This vitamin is best-known for its role in healthy blood formation; it is needed to form red blood cells in the bone marrow. Consequently, a deficiency can result in pernicious anemia. Poor absorption of this vitamin causes a high percentage of older adults to feel fatigued or tired. A deficiency of this vitamin may also be associated with age-related hearing loss and age-related cognitive impairment, and has been implicated in a variety of neuropsychiatric disorders and mild to severe physical symptoms, including confusion, memory loss, leg and finger incoordination, peripheral neuritis, depression, moodiness and psychosis. Fortunately, oral supplementation has been shown to be an effective alternative to painful intramuscular injections commonly used in the treatment of this vitamin deficiency. What is this vitamin?