Summary: Vitamin B12 (also known as cobalamin) is a B vitamin that plays an important role in cellular metabolism, especially DNA synthesis, methylation, and mitochondrial metabolism. Clinical B12 deficiency with classic hematologic and neurologic manifestations is relatively rare. In clinical practice, many cases of vitamin B12 deficiency are overlooked and sometimes even misdiagnosed. Vitamin B12 absorption deficiency is not infrequently encountered in patients with gastrointestinal diseases or after gastrointestinal surgery. Similarly, long-term use of acid-suppressing drugs has been linked to vitamin B12 deficiency. Such patients require chronic treatment and, as shown, oral administration of vitamin B12 has been no less effective than intramuscular administration.