Vitamin B12 Dosage Calculator
An essential cobalamin vitamin required for DNA synthesis, myelin sheath formation, and red blood cell production. Deficiency is common in vegans, elderly adults, and metformin users — and progresses silently for years before becoming irreversible.
What is Vitamin B12?
Vitamin B12 (cobalamin) is a water-soluble vitamin found exclusively in animal products, required for methionine synthase (homocysteine → methionine methylation), methylmalonyl-CoA mutase (odd-chain fatty acid catabolism), and myelin synthesis. Deficiency develops slowly over 3–5 years as liver stores deplete, then presents as megaloblastic anaemia (large, immature red blood cells causing fatigue, pallor, dyspnoea), subacute combined degeneration of the spinal cord (symmetrical paraesthesia, ataxia, spasticity — potentially irreversible), and neuropsychiatric symptoms (depression, cognitive decline, psychosis). Serum homocysteine elevates early in deficiency, increasing cardiovascular and thrombotic risk. **High-risk populations requiring physician-supervised testing and supplementation:** strict vegans and vegetarians (B12 is absent from plant foods); adults over 60 (gastric acid and intrinsic factor decline with age, impairing absorption); individuals on metformin (reduces ileal B12 absorption; mandatory monitoring after 4+ years of use); individuals on proton pump inhibitors > 2 years; those with pernicious anaemia (autoimmune destruction of intrinsic factor — requires B12 injections, not oral supplements, as absorption is intestinal-factor-independent). Always test serum B12, MMA (methylmalonic acid), and homocysteine before supplementing to establish baseline and confirm deficiency.
How to Take Vitamin B12
**RDA:** 2.4 mcg/day for adults; 2.6–2.8 mcg during pregnancy/lactation. **No established UL** (water-soluble, renally excreted). **Supplemental doses are intentionally supraphysiological** because oral B12 absorption is passive at high doses (~1% of dose) once intrinsic factor–mediated absorption is saturated at ~2 mcg: 500–1,000 mcg/day oral cyanocobalamin or methylcobalamin achieves sufficient passive absorption without intrinsic factor, making this effective even in partial malabsorption states. Methylcobalamin and adenosylcobalamin are the bioactive forms — preferred over cyanocobalamin for individuals with MTHFR variants or detoxification concerns. **Pernicious anaemia:** oral high-dose B12 (1,000–2,000 mcg/day) is equivalent to injections in controlled trials when adherence is assured, but injections remain first-line for confirmed neurological symptoms.
Timing Recommendations
No timing requirement. Take with or without food. Sublingual B12 bypasses gut absorption entirely and is preferred for individuals with atrophic gastritis or suspected malabsorption.
Potential Side Effects & Safety
B12 is remarkably safe — no toxicity has been established even at doses thousands of times the RDA (no UL set by the IOM). Rare: acneiform rash (cobalt-related) at very high doses in sensitive individuals. Cyanocobalamin releases small amounts of cyanide upon metabolism — clinically irrelevant at standard doses but methylcobalamin is preferred in renal failure or smokers.
Who should avoid Vitamin B12?
No absolute contraindications. Leber's hereditary optic neuropathy (LHON): cyanocobalamin is specifically contraindicated — use methylcobalamin or hydroxocobalamin. Polycythaemia vera or active malignancy: B12 stimulates cell proliferation; consult a haematologist or oncologist before high-dose supplementation.
Best Stacks with Vitamin B12
B12 + methylfolate (5-MTHF) + B6 is the homocysteine-lowering triad. B12 + iron for comprehensive anaemia management. Sublingual B12 + intrinsic factor lozenges for elderly individuals with absorption impairment.
Scientific References
All dosage recommendations are grounded in peer-reviewed research.
- 1Vitamin B12 deficiency: recognition and management
American Family Physician · 2017
- 2Metformin use and vitamin B12 deficiency: a systematic review and meta-analysis
Diabetes & Metabolism · 2019
Vitamin B12 Dosage Calculator
Fixed dosage — independent of body weight
Your recommended daily dosage
Formula: RDA: 2.4 mcg/day | Supplemental: 500–2,000 mcg/day oral (passive absorption ~1% of dose) | No established UL
Safety notes
- Test serum B12, MMA, and homocysteine BEFORE supplementing — deficiency and insufficiency have different treatment protocols.
- Pernicious anaemia: injections may be required — oral supplementation alone is not sufficient for all cases.
- Metformin users: B12 monitoring every 2 years is recommended — metformin reduces B12 absorption.
- Leber's optic neuropathy (LHON): cyanocobalamin is contraindicated — use methylcobalamin only.
- Vegans: routine B12 supplementation is not optional — it is mandatory for long-term neurological health.
This calculator provides general guidance only. Always consult a qualified healthcare professional before starting any supplement.