Deficiency care may use oral 1,000–2,000 µg/day or injections initially; duration depends on cause
Reviewed · Published
Vitamin B12
Required for DNA synthesis, red-cell formation, and maintenance of nerve myelin.
Established functions
- Required for DNA synthesis, red-cell formation, and maintenance of nerve myelin.
Dietary sources
- clams
- beef
- dairy foods
Claims and evidence
Claims that Vitamin B12 alone prevents or treats disease, or invariably improves health without deficiency, are unsupported.
Japanese reference amounts
- Japanese adults (age and sex as specified): Adults 4.0 µg/day; pregnancy 4.5 µg/day; lactation 5.5 µg/day (RDA)
Amounts studied
- Deficiency care may use oral 1,000–2,000 µg/day or injections initially; duration depends on cause
Tolerable upper limits
- No UL is established; the need for high doses should be diagnosis-based.
Forms and bioavailability
Supplement forms: cyanocobalamin, methylcobalamin
- Intrinsic-factor absorption saturates; a small fraction of high doses diffuses passively.
Safety and interactions
Adverse effects
- rare acneiform eruptions and injection hypersensitivity
Medicine and nutrient interactions
- metformin, acid suppressants, nitrous oxide
Contraindications and cautions
- People with Leber hereditary optic neuropathy should seek specialist advice about cyanocobalamin.
Practical selection
- Compare Vitamin B12 amount per serving, form, allergens, and batch testing; prefer a single-ingredient product suited to the intended use.
Sources and review
- Dietary Reference Intakes for Japanese (2025)Ministry of Health, Labour and Welfare (Japan)
- NIH ODS vitamin fact sheetsNIH Office of Dietary Supplements
Daiki Minaki (editorial and evidence review) ·