Vitamin B12: who needs a supplement and why

Vitamin B12 is reliably found only in foods of animal origin. If you are vegan, supplementing is not optional: Santé publique France (the French public health agency) considers it necessary. If you are vegetarian, flexitarian, elderly or on certain medicines (metformin, antacids), your status also deserves attention. Anses (the French food safety agency) sets the reference at 4 µg a day for an adult. Here is what it does, where to find it, who should supplement and how, with the official figures and what the studies say.

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What does vitamin B12 do?

Vitamin B12, or cobalamin, is a water-soluble vitamin that contains cobalt. According to Anses, it acts as a cofactor for two key enzymes:

• methionine synthase, which recycles homocysteine into methionine in close partnership with vitamin B9 (folate). This is why a lack of B12 makes homocysteine rise in the blood;
• methylmalonyl-CoA mutase, in the mitochondria. When B12 is lacking, methylmalonic acid (MMA) builds up, which makes it a marker of deficiency.

In practical terms, B12 is essential for making red blood cells, for DNA synthesis and for a healthy nervous system.

The consequences of a deficiency

Anses describes two main pictures:

• Megaloblastic anaemia: abnormally large red blood cells, with tiredness and breathlessness caused by poorer oxygen transport. It resembles the anaemia of a B9 deficiency, but appears later.
• Neurological damage: gradual demyelination of the brain and spinal cord, problems with movement and sensation, sometimes irritability, memory or mood problems. These signs improve with B12, but Anses points out that in most cases lasting damage remains.

This is why you don’t wait for symptoms to act when you are at risk.

How much do you need?

In 2021 Anses adopted the EFSA values. They are based on four blood markers (cobalamin, holotranscobalamin, MMA and homocysteine): from 4 µg a day, these markers stay in the ranges that indicate good status.

Population	Adequate intake (µg/day)
Infants under 6 months	0.4
Infants aged 6 months and over	1.5
Children aged 1 to 10	1.5
Teenagers aged 11 to 17	2.5
Adults (men and women)	4
Pregnant women	4.5
Breastfeeding women	5

Two important points:

• There is no upper safe limit. EFSA has not set one, as no adverse effect has been observed, and Anses points this out. That doesn’t mean you should take it at random, but according to EFSA no harmful effect of a high intake has been observed to date.
• Needs during pregnancy are calculated from the B12 accumulated by the foetus (0.1 to 0.2 µg a day) and an absorption rate of 40%. For breastfeeding, EFSA takes into account the 0.4 µg a day secreted in milk.

For people who eat 100% plant-based, the Observatoire national des alimentations végétales (ONAV), cited by the French public health information service (sante.fr), recommends intakes of at least 25 µg a day for adults, as a supplement or fortified foods.

Where can you find vitamin B12?

B12 is made only by micro-organisms (bacteria and archaea), Anses reminds us. Animals concentrate it in their tissues. Hence its sources: offal (especially liver), fish and seafood, eggs, meat, milk and dairy products. The detailed ranking, with portions, is in our guide to foods high in vitamin B12.

Food (Ciqual)	Vitamin B12 (µg per 100 g)	Share of the adult AI (4 µg)
Calf’s liver, cooked	52.6	more than 13 times the AI
Pacific oyster, raw	28.6	more than 7 times the AI
Mackerel, roasted or baked	19	475%
Mussel, boiled	17.6	440%
Salmon, steamed	3.05	76%
Lean beef mince (5% fat), cooked	2.47	62%
Yellowfin tuna in brine, drained	2.05	51%
Emmental	1.84	46%
Hard-boiled egg	1.11	28%
Grilled chicken breast	0.18	4.5%
Semi-skimmed UHT milk	0.16	4%
Tempeh	0.019	less than 1%
Dried spirulina	0	0%

The takeaway: an omnivore who eats fish, meat, eggs and dairy generally meets 4 µg easily. Dairy products and eggs can also supply B12 for a vegetarian, as Manger Bouger points out, but the amounts are smaller: you need several portions a day to get close to the reference.

