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How much vitamin B12 do vegans need? Dosage by frequency

The correct B12 dose for vegans: 25–100 mcg/day or 2,000 mcg/week of cyanocobalamin. Based on NIH and Vegan Society guidelines. Includes dosage table, absorption math and monitoring schedule.

🗓️ Updated June 2026 Reviewed by Martín Rodríguez

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Reviewed by: Martín Rodríguez (editorial policy ) · Last reviewed: Jun 20, 2026

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Vitamin B12 is the one nutrient that every vegan must supplement — no plant food reliably provides it in absorbable quantities. This calculator applies the dosage schedules recommended by the NIH Office of Dietary Supplements and the Vegan Society: 25–100 mcg daily or 2,000 mcg once a week. Choose your preferred supplement frequency and get the correct target dose, the best form to take, and the monitoring schedule to confirm it's working.

When to use this calculator

A new vegan who wants to know exactly how much B12 to buy and take from day one
A long-term vegan switching from daily to weekly dosing who wants to confirm the correct weekly amount
A dietitian or GP advising plant-based patients on evidence-based supplement schedules
A parent of a vegan child checking whether the adult schedule applies (it does not — see FAQ)

Vegan B12 Supplementation: Dose by Frequency & Reference Blood Markers

Parameter	Value	Notes
Daily dose (oral)	25–100 mcg	Absorbed via active transport (intrinsic factor); efficient at low doses
Weekly dose (oral)	2,000 mcg	~20 mcg absorbed via passive diffusion (~1% of dose)
Monthly dose	Not recommended	Liver stores cannot be replenished from a single oral dose
Active transport capacity	~1.5–2 mcg per dose	Saturates quickly; applies to low-dose or food B12
Passive diffusion absorption	~1% of dose	Applies to high-dose supplements; unlimited capacity
Adult RDA (NIH)	2.4 mcg/day	2,000 mcg/week yields ~2.9 mcg/day equivalent ✓
Serum B12 (normal)	200–900 pg/mL	Low sensitivity for functional deficiency
Holotranscobalamin (active B12)	≥35 pmol/L	Earlier marker of depletion than serum B12
Methylmalonic acid (MMA)	<0.4 μmol/L	Most sensitive functional marker of B12 status
Homocysteine	<12 μmol/L	Elevated when B12 or folate is insufficient

Fuente: NIH Office of Dietary Supplements — Vitamin B12 Health Professional Fact Sheet; The Vegan Society — What Every Vegan Should Know About Vitamin B12

How it works

How much B12 does a vegan need?

This calculator returns the evidence-based dosage for each supplementation frequency, grounded in the physiology of B12 absorption — not a mathematical formula.

Vegan B12 dosage table by frequency

Frequency	Recommended dose	Estimated absorption	Basis
Daily	25–100 mcg	~2–3 mcg/day	Active route (intrinsic factor), efficient at low doses
Weekly	2,000 mcg	~20 mcg/dose	Passive diffusion (~1% of dose)
Monthly	Not recommended	Insufficient	Liver stores cannot be replenished from a single oral dose

Why the dose varies by frequency

The gut absorbs B12 by two routes:

Route	Mechanism	Capacity	When it applies
Active transport	Intrinsic factor (IF) glycoprotein	~1.5–2 mcg per dose	Low-dose supplements, food
Passive diffusion	Direct mucosal absorption	~1% of dose, unlimited	High-dose supplements

Weekly dose vs. daily absorbed B12 — reference table

Weekly dose	Absorbed via diffusion	Daily equivalent
500 mcg	~5 mcg	~0.7 mcg/day
1,000 mcg	~10 mcg	~1.4 mcg/day
1,500 mcg	~15 mcg	~2.1 mcg/day
2,000 mcg	~20 mcg	~2.9 mcg/day ✓
2,500 mcg	~25 mcg	~3.6 mcg/day

Adult RDA: 2.4 mcg/day (NIH). A weekly 2,000 mcg dose exceeds this threshold.

Why vegans need supplemental B12

Vitamin B12 (cobalamin) is synthesised only by certain bacteria and archaea. It accumulates in animal tissues, making meat, fish, eggs and dairy the main dietary sources. Whole plant foods contain no reliably absorbable B12; fermented foods, algae and mushrooms provide amounts too small and variable to count on. Vegans who don't supplement consistently develop deficiency — sometimes years before symptoms appear — because the liver stores 2–5 mg of B12 and depletion is slow.

Cyanocobalamin vs. methylcobalamin

Cyanocobalamin is the standard recommendation for vegan supplementation because:

It is the most chemically stable form (shelf-stable at room temperature)

It has the largest evidence base from clinical trials

It is the most cost-effective form

The body readily converts it to methylcobalamin and adenosylcobalamin (the two active forms)

Methylcobalamin is bioactive without conversion, but is less stable, more expensive, and has fewer controlled trials in vegans. It is an acceptable alternative but not superior for most healthy adults.

Reference blood values

Biomarker	Normal range (adults)	Notes
Serum B12	200–900 pg/mL	Low sensitivity for functional deficiency
Holotranscobalamin (active B12)	>35 pmol/L	Earlier marker of depletion
Methylmalonic acid (MMA)	<0.4 μmol/L	Most sensitive functional marker
Homocysteine	<12 μmol/L	Elevated when B12 or folate is low

> Disclaimer: This calculator is for educational reference only and does not replace personalised medical advice. If you have symptoms of deficiency (fatigue, tingling, anaemia, neurological changes) or abnormal blood results, consult a healthcare professional.

