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A vitamin from animal foods that the body needs for red blood cells and nerves. Most studied for preventing and treating a low B12 level.
Ask a doctor or pharmacist before taking vitamin B12 if you:
Vitamin B12 (cobalamin) is a water-soluble vitamin that contains the mineral cobalt. The body needs it to make red blood cells and DNA and for the nervous system.[2] It is found naturally only in animal foods, and it is added to some cereals and nutritional yeasts.[2] Supplements contain cyanocobalamin, methylcobalamin, adenosylcobalamin or hydroxocobalamin, and doctors also give it as injections.[2]
Mainly to prevent and treat a low B12 level: in 3 small trials, tablets of 1,000 to 2,000 µg a day raised low levels about as well as injections.[5] For nerve damage in diabetes the results are mixed.[6] A Cochrane review of B vitamins found no effect on heart attacks.[7] A 2021 review found no effect on memory or mood in people without a clear deficiency.[8]
Yes, for most people, even at high doses: the US and the EU set no upper limit, because no harm has been found.[2][9] A review of B12 and the skin lists acne and allergic skin reactions as side effects of B12 treatment.[4] People with kidney disease from diabetes should ask a doctor first. In one trial, high doses of B12 with folic acid and vitamin B6 made their kidney function fall faster.[3]
Vitamin B12, also called cobalamin, comes bound to protein in food. The body needs the stomach and the small intestine to take it up, and it keeps a store that lasts for years.[2]
Stomach acid and enzymes free vitamin B12 from the protein in food. The B12 in supplements and fortified foods is already free, so it skips this step.[2]
In the first part of the small intestine, the free vitamin binds to intrinsic factor, a protein that cells of the stomach make.[2]
The ileum, the last part of the small intestine, takes up B12 with its intrinsic factor. Intrinsic factor carries only 1 to 2 µg at a time, so the body absorbs about 2% of a 500 µg dose and 1.3% of a 1,000 µg dose.[2]
In cells, B12 helps two enzymes. One of them turns homocysteine into the amino acid methionine. The body needs B12 to make red blood cells and DNA, and to build and keep the nervous system.[2]
The body stores 1 to 5 mg of B12, about 1,000 to 2,000 times what a person eats in a day. This is why signs of a deficiency can take several years to appear.[2]
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In a Cochrane review, the trials that treated a low B12 level with tablets gave 1,000 to 2,000 µg a day.[5] The daily need from food is much smaller: 2.4 µg for adults in the US and 4 µg in the EU.[2][10]
Cyanocobalamin is the most common form in supplements. Others contain methylcobalamin, adenosylcobalamin or hydroxocobalamin. No evidence shows that the body absorbs one form better than another.[2] Studies suggest that tablets held under the tongue work as well as tablets that are swallowed.[2] Injections need a prescription.[2]
The US and the EU set no upper limit, because no harm has been found at high intakes.[2][9] Germany's food safety institute (BfR) proposes at most 25 µg per daily dose of a food supplement.[12] People with kidney disease from diabetes, and people who take metformin or stomach acid medicines, should ask a doctor or pharmacist first.[3][2]
Most studies gave 0.25 to 0.5 mg a day, for 1 to 3 months.
The counts are study groups. A study group is one set of people in a study who took the same dose. One study can have several groups. These are amounts from studies, not advice for you.
271 of 359 study groups took vitamin B12 with something else. This shows what studies gave together, not what works together.
What studies used when they gave vitamin B12 for one goal, or to a group of people.
Each protocol describes what studies used, for one goal or for one group of people. It is not advice for you: talk to a doctor before you start a supplement.
12 questions in 4 groups. Open one for the short answer; each has its own page with the full answer and sources.
Older adults, vegans and vegetarians, people with autoimmune gastritis or stomach or bowel surgery, and people who take metformin or stomach acid medicines.[2][1] In the US, about 3.6% of adults are deficient and 12.5% have an insufficient level.[2]
Only animal foods contain B12 naturally: liver, clams, fish, meat, eggs and dairy. Plant foods have none, unless B12 is added, as in many breakfast cereals and nutritional yeasts.[2] For example, 3 ounces of cooked salmon have 2.6 µg, 1 cup of milk has 1.3 µg and 1 egg has 0.5 µg. The adult need is 2.4 µg a day.[2]
In people with a deficiency, symptoms may start to improve within 2 weeks of treatment, but this can take up to 3 months, and longer to go away fully.[1] In a trial of high-dose tablets and injections, blood counts and nerve problems got better quickly in both groups.[13] In people without a clear deficiency, trials found no effect on memory or mood.[8]
Only when tiredness comes from a deficiency. Tiredness without a clear cause is a common sign of a low B12 level, and treatment works in most people with a deficiency.[1] In people who have enough B12, supplements do not appear to improve endurance or sports performance.[2] A 2021 review found only one trial on unexplained tiredness, too few to analyze.[8] See the goal energy.
Not in people without a clear deficiency: a 2021 review of 16 trials with 6,276 people found no effect on memory or thinking.[8] A real deficiency can cause trouble concentrating and memory loss, and NICE lists these as reasons to test for it.[1]
No. B12 lowers homocysteine, but a 2017 Cochrane review of 15 trials with 71,422 people found that B vitamins did not prevent heart attacks or deaths. Strokes were slightly fewer: 4.3% against 5.1%.[7] The trials gave B12, folic acid or vitamin B6, alone or together.[7] See the goal cardiovascular.
The evidence is mixed. A 2020 review of 15 trials in nerve damage from diabetes or shingles found more overall improvement with methylcobalamin, a form of B12, but no effect on pain scores. Most of the trials had a high risk of bias.[6] A 1-year trial included 90 people with nerve damage who took metformin and had B12 below 400 pmol/L. Nerve tests and pain improved with 1,000 µg a day and got worse with placebo.[11] The protocol for diabetic nerve damage lists the trial doses.
Not in people without a clear deficiency: a 2021 review of trials found no overall effect of B12 on depression scores.[8] A B12 deficiency can come with depression, anxiety or psychosis, so NICE asks doctors to keep it in mind.[1] See the goal stress and mood.
Yes. A 2016 meta-analysis found B12 levels 57 pmol/L lower after 6 weeks to 3 months of metformin, and 10 of 17 observational studies found lower levels in people who took it.[14] NICE lists metformin as a risk factor, and says that a deficiency caused by the medicine can be treated with tablets or injections while a person takes it.[1] In a 2022 review of 7 trials in people on metformin, B12 supplements raised blood B12 in all 5 trials that measured it.[15]
A little more: the US amount rises from 2.4 µg a day to 2.6 µg in pregnancy and 2.8 µg while breastfeeding.[2] A 2024 Cochrane review of 5 trials found that supplements may lower the share of mothers with a deficiency: 25.9% against 67.9%. The evidence was very uncertain, and effects on the baby were not clear.[16] Babies breastfed by mothers who eat no animal foods are at risk of a deficiency.[2]
No evidence shows that it is. NIH ODS states that no evidence shows that the body absorbs one form of B12 in supplements better than another.[2] NICE lists cyanocobalamin, methylcobalamin and adenosylcobalamin as forms to choose in a supplement.[1] Cyanocobalamin is the most common form in supplements.[2]
For most people with a low level, yes. In a Cochrane review, tablets of 1,000 to 2,000 µg a day raised blood B12 about as well as injections. The evidence was of low quality.[5] NICE still advises lifelong injections when the cause is autoimmune gastritis or removal of the stomach.[1]
