Vitamin B12, or cobalamin, is found almost exclusively in foods of animal origin. Only a few micrograms a day are needed, but a deficiency can go unnoticed for a long time and then affect the blood and the nerves.
Key points
- EFSA sets an adequate intake of 4µg a day for adults, 4.5µg during pregnancy and 5µg during breastfeeding.
- Vegans who do not take a supplement, older people and people on long-term metformin or antacids are the most exposed to deficiency.
- A deficiency can affect the nerves without anaemia: in a landmark study, 28% of the 141 patients with neuropsychiatric disorders were not anaemic.
- No form of B12 (cyano-, methyl-, hydroxo- or adenosylcobalamin) has shown clear superiority in humans.
What does vitamin B12 do?
Vitamin B12 is the cofactor of two enzymes. Methionine synthase recycles homocysteine into methionine, a step linked to folate (vitamin B9). Methylmalonyl-CoA mutase is involved in the metabolism of certain fatty acids and amino acids, in the mitochondria. Through these two enzymes, B12 is involved in DNA synthesis, methylation reactions and mitochondrial metabolism.
How much vitamin B12 do you need?
In 2015, the European Food Safety Authority (EFSA) set an adequate intake of 4µg a day for adults, 4.5µg during pregnancy and 5µg during breastfeeding. For children, it ranges from 1.5µg between 7 and 11 months to 4µg between 15 and 17 years. In the European Union, average intakes range from 4.2 to 8.6µg a day.
How is B12 absorbed?
Released from food proteins by stomach acid, B12 binds to intrinsic factor, a protein secreted by the stomach lining, which allows it to be absorbed at the end of the small intestine. This system quickly becomes saturated:
- about 50% of B12 is absorbed at doses below 1 to 2µg;
- only about 2% at 500µg, and 1.3% at 1,000µg.
Hence the oral doses of several hundred micrograms used to correct a deficiency. The body stores 1 to 5mg of B12, or 1,000 to 2,000 times the usual daily intake: a dietary deficiency develops slowly.
Which foods are rich in vitamin B12?
Offal and seafood top the list (levels reported by the US National Institutes of Health).
| Food | Portion | Vitamin B12 |
|---|---|---|
| Beef liver, cooked | 85g | 70.7µg |
| Clams, cooked | 85g | 17µg |
| Salmon, cooked | 85g | 2.6µg |
| Canned tuna | 85g | 2.5µg |
| Milk, 2% fat | 1 cup (about 240mL) | 1.3µg |
| Plain yoghurt, 0% fat | 170g | 1.0µg |
| Whole egg, cooked | 1 large egg | 0.5µg |
| Cheddar | 42g | 0.5µg |
Plant foods do not provide reliable B12. Spirulina is often presented as a source, but in tablets analysed in 1999, 83% of the compounds that bind to intrinsic factor were pseudovitamin B12, considered inactive in humans, compared with 17% genuine vitamin B12.
Who is at risk of vitamin B12 deficiency?
- Vegans and vegetarians. According to a review of 40 studies, deficiency affects 0 to 86.5% of vegetarian adults depending on the study, and up to 45% of infants. The risk is highest among vegans who do not take a supplement.
- Older people. About 6% of people over 60 are deficient in large American and British surveys, and nearly 20% have borderline status. The main cause is poorer absorption of B12 from food, partly due to atrophic gastritis.
- People taking metformin. In a large American study, low or borderline B12 affected 19.1% of participants on metformin versus 9.5% on placebo after 5 years, and each year of treatment was associated with a higher risk (odds ratio of 1.13 per year).
- Long-term antacid treatment. Two years or more of proton pump inhibitors (PPIs) were associated with a higher risk of deficiency (odds ratio of 1.65) in a study of nearly 26,000 cases, without proof of causality.
- Obesity surgery and certain digestive diseases.
- Pernicious anaemia (Biermer's disease), an autoimmune disease that eliminates intrinsic factor. It is found in about 15 to 25% of older people with a deficiency.
For more on metformin, our article on insulin resistance puts this treatment in context.
What are the symptoms of vitamin B12 deficiency?
They develop gradually and are not specific:
- blood signs: anaemia with large red blood cells (macrocytic anaemia), with tiredness, breathlessness and pallor;
- neurological signs: tingling and numbness in the feet and hands, loss of sensation, balance problems;
- cognitive and psychological signs: memory or mood problems and, in some cases, even dementia;
- sometimes a red, sore tongue (glossitis).
Nerve damage can occur without anaemia. In a study published in 1988, 40 of the 141 patients (28%) with neuropsychiatric disorders caused by deficiency had neither anaemia nor large red blood cells; their symptoms improved with treatment in the 39 patients assessed. Persistent tiredness has many other possible causes: our guide why am I always tired helps you narrow them down, and our article on brain fog sets out the avenues to explore with a doctor.
How can you tell if you are low in vitamin B12?
