Quick answer

Vitamin B12 keeps nerves and red blood cells working and is needed to make DNA. Adults need 2.4 mcg a day, found naturally only in animal foods. Most people get enough, but adults over 50, vegans and people on some medicines are more likely to run low. Extra B12 does not add energy.^1,2

Key facts

B12 comes naturally only from animal foods; plant foods have none unless it is added.¹
Your body stores 1,000 to 2,000 times the B12 you eat in a day, so a shortage can take years to show.¹
The share absorbed drops as the dose rises: about half of 1 to 2 mcg, but about 2% of a 500 mcg tablet.²
No upper limit is set, because B12 has a low potential for harm.²
Metformin and acid-reducing medicines can lower B12 over time.¹

What vitamin B12 does

Vitamin B12 is required for the development, myelination and function of the central nervous system, for healthy red blood cell formation and for making DNA.²

B12 is a helper for two enzymes. One turns homocysteine into methionine, which feeds a methyl donor the body uses on DNA, RNA, proteins and fats. The other is part of how the body breaks down propionate, a short-chain fatty acid.²

The body absorbs B12 from food in two steps: stomach acid separates it from the protein it is attached to, and it then combines with intrinsic factor, a protein made by the stomach.¹

What the evidence shows

Each statement below is an established nutrient function or an official intake recommendation, graded moderate. They describe what B12 does when you get enough, not extra effects from large doses.

Contributes to normal energy-yielding metabolism*^1,2,3

Moderate evidence

B12 is the helper for an enzyme that processes propionate, and NIH describes a role for it in energy metabolism. 'Contributes to normal energy-yielding metabolism' is the wording European regulators permit for vitamin B12. It does not mean extra B12 gives more energy: NIH notes that B12 supplements do not add energy, athletic performance or endurance in people who get enough from their diet.

Amount studied: Recommended intake: 2.4 mcg a day for adults, from all sources

Contributes to normal functioning of the nervous system*^2,3

Moderate evidence

NIH describes B12 as required for the development, myelination and function of the central nervous system; myelin is the coating around nerve fibers. The EU permits this statement. In trials summarized by NIH, B12 supplements given alone or with other B vitamins for 1 to 5 years did not change measures of memory and thinking in older adults.

Amount studied: Recommended intake: 2.4 mcg a day for adults, from all sources

Contributes to normal red blood cell formation*^2,3

Moderate evidence

NIH lists healthy red blood cell formation among B12's core jobs, and the US recommended amount was set as the intake needed to keep blood counts and blood B12 levels normal. Without enough B12, the body makes large, abnormal red blood cells. The EU permits this statement.

Amount studied: Recommended intake: 2.4 mcg a day for adults, from all sources

Supplies vitamin B12 in a free form that most adults over 50 absorb well*^1,2

Moderate evidence

Many older adults make too little stomach acid to release the B12 bound to protein in food. B12 in supplements and fortified foods is not bound to protein, so it does not need that step, and NIH advises people over 50 to get most of their B12 from these sources. Depending on the definition, 3% to 43% of older adults living at home have low B12.

Amount studied: Recommended intake: 2.4 mcg a day for adults over 50, mostly from fortified foods or supplements

Supplies vitamin B12 for people who eat little or no animal food*^1,2,4

Moderate evidence

B12 occurs naturally only in animal foods, and NIH lists vegans and vegetarians among the groups more likely to run low, adding that fortified foods and B12 supplements help close the gap. In a 90-day trial in 40 vegans and vegetarians with marginal B12 status, 50 mcg a day under the tongue brought blood B12 back to adequate levels and lowered methylmalonic acid, a marker of shortage; 2,000 mcg once a week did too.

