Patients with a vitamin B12 deficiency classically present with megaloblastic anemia, atrophic glossitis, localized or generalized skin pigmentation, neurological changes characterized by symmetrical lower extremity neuropathy greater than upper extremity neuropathy with paresthesia and ataxia that may progress to weakness, spasticity, cerebellar ataxia, and personality changes. Concomitant folate deficiency is often present and may mimic many symptoms of vitamin B12 deficiency.

Vitamin B12, or cobalamin, is a water-soluble vitamin found in meat, fish, and dairy products and, once combined with intrinsic factor in the stomach, is absorbed in the ileum. Patients at risk for vitamin B12 deficiency include those with a lack of intrinsic factor (pernicious anemia) or antibodies to intrinsic factor, malabsorption syndromes (especially those who have had a gastrectomy, gastric bypass, or chronic pancreatitis), or infections such as Helicobacter pylori or tapeworm infections. In addition, strict vegans, older adults, people with alcohol use disorder, and people with selective eating are particularly vulnerable to vitamin B12 deficiency. Elderly patients are at risk due to the high prevalence of atrophic gastritis. Patients with autoimmune conditions such as Graves disease, thyroiditis, and vitiligo are also at risk.