Water-soluble vitamin

Vitamin B12

cobalamin

Red blood cell formation, nerve myelin, DNA synthesis, homocysteine metabolism

Essential nutrient

Reference intakes (RDA or AI) are set by the National Academies of Sciences, Engineering, and Medicine. Deficiency thresholds come from clinical and WHO guidance.

At a glance

Women, 19-50

2.4 mcg (2.6 mcg pregnancy)

Men, 19-50

2.4 mcg

Upper limit (adults)

Not established

How common is deficiency?

COMMON - especially in older adults, vegans/vegetarians, and people on metformin or acid-suppressing drugs

Food sources

Animal sources

Clams, beef liver, sardines, salmon, tuna, beef, eggs, milk, yogurt, cheese

Plant sources

NO reliable unfortified plant source. Fortified nutritional yeast, fortified plant milks/cereals; some algae contain inactive analogs

Deficiency and testing

Signs of deficiency

Fatigue, pallor, megaloblastic anemia, tingling/numbness (paresthesia), balance problems, memory loss, glossitis, depression, irreversible nerve damage if untreated

Who is at higher risk

Adults 50+ (low stomach acid), vegans, pernicious anemia, metformin / PPI / H2 blocker users, bariatric surgery, Crohn's disease, nitrous oxide misuse

How it is tested

Serum vitamin B12; methylmalonic acid (MMA); plasma homocysteine; holotranscobalamin (active B12); CBC/MCV; anti-intrinsic-factor antibodies

Cut-offs

Serum B12 <200 pg/mL (<148 pmol/L) = deficient; 200-300 pg/mL = borderline - confirm with MMA (>0.4 umol/L supports deficiency)

Symptoms are not specific. Only a clinician with the right tests can tell whether you are deficient.

Supplement forms

Common forms

cyanocobalamin, methylcobalamin, hydroxocobalamin, adenosylcobalamin, oral tablets, sublingual tablets/sprays, intramuscular (IM) injection, high-dose oral.

More effective

cyanocobalamin (most studied, stable, effective) or methylcobalamin (comparable in trials) taken orally; high-dose oral 1,000-2,000 mcg daily corrects deficiency even in many people with poor absorption, since about 1% is absorbed passively; IM injection (hydroxocobalamin or cyanocobalamin) for severe deficiency with neurological signs, pernicious anemia that does not respond to oral, or after bariatric surgery.

Less effective

sublingual and oral forms show no proven superiority over each other; spirulina, algae, and unfortified plant foods (contain inactive B12 analogs); low-dose oral (about 2.4 mcg) when absorption is impaired; claims that methylcobalamin is vastly superior (not supported).

Safety, interactions and halal notes

Safety and interactions

Upper limit for adults: Not established. Going above the upper limit over time can cause harm, so check labels and talk to a clinician before high-dose supplements.

Halal note

The nutrient itself is not a halal question. For supplements, check the delivery form: gelatin (pork or non-halal bovine), capsule shells, glycerin, and alcohol-based carriers need halal certification. Ask for the halal certificate, not just a label claim. This is general guidance, not a certification.

Frequently asked questions

How much Vitamin B12 do adults need per day?

Women 19-50: 2.4 mcg (2.6 mcg pregnancy). Men 19-50: 2.4 mcg. Upper limit: Not established. Needs change with age, pregnancy and health conditions.

What are the signs of Vitamin B12 deficiency?

Fatigue, pallor, megaloblastic anemia, tingling/numbness (paresthesia), balance problems, memory loss, glossitis, depression, irreversible nerve damage if untreated. These signs are not specific, so only a clinician with the right tests can confirm a deficiency.

What are the best food sources of Vitamin B12?

Animal sources: Clams, beef liver, sardines, salmon, tuna, beef, eggs, milk, yogurt, cheese. Plant sources: NO reliable unfortified plant source. Fortified nutritional yeast, fortified plant milks/cereals; some algae contain inactive analogs.

Which form of Vitamin B12 supplement is best?

Sources

NIH Office of Dietary Supplements fact sheet

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This page is for education only and is not medical advice. These statements have not been evaluated by the Food and Drug Administration. Supplements are not intended to diagnose, treat, cure or prevent any disease. Talk to a qualified clinician before starting a supplement, especially if you are pregnant, breastfeeding, take medicine or have a medical condition. Daily needs are US reference intakes for adults 19-50.