250 µg–1 mg/day orally if deficient or on a vegan diet.
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Vitamin B12, or cobalamin, is a water-soluble vitamin with an exceptionally complex chemical structure (it includes a cobalt atom at its core, a feature unique among vitamins) that is synthesized exclusively by microorganisms (bacteria and archaea) in the environment, not by plants or higher animals; humans obtain B12 mainly from animal products (animals accumulate B12 synthesized by bacteria in their gut microbiota and/or by environmental bacteria), or from fortified products and supplements in people on strict plant-based diets.
Intestinal absorption of B12 is exceptionally complex and depends on several factors: gastric acid (which separates B12 from food proteins), intrinsic factor (a glycoprotein secreted by gastric parietal cells that binds B12 to allow its specific absorption in the terminal ileum), and the integrity of the distal ileum; these particularities explain why clinical B12 deficiency can arise not only from insufficient intake but also from atrophic gastritis (common in older people), gastrectomy, prolonged use of proton pump inhibitors or metformin, pernicious anemia (autoimmune against intrinsic factor), ileal disease, or gastric bariatric surgery.
The clearest indication is the treatment of clinical B12 deficiency, which causes megaloblastic anemia (with large red blood cells), neurological manifestations (paresthesias, ataxia, cognitive decline, and in advanced cases irreversible myelopathy), and other manifestations; the historical standard treatment was parenteral (intramuscular) cyanocobalamin or hydroxocobalamin, but modern data support that high-dose oral supplementation (1,000 micrograms/day) is equally effective for most causes of deficiency (except severe ileal malabsorption), even in pernicious anemia.
In people on strict vegan diets or vegetarian diets with very few animal products, regular B12 supplementation is absolutely essential because plant sources do not provide physiologically significant amounts (although some products such as spirulina and "algae B12" are promoted as alternative sources, they contain predominantly pseudovitamin B12, an analogous form with no biological activity for humans); typical recommended doses are 50-100 micrograms daily or 2,000 micrograms weekly, depending on the formulation.
In people over 50, atrophic gastritis with reduced gastric acid secretion affects up to 30 percent of the population and significantly reduces the absorption of B12 from dietary sources (especially protein-bound B12 in meat), whereas crystalline B12 from supplements and fortified foods is absorbed normally because it does not require the gastric separation step; international nutritional guidelines recommend that older adults meet their B12 needs preferably with fortified foods or supplements to avoid the subclinical deficiency common in this group.
In older adults with low or suboptimal B12 levels, several studies have documented modest improvements in cognitive function with supplementation over months; in people with normal levels, the benefits are quantitatively small or absent; correcting subclinical B12 deficiency is one of the few nutritional components with demonstrable potential cognitive impact in people with a demonstrated deficiency.
Nutritional doses are 2.4 micrograms daily in adults; therapeutic doses for clinical deficiency range from 1,000 to 2,000 micrograms/day orally or parenteral doses according to protocol; for preventive supplementation in vegans or older adults, 50-100 micrograms daily; the most common commercial forms are cyanocobalamin (the most stable and cheapest form, perfectly effective for most people), methylcobalamin (the active form, more expensive but theoretically more bioavailable although the clinical differences are modest), and hydroxocobalamin (the form used parenterally in acute conditions).
It is one of the vitamins with an exceptional safety profile: practically no adverse effects at nutritional, therapeutic, or even very high doses; excess is efficiently excreted in urine without significant accumulation; no clinically relevant toxicities have been described with prolonged use; it has no known specific contraindications; people with MTHFR gene polymorphisms may have slightly higher requirements; those taking metformin, proton pump inhibitors, H2 antagonists, or who have had a gastrectomy or bariatric surgery are at increased risk of subclinical deficiency and should monitor B12 levels periodically; in pregnancy and breastfeeding, adequate nutritional doses are critical for fetal and neonatal development of the nervous system, especially in strictly vegetarian mothers; monitoring of clinical deficiency is done with serum B12 (although "normal" levels may mask functional deficiencies, which is why methylmalonic acid and homocysteine are more sensitive functional markers); B12 supplementation is one of the safest, most effective, and most economical nutritional interventions available for clearly established specific indications.
Information, not medical advice. Scientific evidence reflects the quality of the studies.