Vitamin B12 deficiency is a condition in which the body has inadequate stores of vitamin B12. This vitamin is essential for many aspects of health, including the production of red blood cells.Healthy numbers of red blood cells are critical for delivering oxygen and nutrients to the body’s cells and tissues.itamin B12 deficiency is most common in people who have a disease or condition in which the body is unable to absorb vitamin B12 from ingested foods. It is also relatively common in older adults.