological function, and DNA synthesis. This vitamin is essential to protect the body against anemia and support a healthy central nervous system. Vitamin B12  is a water-soluble and is naturally present in some foods. Vitamin B12 is available as a dietary supplement and a prescription medication, it exists in several forms and contains the mineral cobalt. That's why compounds with vitamin B 12 activity are collectively called "cobalamins".

Form of Vitamin B12.

Methyl and Deoxyadenosy (lcobalamins) are the forms of vitamin B 12 that are active in human metabolism. Vitamin B 12, bound to protein in food. It is released by the activity of hydrochloric acid and gastric protease in the stomach. When synthetic vitamin B 12 is added to fortified foods and dietary supplements, it is already in free form and, thus, does not require this separation step. Free vitamin B 12 then combines with intrinsic factor, a glycoprotein secreted by the stomach's parietal cells, As a result, the complex undergoes absorption within the distal ileum by receptor-mediated endocytosis. Approximately 56% of a 1 mcg oral dose of vitamin B 12 is absorbed. but absorption decreases drastically when the capacity of intrinsic factor is exceeded (at 1–2 mcg of vitamin B 12).

Pernicious anemia is an autoimmune disease that affects the gastric mucosa and results in gastric atrophy. This leads to the destruction of parietal cells, achlorhydria, and failure to produce intrinsic factor, resulting in vitamin B12 malabsorption. Therefore, if pernicious anemia is left untreated, it causes vitamin B12 deficiency. This may lead to megaloblastic anemia and neurological disorders, even in the presence of adequate dietary intake of vitamin B12.