Blood formation, myelin/nerve function and DNA synthesis.
Switzerland age 18–65: 4 µg/day.
Fish, meat, eggs, dairy and reliably fortified vegan foods.
Fatigue, anaemia, inflamed tongue, tingling/numbness, balance, memory and mood symptoms.
Neurological injury can become permanent without treatment and can occur without marked anaemia.
Pallor and tongue changes are possible; hair/nails are not a reliable B12 test.
Vegan diet, older age, pernicious anaemia, gastric/intestinal surgery, malabsorption, metformin and acid suppressants.
B12 with full blood count; use methylmalonic acid and clinical context in unclear cases.
People following a vegan diet need a reliable B12 source, usually a supplement or fortified foods. Treat confirmed deficiency specifically.
No usual toxicity limit established; high doses without indication have no proven added benefit.