Work it out with the Ciqual figures for a typical vegetarian day: 100 g of hard-boiled egg (1.11 µg), 40 g of Emmental (0.74 µg) and 250 g of semi-skimmed milk (0.4 µg) provide about 2.25 µg. That is barely more than half of the 4 µg. Note that the 4 µg reference relates to what you eat, not what you absorb: the agencies have already built absorption rates of 40% (EFSA) to 50% (US IOM) into their calculations. So there is nothing to « add », but nothing to subtract either: a vegetarian who eats few eggs and little cheese can therefore stay below the reference without realising it.

What about plant foods?

The public site sante.fr states that active forms of B12 have been found in small amounts in some mushrooms (shiitake), some algae (nori) and fermented plant foods (tempeh, black tea), but that these amounts are insufficient to meet daily needs. Ciqual confirms this: 0.019 µg per 100 g of tempeh, and a zero value for spirulina. If someone sells you spirulina as a source of B12, be wary. Many plant foods mainly contain inactive forms of the vitamin.

Who should supplement?

Vegans: always

This is the clearest rule in all of nutrition. Manger Bouger (Santé publique France) writes: « Vegans must therefore take a vitamin B12 supplement ». Anses notes that B12 deficiency is particularly common among vegans. No combination of unfortified plant foods replaces a supplement. If you are just starting out, our guide to becoming vegetarian covers the other points to watch (iron, zinc, calcium, iodine).

Manger Bouger adds two warnings: a vegan diet is not suitable for children, given the risk of insufficient intakes of iron, protein, calcium and vitamins; and a pregnant vegan woman needs particular medical attention. Anses also points out that neurological problems have been observed in children of mothers following a strict vegetarian diet.

Vegetarians and flexitarians: often

According to sante.fr, flexitarians, vegetarians and vegans « would do well to supplement their diet with vitamin B12 », through fortified foods or supplements. It all depends on how many eggs and dairy products you actually eat and, for flexitarians, how much fish and meat. If in doubt, talk to your doctor.

The elderly

The same sante.fr document notes that older people are particularly at risk because of a reduced capacity to absorb it. The Cochrane review on B12 also notes that deficiency is common and that its incidence increases with age. See our guide to nutrition for older people.

Certain medicines and digestive conditions

• Metformin (a treatment for type 2 diabetes): since June 2022 the UK medicines agency (MHRA) has considered reduced B12 a common side effect, which can affect up to 1 in 10 people, especially at high doses or over a long period. A meta-analysis of 17 observational studies (Kakarlapudi, 2022) found 23.16% deficiency on metformin against 17.4% without (OR 2.95). Never stop it on your own: the deficiency is corrected, the treatment is not withdrawn.
• Proton pump inhibitors (antacids of the « -prazole » type) and colchicine are among the risk factors identified by the MHRA in patients taking metformin. If you take a PPI long term, for example for acid reflux, tell your doctor.
• Absorption disorders: pernicious anaemia (Biermer’s disease), gastrectomy, Imerslund disease (cited by ANSM, the French medicines agency), Crohn’s disease and inflammatory bowel diseases (cited by the MHRA). In these situations, ANSM points out that intramuscular injection is sometimes essential, and that going untreated exposes you to severe anaemia.

How to supplement

Tablet or injection?

For the vast majority of people, the oral route is enough. The 2018 Cochrane review (Wang et al.) compared oral and intramuscular B12 in deficient people: with 1,000 µg a day by mouth, there was no clinically relevant difference in blood B12 levels in two trials; one trial at 2,000 µg a day even showed an advantage for oral. Oral treatment cost less. The level of evidence remains low (3 trials, 153 participants). A 2025 meta-analysis (Mazur et al., 16 studies, 6,098 participants) also concludes that the oral and sublingual routes appear as effective as injection for improving status and lowering homocysteine.

