Vitamin B12 deficiency may present symptoms in several systems including skin, blood, nerve, digestive, and cardiovascular. 1. Skin manifestations: there may be widespread and symmetrical hyperpigmentation. Especially in the curved parts of the body, palms of the hands, soles of the feet, nails and mouth, the hair becomes gray and the nails may be hyperpigmented. 2. Hematological system: Vitamin B12 deficiency may result in anemia, which is visible on physical examination. The liver and spleen are enlarged, and in severe cases, there may be skin petechiae and ecchymosis. 3. Nervous system: peripheral neuropathy, demyelinating changes, subacute combined degeneration, or even dementia and coma may occur. Sometimes it is manifested as dull expression, reduced sensitivity to the environment, hypotonia, paralysis and so on. 4. Digestive system: mainly nausea, vomiting, loss of appetite, abdominal pain and diarrhea. 5. Cardiovascular system: Vitamin B12 deficiency may induce anemic heart disease and heart failure when it is more serious. Whether the lack of vitamin B12 needs to be diagnosed by a doctor, if you feel unwell, you should consult a doctor in a timely manner, under the professional guidance of the doctor for diagnosis and treatment.