Vegan B12 Dosage Calculator: Daily vs Weekly
Vitamin B12 (cobalamin) does not occur reliably in plant foods. All B12 on Earth is synthesized by bacteria; animals accumulate it by consuming bacteria-rich food or water. Vegans must supplement — deficiency causes megaloblastic anemia, peripheral neuropathy, and irreversible neurological damage. The adult RDA is 2.4 mcg/day, but supplement doses are far higher because absorption drops sharply as the oral dose increases. Evidence-based guidance (Vegan Society / NIH) recommends 250 mcg/day (adult & pregnancy) or 2000 mcg/week as a single dose. Adults over 65 need 500 mcg/day because intrinsic-factor secretion declines with age. Select your dosing frequency and life stage below.
Vegan adults need **250 mcg of cyanocobalamin per day** (sublingual or chewable) or **2000 mcg once a week**. Adults 65+: 500 mcg/day. The RDA is only 2.4 mcg/day, but passive absorption is about 1% of the dose — so high supplement doses are required to cover a week's need in one shot. There are no reliable plant-food sources of B12; supplementation is mandatory.
When to use this calculator
- New vegan calculating their first supplement purchase.
- Vegan woman who is pregnant or breastfeeding.
- Adult over 65 switching to a plant-based diet.
- Person deciding between a daily pill and a weekly high-dose strategy.
Example: Adult vegan choosing weekly dosing
- Profile: 34-year-old vegan adult, prefers one dose per week.
- RDA: 2.4 mcg/day. Total weekly need ≈ 16.8 mcg from food — but the diet provides essentially zero.
- Why 2000 mcg for a 16.8 mcg need? B12 absorption is dose-dependent. Intrinsic factor saturates at ~1.5 mcg per meal. Above that, only ~1% is absorbed passively (~5 mcg at 500 mcg dose; ~20 mcg at 2000 mcg dose). So 2000 mcg covers a full week in one shot.
- Result: 2000 mcg cyanocobalamin sublingual or chewable, once per week with food.
How it works
2 min readHow the dosage is calculated
The RDA for adults is 2.4 mcg/day (2.6 mcg in pregnancy, 2.8 mcg when breastfeeding). Vegans obtain essentially zero B12 from food, so the entire requirement must come from supplements.
Vegan B12 Dosage Table
| Strategy | Dose | Mechanism | Weekly absorbed (est.) |
|---|---|---|---|
| Daily (adult/pregnancy) | 250 mcg | IF + passive diffusion | ~2.5–3 mcg/day |
| Weekly | 2000 mcg | Mostly passive diffusion | ~18–22 mcg total |
| Daily, age 65+ | 500 mcg | Passive diffusion dominant | ~5 mcg/day |
Why supplement doses are so much larger than the RDA
B12 absorption follows two mechanisms:
1. Intrinsic factor (IF)-mediated: active, saturable at ~1.5 mcg per meal. Taking a 1 mcg pill absorbs ~0.5 mcg via IF.
2. Passive diffusion: only ~1–2% of the dose, but unlimited quantity. Taking 2000 mcg absorbs ~20 mcg passively — enough for a week.
This non-linear relationship is why weekly doses are enormous compared to daily doses, and why a 25 mcg tablet is not enough for a vegan.
Life-stage adjustments
| Life stage | Daily dose | Weekly dose |
|---|---|---|
| Adult 18–64 | 250 mcg | 2000 mcg |
| Pregnancy / breastfeeding | 250 mcg | Not recommended (prefer daily for steady fetal supply) |
| Age 65+ | 500 mcg | Less well-studied in this group |
Preferred B12 forms
| Form | Pros | Best for |
|---|---|---|
| Cyanocobalamin | Most studied, stable, cheap | All vegans |
| Methylcobalamin | Active form, may suit MTHFR variants | Severe deficiency / specialist use |
| Hydroxocobalamin | Long-acting depot | Injectable, medical use |
For healthy vegans, cyanocobalamin is the standard recommendation (NIH, Vegan Society). Sublingual or chewable tablets are preferred over swallowed capsules — they allow partial mucosal absorption, bypassing intrinsic factor.
What does NOT work as a B12 source
Monitoring
> Medical disclaimer: this calculator is educational, not medical advice. If you have symptoms of deficiency, absorption disorders (Crohn's, celiac, prior bariatric surgery), or take metformin or PPIs (which reduce B12 absorption), consult a physician for individual dosing.
Frequently asked questions
How much B12 should a vegan take per day?
250 mcg of cyanocobalamin (sublingual or chewable), once daily with food. This covers the 2.4 mcg/day RDA via passive absorption plus a safety margin. A 25–50 mcg tablet is not enough — passive absorption at that dose is less than 1 mcg.
What is the correct weekly B12 dose for vegans?
2000 mcg of cyanocobalamin in a single dose once per week. About 1% is absorbed passively (~20 mcg), covering the week's requirement. It is equally valid to the daily strategy — choose whichever you'll stick to.
Is B12 supplementation truly mandatory for vegans?
Yes — it is the one supplement that experts universally agree all vegans must take. There is no reliable plant source. The body stores 2–5 years of B12, which is why deficiency can be silent for a long time before causing irreversible nerve damage.
Why is the weekly dose 2000 mcg if the daily need is only 2.4 mcg?
Because B12 absorption is non-linear. Intrinsic factor saturates at about 1.5 mcg per meal. Above that, only ~1% is absorbed passively. So to get ~20 mcg (a week's supply) in one dose, you need 2000 mcg. It sounds paradoxical but it is backed by clinical pharmacokinetics.
Does spirulina or chlorella count as a B12 source?
No. Spirulina and many algae contain B12 analogues (inactive corrinoids) that can actually compete with real B12 and worsen your status. Several studies have shown vegans consuming spirulina had higher MMA levels — the opposite of what you want. Do not rely on algae for B12.
Is sublingual B12 better than swallowing a capsule?
For vegans, sublingual or chewable is generally preferred. Dissolving under the tongue allows small amounts to be absorbed directly through the mucosa, partially bypassing intrinsic factor. That said, swallowing a high-dose tablet (250 mcg) still works fine for most healthy adults — passive diffusion takes over at those doses.
Why do adults over 65 need a higher dose (500 mcg/day)?
Gastric atrophy is common with age, reducing intrinsic factor secretion. This means even the normal IF-mediated absorption pathway becomes less reliable. A higher daily dose ensures enough is absorbed via passive diffusion alone. Weekly dosing at 2000 mcg is less well-studied in this age group.
Cyanocobalamin or methylcobalamin — which B12 form is best?
Cyanocobalamin is the most studied form with the strongest clinical evidence base. It is stable at room temperature and inexpensive. Methylcobalamin is the active coenzyme form and is preferred by some practitioners for people with MTHFR variants or severe deficiency. Both work; cyanocobalamin is the standard recommendation.
How do I know if I am already B12 deficient?
Ask your doctor for: serum B12 (target >300 pg/mL for vegans), methylmalonic acid (MMA — the most sensitive functional marker), and homocysteine. Symptoms of established deficiency include fatigue, tingling in hands/feet, memory problems, glossitis (sore tongue), and macrocytic anemia (MCV >100 fL on a CBC).
Does metformin or a PPI affect B12 absorption?
Yes. Metformin (used for diabetes and PCOS) reduces B12 absorption in about 10–30% of long-term users by interfering with the calcium-dependent IF-B12 receptor in the ileum. Proton pump inhibitors (omeprazole, pantoprazole) reduce stomach acid, impairing protein-bound B12 release — though this mainly affects food B12, not supplement B12. If you take either drug, mention it to your doctor and consider more frequent monitoring.