Classically presents with megaloblastic anemia, but can also present with peripheral neuropathy and neuropsychiatric complaints.
Older people, patients with chronic malabsorption, patients with a history of gastric resection or bypass, and those taking certain medications (metformin, proton-pump inhibitors) are at risk.
Early diagnosis is critical in preventing and halting the progression of neurologic disorders such as peripheral neuropathy and dementia.
Methylmalonic acid and homocysteine levels may help to diagnose vitamin B12 deficiency at an early, asymptomatic state.
Cause of vitamin B12 deficiency should be searched for once a diagnosis is confirmed.
Treatment with high-dose oral vitamin B12 therapy may be as effective as intramuscular vitamin B12 therapy.
Vitamin B12 deficiency is a common condition that can manifest with neurologic, psychiatric, and hematologic disorders. Vitamin B12 is an essential vitamin, and deficiency generally occurs with inadequate absorption or lack of dietary intake. While severe deficiency can cause permanent neurologic damage, earlier manifestations are generally subtle or asymptomatic.
The likelihood of vitamin B12 deficiency is defined according to the serum vitamin B12 level as follows: <200 picograms/mL indicates probable deficiency, 201 to 350 picograms/mL indicates possible deficiency, and >350 picograms/mL indicates that deficiency is unlikely.  
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Madigan Army Medical Center
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