Only a blood test can tell. There is no single reference test, and thresholds vary between laboratories.
| Test | What it measures | Role in the work-up |
|---|---|---|
| Serum vitamin B12 | Total circulating B12 | First test; deficiency is often defined as below 148pmol/L (200pg/mL) |
| Holotranscobalamin | The share of B12 available to cells | Could become a first-line test, with a grey zone |
| Methylmalonic acid | A metabolite that builds up when B12 is lacking | Second line, when in doubt |
| Homocysteine | An amino acid that rises when B12 or folate is lacking | Second line, less specific |
According to the 2014 British guidelines, when the clinical signs are suggestive, a discordant result should not delay treatment.
How is vitamin B12 deficiency corrected: tablets or injections?
Correcting a confirmed deficiency is a matter for a doctor, who will also look for its cause, using intramuscular injections or high oral doses.
In a trial conducted in 1998, 38 patients were randomly assigned to receive either 1mg of cyanocobalamin by injection on nine occasions over 90 days, or 2mg orally every day for 120 days. Blood and neurological abnormalities were corrected rapidly and to a similar extent. After 4 months, mean blood B12 reached 1,005pg/mL in the oral group versus 325pg/mL in the injection group.
A 2018 Cochrane review (3 trials, 153 participants, 3 to 4 months) found no clinically relevant difference in blood B12 between the two routes, with low-certainty evidence. No trial measured symptoms.
Cyanocobalamin or methylcobalamin: which form should you choose?
Four forms are used. Methylcobalamin and adenosylcobalamin are the two active forms in cells; cyanocobalamin is a synthetic form; hydroxocobalamin is used in particular for injections.
Inside the cell, all of them are converted into a common intermediate, from which the body rebuilds its two active forms. According to a 2015 biochemical review, the “coenzyme” forms are therefore probably not superior to cyanocobalamin or hydroxocobalamin for preventing or correcting a deficiency. The National Institutes of Health fact sheet points the same way: no data show that absorption differs according to form. A 2017 review suggests, without clinical data, that certain forms may be of interest depending on genetics.
In practice, the dose, regularity and the cause of the deficiency matter more than the form. Our article on capsules, softgels and tablets compares dosage forms and their excipients.
Can you take too much vitamin B12?
The European Commission's Scientific Committee on Food was unable to set a tolerable upper intake level, as no adverse effects have been seen in healthy people. No adverse effects have been reported at up to 1,000µg a day orally, over long periods, in patients who absorb B12 poorly, although such effects were not systematically looked for.
A high blood B12 level without supplements is a separate case. In a Danish cohort of 333,667 people not treated with B12, a high level was associated with an increased risk of cancer diagnosed afterwards, especially in the following year. The authors see it as a marker, not a cause. An unexplained high level should be discussed with a doctor. The situation is different for vitamin B6, whose overdose is covered in a dedicated article.
Vitamin B12 and folate: why monitor them together?
A high intake of folic acid can correct the anaemia of B12 deficiency without correcting the nerve damage, which may continue to worsen: this is known as “masking”. It is precisely this risk that EFSA used in 2023 to set the upper limit for added folate at 1,000µg a day for adults. Before taking high doses of folic acid, it is best to know your B12 status.
What can a vitamin B12 supplement claim?
European Regulation (EU) No 432/2012 authorises eight health claims for vitamin B12, which may only be used for a food that is at least a source of vitamin B12 within the meaning of Regulation (EC) No 1924/2006:
- “Vitamin B12 contributes to normal energy-yielding metabolism”;
- “Vitamin B12 contributes to normal functioning of the nervous system”;
- “Vitamin B12 contributes to normal homocysteine metabolism”;
- “Vitamin B12 contributes to normal psychological function”;
- “Vitamin B12 contributes to normal red blood cell formation”;
- “Vitamin B12 contributes to the normal function of the immune system”;
- “Vitamin B12 contributes to the reduction of tiredness and fatigue”;
- “Vitamin B12 has a role in the process of cell division”.
The studies cited above, for their part, concern people with a deficiency. See also our comparison which vitamin for tiredness and our article on vitamin B3, another B-group vitamin.
Frequently asked questions
How long does it take to correct a vitamin B12 deficiency?
In the 1998 trial, blood and neurological abnormalities were corrected rapidly, and the trials in the Cochrane review lasted 3 to 4 months. Depending on the cause, long-term intake may be needed.
Should vegetarians and vegans take a vitamin B12 supplement?
The authors of the review of 40 studies recommend that vegetarians seriously consider a B12 supplement and have their status checked. For vegans, fortified foods or a supplement are the only reliable source.
Does spirulina provide vitamin B12?
Very little in active form: in the tablets analysed, most of the “B12” it contains consists of inactive analogues.
Is a high vitamin B12 level in the blood serious?
After a supplement or an injection, a high level is to be expected. Without supplements, a high level may be the marker of another disease that should be looked for: it warrants medical advice.
Should vitamin B12 be monitored when taking metformin?
The authors of the large American metformin study conclude that regular B12 testing should be considered in treated patients. Do not change your treatment without medical advice.
