Vitamin B12

vitamin

Take with food

What is it

Vitamin B12 (cobalamin) is a water-soluble vitamin essential for red blood cell formation, neurological function, and DNA synthesis. It is found naturally only in animal foods, so vegans and many older adults are at risk of deficiency.

How it works

Vitamin B12 acts as a cofactor for two key enzymes: methionine synthase, which converts homocysteine to methionine and supports DNA methylation, and methylmalonyl-CoA mutase, which is involved in fatty acid and amino acid metabolism. These reactions are critical for red blood cell maturation and for maintaining the myelin sheath that insulates nerves. Absorption is a multi-step process. Stomach acid frees B12 from food proteins, then intrinsic factor — a protein made by the stomach — binds B12 so it can be absorbed in the lower small intestine. Anything that disrupts this chain (low stomach acid, autoimmune destruction of intrinsic factor, gut surgery) impairs absorption.

Evidence for 5 uses

AI-assisted evidence assessment — talk to your doctor before relying on any single supplement.

Treatment of B12 deficiency anemia

Grade A

Strong evidence

B12 supplementation reliably reverses megaloblastic anemia caused by deficiency. This is the foundational, well-established use.

Prevention of neurological damage from deficiency

Grade A

Strong evidence

Adequate B12 prevents the peripheral neuropathy, balance problems, and cognitive symptoms that can occur with severe deficiency. Untreated deficiency can cause permanent nerve damage.

Energy in deficient individuals

Grade B

Good evidence

People who are actually deficient often notice substantial energy improvement with supplementation. In people with normal B12 levels, additional B12 does not boost energy.

Lowering homocysteine

Grade B

Good evidence

B12 alongside folate and B6 lowers homocysteine levels. Whether this translates to lower cardiovascular events is uncertain — large trials have been mostly negative.

Cognitive function in older adults

Grade C

Moderate evidence

Observational studies link low B12 to cognitive decline, but trials of supplementation in non-deficient older adults have shown limited benefit.

4 commercial forms

Cyanocobalamin

stable, inexpensive, most common

The standard form in most supplements. The body converts it to active forms. Some critics question its trace cyanide content, but the amount is negligible for healthy people.

Methylcobalamin

already in active form, may be preferred for some

An active form that bypasses one conversion step. Marketed as superior, but evidence for clinically meaningful differences in most people is limited.

Hydroxocobalamin

long-acting, often used in injections

Converted to active forms in the body and has a longer half-life than cyanocobalamin. The standard injectable form in many countries.

Adenosylcobalamin

active form for cellular energy

The cofactor for methylmalonyl-CoA mutase. Less common in supplements, sometimes paired with methylcobalamin.

Dosage

The RDA is 2.4 mcg per day for adults, with slightly higher needs during pregnancy (2.6 mcg) and breastfeeding (2.8 mcg). Common supplemental doses range from 100 mcg to 1,000 mcg or more. There is no established tolerable upper limit because excess is excreted in urine and toxicity is not seen even at high doses. People with malabsorption may need 1,000 to 2,000 mcg daily or periodic injections.

When and how to take it

Vitamin B12 absorption does not depend on time of day, and it can be taken with or without food. Sublingual tablets and oral tablets are equally effective for most people, since passive diffusion in the mouth and gut accounts for most absorption at supplemental doses. Splitting very high doses across the day is not necessary because excess B12 is stored in the liver in significant amounts. A single daily dose works fine.

Food sources

Food	Amount	%DV
Clams, 3 oz cooked	84 mcg	3500%
Beef liver, 3 oz cooked	70 mcg	2917%
Sockeye salmon, 3 oz cooked	4.8 mcg	200%
Tuna (canned), 3 oz	2.5 mcg	104%
Beef (top sirloin), 3 oz cooked	1.4 mcg	58%
Milk (low fat), 1 cup	1.2 mcg	50%
Yogurt (plain low-fat), 1 cup	1.1 mcg	46%
Egg, 1 large	0.5 mcg	21%
Fortified breakfast cereal, 1 serving	1.5 to 6 mcg	100%
Nutritional yeast (fortified), 1 Tbsp	2.4 mcg	100%

Safety

Vitamin B12 has an excellent safety profile. No toxicity has been reported even at very high oral doses, and the body simply excretes what it does not use. Rare allergic reactions have been reported with injectable forms. Some people report acne flares with high-dose B12, though the evidence is anecdotal. Oral high doses are absorbed less efficiently but enough crosses by passive diffusion to correct most deficiencies, which is why even people with absent intrinsic factor can sometimes be treated with high-dose oral B12 instead of injections.

Who should be cautious

Vegans and strict vegetarians should supplement B12, since plant foods contain virtually none. Adults over 50 often have reduced stomach acid and benefit from supplemental B12, which is absorbed more easily than food-bound forms. People taking metformin or acid-suppressing drugs long-term should have levels monitored. Pregnant vegans are at particular risk of passing deficiency to their infants.

Interactions

Metformin reduces B12 absorption over years of use — long-term metformin users should have B12 levels checked periodically. Proton pump inhibitors and H2 blockers reduce stomach acid needed to free B12 from food, increasing deficiency risk. Nitrous oxide (used in dental procedures and abused recreationally) inactivates B12 and can cause severe neurological problems in people with low baseline levels.

Frequently asked questions

How do I know if I am B12 deficient?⌄

A blood test for serum B12 is standard. Methylmalonic acid (MMA) is a more sensitive marker because it accumulates when B12 is low at the cellular level. Symptoms include fatigue, tingling in hands and feet, balance problems, and a sore tongue.

Do vegans need to supplement B12?⌄

Yes. Plant foods contain virtually no bioavailable B12. Fortified nutritional yeast, fortified plant milks, or a B12 supplement is essential.

Is methylcobalamin better than cyanocobalamin?⌄

For most people, no meaningful difference in clinical effect. Cyanocobalamin is cheaper and stable. Methylcobalamin may be preferable for people with rare genetic conditions affecting B12 conversion.

Can I take too much B12?⌄

Toxicity has not been reported even at very high oral doses. Excess is excreted in urine. There is no established upper limit.

Do I need injections or do oral pills work?⌄

Oral high-dose B12 (1,000 to 2,000 mcg daily) works for most people, even many with poor absorption, because passive diffusion provides some absorption regardless of intrinsic factor. Severe cases or those who prefer it use injections.

References

NIH ODS Vitamin B12 Fact Sheet — NIH Office of Dietary Supplements link

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Disclaimer: These statements have not been evaluated by the FDA. This page is educational, not a substitute for personalized medical advice. Evidence grades are AI-assisted assessments — talk to your doctor before starting any new supplement, especially if you're pregnant, breastfeeding, on medications, or managing a chronic condition.