Vitamin B12 (Cobalamin) — Complete Supplement Guide (vegan, forms, dose)

SupplementsVitaminsB12 CobalaminWater-soluble

It comes essentially only from animal foods and relies on a fragile, multi-step absorption route — which is why vegans and many older adults genuinely need a supplement. For the short dietary version, see the Vitamins page.

Read first. B12 is very safe even at high doses. The real risk is missing a deficiency: untreated, it causes permanent nerve damage. Vegans must supplement, and folic acid can hide the anaemia — so test B12 if in doubt. Information, not a prescription.

Key takeaways

Comes almost only from animal foods — vegans and many older adults need a supplement.
Adults need ≈ 2.4–4 µg/day (vegans ~10–50 µg/day, or 1,000 µg weekly).
No upper limit — very low toxicity.

Function

Vitamin B12 works closely with folate:

Nerve function: maintains the myelin sheath around nerves — deficiency causes numbness, tingling and, untreated, permanent damage.
Red blood cells: needed to form healthy red cells; lack causes megaloblastic anaemia.
DNA synthesis: essential for making DNA in dividing cells.
Folate cycle: regenerates active folate — which is why B12 deficiency mimics folate deficiency.
Brain & mood: supports normal cognitive function and energy.

Different forms — and which to choose

CyanocobalaminStable, cheap and well-studied; the body converts it to the active forms.

Methyl- / adenosylcobalaminThe active forms; preferred by some, though evidence of superiority is limited.

HydroxocobalaminThe injection form used medically for pernicious anaemia and severe deficiency.

Absorption in the body

How muchComplex: needs stomach acid to release it and intrinsic factor to absorb it. This system declines with age and with PPIs/metformin.

High-dose oralAbove ~500–1000 µg, about 1% is absorbed passively without intrinsic factor — why high-dose oral tablets work even in mild malabsorption.

Deficiency symptoms

SymptomsMegaloblastic anaemia, fatigue, tingling/numbness, balance problems, memory and mood changes; if prolonged, irreversible nerve damage.

Who's at riskVegans/vegetarians, older adults, pernicious anaemia, and people on PPIs or metformin.

Recommended intake

Adults ≈ 2.4–4 µg/day

Reference intakes:

Adults: ~2.4 µg (US) to ~4 µg (EFSA) per day

Best sources: essentially only animal foods — meat, fish, eggs, dairy — plus fortified foods and supplements.¹ ²

Therapeutic use

DeficiencyHigh-dose oral (~1000 µg/day) or injections for pernicious anaemia and severe deficiency.

VeganA routine supplement (or reliably fortified foods) — non-negotiable on a plant-based diet.

Therapeutic doses

Vegan maintenance: ~10–50 µg/day or 1000 µg weekly

EverydayVegans: a daily low dose or a larger weekly dose; check with your practitioner.

RepletionEstablished deficiency is treated with high-dose oral or injections under care.

Toxicity & upper limit

No UL — very low toxicity

Upper limitNone established; excess is excreted and toxicity is very low even at high doses.¹

Real riskThe danger is under-treatment, not overdose — don't let a deficiency go unaddressed.

Bottom line: if you eat animal foods and digest normally, you likely get enough B12. Vegans and vegetarians must supplement, and older adults and PPI/metformin users should check their levels — because the nerve damage from prolonged deficiency can be permanent.

← All supplements Short dietary version

References

Reference and upper-limit values are general adult figures from official bodies; individual needs vary. Therapeutic doses describe clinical/research use, not prescriptions.

US NIH Office of Dietary Supplements. Vitamin B12 — Health Professional Fact Sheet (function, forms, intakes, food sources, deficiency, therapeutic uses, upper limit, interactions). ods.od.nih.gov
EFSA. Dietary Reference Values & Tolerable Upper Intake Levels for vitamins (European reference intakes and ULs). efsa.europa.eu

This article summarizes nutrition science from official health authorities and peer-reviewed sources for education. It is not a substitute for individual medical advice, and the doses given are not prescriptions — always consult a professional before therapeutic supplement use, in pregnancy, or alongside medication.

Vitamin B12 (Cobalamin) — The Complete Supplement Guide