What is the difference between vitamin B12 and vitamin B9?
They work together in the same reaction: a deficiency of either one causes the same type of anaemia, but nerve damage is mainly characteristic of B12 deficiency.
Conclusion
Vitamin B12 is essential for the blood and the nervous system, and a diet that includes animal products generally provides enough of it. The risk of deficiency concerns a few profiles: vegans who do not take supplements, older people, people on metformin or antacids, and people with digestive diseases. If in doubt, a blood test settles the question, and a deficiency can be corrected effectively with medical follow-up. As for form, there is no evidence that the so-called active forms do better than cyanocobalamin.
Sources
- National Institutes of Health, Office of Dietary Supplements (2025). Vitamin B12 — Fact Sheet for Health Professionals. ods.od.nih.gov
- Green R et al. (2017). Vitamin B12 deficiency. Nature Reviews Disease Primers, 3, 17040. doi:10.1038/nrdp.2017.40
- Allen LH (2009). How common is vitamin B-12 deficiency? American Journal of Clinical Nutrition, 89(2), 693S-696S. doi:10.3945/ajcn.2008.26947A
- Watanabe F et al. (1999). Pseudovitamin B12 is the predominant cobamide of an algal health food, spirulina tablets. Journal of Agricultural and Food Chemistry, 47(11), 4736-4741. PubMed 10552882
- Pawlak R, Lester SE, Babatunde T (2014). The prevalence of cobalamin deficiency among vegetarians assessed by serum vitamin B12: a review of literature. European Journal of Clinical Nutrition, 68(5), 541-548. doi:10.1038/ejcn.2014.46
- Aroda VR et al. (2016). Long-term metformin use and vitamin B12 deficiency in the Diabetes Prevention Program Outcomes Study. Journal of Clinical Endocrinology & Metabolism, 101(4), 1754-1761. doi:10.1210/jc.2015-3754
- Lam JR et al. (2013). Proton pump inhibitor and histamine 2 receptor antagonist use and vitamin B12 deficiency. JAMA, 310(22), 2435-2442. doi:10.1001/jama.2013.280490
- EFSA NDA Panel (2015). Scientific opinion on dietary reference values for cobalamin (vitamin B12). EFSA Journal, 13(7), 4150. doi:10.2903/j.efsa.2015.4150
- Scientific Committee on Food (2000). Opinion on the tolerable upper intake level of vitamin B12. European Commission. ec.europa.eu
- EFSA NDA Panel (2023). Scientific opinion on the tolerable upper intake level for folate. EFSA Journal, 21(11), e08353. doi:10.2903/j.efsa.2023.8353
- Lindenbaum J et al. (1988). Neuropsychiatric disorders caused by cobalamin deficiency in the absence of anemia or macrocytosis. New England Journal of Medicine, 318(26), 1720-1728. doi:10.1056/NEJM198806303182604
- Devalia V, Hamilton MS, Molloy AM (2014). Guidelines for the diagnosis and treatment of cobalamin and folate disorders. British Journal of Haematology, 166(4), 496-513. doi:10.1111/bjh.12959
- Wang H et al. (2018). Oral vitamin B12 versus intramuscular vitamin B12 for vitamin B12 deficiency. Cochrane Database of Systematic Reviews, 3, CD004655. doi:10.1002/14651858.CD004655.pub3
- Kuzminski AM et al. (1998). Effective treatment of cobalamin deficiency with oral cobalamin. Blood, 92(4), 1191-1198. doi:10.1182/blood.V92.4.1191
- Obeid R, Fedosov SN, Nexo E (2015). Cobalamin coenzyme forms are not likely to be superior to cyano- and hydroxyl-cobalamin in prevention or treatment of cobalamin deficiency. Molecular Nutrition & Food Research, 59(7), 1364-1372. doi:10.1002/mnfr.201500019
- Paul C, Brady DM (2017). Comparative bioavailability and utilization of particular forms of B12 supplements with potential to mitigate B12-related genetic polymorphisms. Integrative Medicine (Encinitas), 16(1), 42-49. PubMed 28223907
- Mills JL et al. (2003). Low vitamin B-12 concentrations in patients without anemia: the effect of folic acid fortification of grain. American Journal of Clinical Nutrition, 77(6), 1474-1477. doi:10.1093/ajcn/77.6.1474
- Arendt JF et al. (2013). Elevated plasma vitamin B12 levels as a marker for cancer: a population-based cohort study. Journal of the National Cancer Institute, 105(23), 1799-1805. doi:10.1093/jnci/djt315
- Commission Regulation (EU) No 432/2012 of 16 May 2012 establishing a list of permitted health claims made on foods, other than those referring to the reduction of disease risk and to children’s development and health. Official Journal of the European Union, L 136 of 25.5.2012, p. 1-40. eur-lex.europa.eu
Food supplements are not a substitute for a varied and balanced diet and a healthy lifestyle. Ask a healthcare professional for advice if you are taking medication, are pregnant or are breastfeeding.