Worked Example — Weekly Dosing

Selected frequency: Weekly

Intrinsic-factor active transport saturates at ~1.5–2 mcg per dose

Passive diffusion absorbs ~1% of an unbound oral dose

To deliver ≥16.8 mcg/day via passive diffusion once a week: 2,000 mcg × 1% ≈ 20 mcg absorbed per dose, spread over 7 days ≈ ~2.9 mcg/day — above the adult RDA of 2.4 mcg/day

2,000 mcg cyanocobalamin once per week | Serum B12 + methylmalonic acid check every 6–12 months

Frequently asked questions

How much vitamin B12 should a vegan take per day?

A vegan adult should take 25–100 mcg of cyanocobalamin per day if supplementing daily. This range uses the active absorption route (intrinsic factor) efficiently — it saturates at ~1.5–2 mcg per dose, so a small daily pill well above 2.4 mcg ensures the RDA is met. If you prefer a single weekly pill, the correct dose is 2,000 mcg.

Why is 2.4 mcg per day (the adult RDA) not enough for vegans to supplement with?

The RDA assumes the body will absorb B12 from food via intrinsic factor across multiple small meals. A single supplement dose saturates intrinsic factor at about 1.5–2 mcg; anything above that relies on passive diffusion at only 1% efficiency. So a 2.4 mcg supplement pill delivers only about 1.7 mcg total — below the RDA. Supplements must be high-dose to compensate.

Can I take 1,000 mcg of B12 every day instead of 2,000 mcg once a week?

Yes. Taking 1,000 mcg daily delivers about 1,000 × 1% = 10 mcg by passive diffusion plus ~2 mcg via intrinsic factor ≈ 12 mcg/day — well above the RDA and a safe approach. This is often used by people recovering from deficiency or with malabsorption. The 25–100 mcg/day range given by this calculator is the minimum-effective approach for healthy vegans without deficiency.

Why doesn't monthly dosing work?

To supply 2.4 mcg/day for 30 days via passive diffusion alone (≈72 mcg total absorbed), you would need a 7,200 mcg single dose — and no guideline recommends this because absorption efficiency, gut transit, and safety at that dose haven't been established in trials. Monthly injections of hydroxocobalamin are used medically, but that is a prescription treatment for pernicious anaemia, not oral supplementation.

Is cyanocobalamin safe? It contains cyanide.

Yes. The tiny amount of cyanide in a cyanocobalamin supplement (a few micrograms) is far below any toxicological threshold and is rapidly excreted. Heavy smokers may prefer methylcobalamin because their bodies already process more cyanide, but for non-smokers cyanocobalamin is equally safe and better studied. No tolerable upper intake level (UL) has been established for B12 because no adverse effects from high oral doses have been documented.

What are the first signs of B12 deficiency?

Early signs include fatigue, brain fog, and glossitis (sore, inflamed tongue). As deficiency deepens: macrocytic megaloblastic anaemia, peripheral tingling or numbness (subacute combined degeneration of the spinal cord), and mood changes. Neurological damage can be irreversible if deficiency persists for years. Deficiency can take 3–5 years to become symptomatic because of liver stores, making preventive supplementation essential from the start of a vegan diet.

Which blood tests should vegans get to monitor B12 status?

Serum B12 alone is not reliable — it can look normal even when functional B12 is depleted. A complete B12 panel includes: (1) Serum B12, (2) Holotranscobalamin (active B12, the earliest marker of depletion), (3) Methylmalonic acid (MMA) — the most sensitive functional marker, and (4) Homocysteine. Ask your doctor to check all four if you suspect deficiency. Recheck every 6–12 months while on a stable supplement regimen.

Do B12-fortified foods count toward the recommended intake?

Yes, if consumed consistently. The Vegan Society recommends eating fortified foods 2–3 times a day to get at least 3 mcg of B12 daily from food (e.g., fortified plant milks, nutritional yeast, breakfast cereals). However, fortification levels vary by brand and country, and eating habits change — supplementation is more reliable and is the approach recommended by most vegan nutrition experts.

Do vegan children need a different B12 dose?

Yes. The RDA for B12 varies by age: 0.9 mcg/day (1–3 years), 1.2 mcg/day (4–8 years), 1.8 mcg/day (9–13 years), and 2.4 mcg/day (adults). Supplemental doses for children should be adjusted accordingly and ideally confirmed with a paediatrician or registered dietitian specialising in plant-based nutrition. This calculator is designed for adults only.

Can pregnant or breastfeeding vegans use this calculator?

Pregnant and lactating people have higher B12 needs (2.6 mcg/day and 2.8 mcg/day respectively per NIH). The dosage ranges in this calculator provide a safe margin above those levels, but if you are pregnant, breastfeeding, or planning pregnancy on a vegan diet, consult a healthcare provider — they may recommend a higher or more closely monitored supplementation protocol.

Sources & references

Methodology & trust

Editorial

Calculadora de salud revisada por el equipo editorial de Hacé Cuentas, contrastada con NIH Office of Dietary Supplements — Vitamin B12 Health Professional Fact Sheet, según nuestra política editorial y metodología.

Updates

Última revisión: June 20, 2026. Los parámetros se verifican periódicamente con las fuentes citadas.

Privacy

Calculations run 100% in your browser. We do not store or transmit your data.

Limitations

Indicative results. For critical decisions, consult a professional.

📌 How to cite this calculator

Rodríguez, M. (2026). How much vitamin B12 do vegans need? Dosage by frequency. Hacé Cuentas. https://hacecuentas.com/vitamin-b12-dosage-vegan-monthly

Contenido bajo licencia CC-BY 4.0 — reutilizable citando la fuente con enlace a Hacé Cuentas.

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