Amount studied: Recommended intake: 2.4 mcg a day; the trial used 50 mcg a day or 2,000 mcg once a week

Who runs low

Depending on the definition used, 3% to 43% of older adults living at home have low B12.²

About 3.6% of US adults have blood B12 below 200 pg/mL, a common cutoff for deficiency, and about 12.5% are below 300 pg/mL.²

The groups more likely to fall short are adults over 50, who often make less stomach acid; people who eat few or no animal foods; people who have had stomach or weight-loss surgery; and people who take metformin or acid-reducing medicines for a long time.^1,2

How much you need

Adults need 2.4 mcg of vitamin B12 a day, the same amount for men and women at every age from 14 on.¹

The recommended amount is 2.6 mcg a day during pregnancy and 2.8 mcg while breastfeeding.¹

Supplements carry far more than that because absorption falls as the dose rises: a 500 mcg tablet yields roughly 10 mcg.²

B12 for hair, skin and men

None of the sources we use report a trial of B12 for hair or skin in people who already get enough. Pale skin appears in NIH's list of signs of running low.¹

Men and women need the same 2.4 mcg a day. US men average 5.84 mcg a day from food, and 5% fall below the average requirement.²

Forms and food

Cyanocobalamin is the most common form in supplements. The body converts it, like hydroxocobalamin, into the two active forms, methylcobalamin and adenosylcobalamin.²

Research has not shown that any supplement form of B12 is better than the others, and no evidence shows that the share absorbed differs by form.^1,2

Fish, meat, poultry, eggs, milk and other dairy foods contain vitamin B12. Clams, oysters and beef liver are among the best sources.¹

Plant foods have no vitamin B12 unless it is added. Many breakfast cereals and nutritional yeasts are fortified with it, and the B12 they contain is well absorbed.^1,2

What's in ours

What’s in ours

Men's Multi: Vitamin B12, 27 mcg per serving, from the label
Ashwagandha Complex: Vitamin B12, 25 mcg per serving, from the label

Men's Multi lists 27 mcg of B12 as cyanocobalamin and Ashwagandha Complex 25 mcg as methylcobalamin. Both are more than the 2.4 mcg adults need each day, and no upper limit applies to B12.^1,2

Side effects, medicines and who should ask first

No upper limit is set for vitamin B12 because it has a low potential for harm. NIH says it is generally considered safe even at large doses, such as 500 or 1,000 mcg, because the body does not store the excess.^1,2

Acne-like skin eruptions have been reported with high-dose B12 supplements, mostly in case reports, and they generally clear after the supplement is stopped. In one small study, 1 of 10 healthy people given extra B12 developed acne a week later.^5,6

Your doctor may want to check on you

Metformin may lower vitamin B12 levels, possibly by interfering with its absorption. In 29-week trials described on the metformin label, about 7% of people with normal B12 dropped below normal, and the label tells prescribers to measure B12 every 2 to 3 years.^1,2,7,8

On the medicine's label

Your doctor may want to check on you

Proton pump inhibitors such as omeprazole and lansoprazole reduce stomach acid, which the body needs to release B12 from food. The omeprazole label says daily use of any acid-suppressing medicine for a long time, such as more than 3 years, may lead to poor absorption of B12.^1,2,9

On the medicine's label

Your doctor may want to check on you

H2 blockers such as cimetidine also reduce stomach acid and can interfere with absorbing B12 from food, which NIH says can lead to low B12.^1,2

Reported in studies

Low B12 can cause tiredness, weakness, numbness or tingling in the hands and feet, balance problems, memory trouble and megaloblastic anemia. Nerve damage can happen even without anemia, so these signs need a doctor's assessment rather than a guess with supplements.^1,2

Ask first if you take metformin, a proton pump inhibitor or an H2 blocker
Ask first if you have pernicious anemia, atrophic gastritis, celiac disease or Crohn's disease, or have had stomach or weight-loss surgery¹
Ask first if you are pregnant or breastfeeding and eat no animal foods
Tell your doctor if you take large amounts of folic acid, which can hide low B12¹⁰

Questions people ask

Does vitamin B12 give you energy?

Only if you were short of it. B12 is part of how the body releases energy from food, but NIH says extra B12 does not add energy or endurance in people who already get enough. Ongoing tiredness is worth a doctor's visit.¹

Is it better to get B12 from food or supplements?

Either works for most people. B12 from supplements is absorbed about 50% better than B12 from food and does not need stomach acid to be released, which is why NIH advises adults over 50 to rely on fortified foods or supplements.^1,2

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See the Full Measure line Ashwagandha Complex Men's Multi

Sources

*These statements have not been evaluated by the Food and Drug Administration. This product is not intended to diagnose, treat, cure, or prevent any disease.

This page is information, not medical advice. Talk to your doctor or pharmacist about your own situation.