When to check for vitamin B12 deficiency, and how to treat if low?

Vitamin B12 deficiency should be tested for in people who have at least one common symptom or sign (such as anaemia, neurological problems, or cognitive difficulties) and at least one common risk factor (such as dietary restrictions, certain health conditions, or medication use).

Initial tests include either total B12 (serum cobalamin) or active B12 (serum holotranscobalamin), unless during pregnancy or suspected recreational nitrous oxide use, where plasma homocysteine or serum methylmalonic acid (MMA) are used. Blood samples for diagnostic tests should be taken before starting vitamin B12 replacement.

If vitamin B12 deficiency is confirmed, treatment options include oral vitamin B12 supplements (at least 1 mg daily during pregnancy or breastfeeding) or intramuscular injections, especially if rapid deterioration or adherence issues are concerns. Lifelong replacement may be necessary in cases caused by autoimmune conditions like autoimmune gastritis.