In 2024, during a shortage, ANSM asked people to prefer tablets to ampoules for oral use, so as to reserve injectable ampoules for patients who cannot absorb B12 through the digestive tract.

Cyanocobalamin or methylcobalamin?

Cyanocobalamin is the form most used in medicines and studies. The methylcobalamin and adenosylcobalamin forms correspond to the two active coenzymes described by Anses. In the official sources we consulted, we found no evidence that one form of supplement is superior to another in a healthy person. Our comparison of the best vitamin B12 supplements details the forms, doses and prices of the products on the market.

What dose?

There is no single official protocol for the general public. Two reference points frame the thinking: the AI of 4 µg a day (Anses) and, for a 100% plant-based diet, at least 25 µg a day according to ONAV. The doses of 1,000 to 2,000 µg a day used in the Cochrane trials relate to treating a proven deficiency, on prescription. The dose and the frequency (daily or weekly) should be discussed with your doctor or pharmacist, especially if a deficiency is confirmed.

The blood test

No marker is perfect. Anses stresses that no marker of cobalamin activity is sufficient on its own. The target values adopted by the German-speaking countries (D-A-CH), cited in its report, give an idea of the orders of magnitude:

Marker	Target value (D-A-CH)	What it measures
Serum cobalamin	above 221 pmol/L	total circulating B12
Holotranscobalamin (holoTC)	40 pmol/L or more	B12 available to cells
Methylmalonic acid (MMA)	below 270 nmol/L	rises when B12 is lacking in cells
Homocysteine	below 12 µmol/L	rises when B12 or B9 is lacking

The MHRA advises testing B12 in patients on metformin who have severe tiredness, a sore red tongue, pins and needles, or pale or yellow skin. These are signs everyone should know, but they are not specific: only the tests done by your doctor can give a conclusion.

Common myths about vitamin B12

« Algae and spirulina provide B12 »

False in practice. Ciqual gives a zero content for spirulina, and sante.fr reminds us that plant foods mainly contain inactive forms, in insufficient amounts.

« If you eat organic and varied food, a vegan doesn’t need a supplement »

False. The quality of the diet doesn’t change the fact that B12 comes from micro-organisms and animal products. Santé publique France is categorical: supplementation is necessary.

« B12 gives you energy »

Only if you are lacking it. Tiredness is one of the signs of deficiency, but nothing in the sources we consulted shows that a supplement gives energy to someone whose status is normal.

« Injections are more effective than tablets »

Not for most people. The Cochrane review shows similar effects on blood levels at a sufficient oral dose. Injection remains essential in the case of an absorption disorder (pernicious anaemia, gastrectomy).

B12-rich meals for omnivores and vegetarians

• Seafood: light moules marinières or mussels with curry and coconut. With 17.6 µg per 100 g of cooked mussels, one portion more than covers the day.
• Oily fish: mackerel or salmon, once or twice a week, in line with Santé publique France’s guideline of « fish twice a week, one of them oily ».
• Eggs and dairy for vegetarians: baked eggs with spinach or a protein omelette, a piece of Emmental.
• On the plant side, our vegan recipes and the guide to plant proteins help you balance the rest of the plate. The B12 itself will come from the supplement or from fortified foods whose label you check.

Precautions

• Don’t put a child on a vegan diet without medical supervision: Manger Bouger advises against it.
• If you are pregnant or breastfeeding and eat few or no animal products, talk to your midwife or doctor early (see also our pregnancy guide).
• Pins and needles, problems walking or memory problems warrant a prompt consultation: neurological damage can leave lasting effects.

Frequently asked questions

How much vitamin B12 a day?

Anses sets 4 µg a day for an adult, 4.5 µg during pregnancy and 5 µg during breastfeeding. For a 100% plant-based diet, ONAV recommends at least 25 µg a day as a supplement or fortified foods.

Can you overdose on B12?

No upper safe limit has been set: EFSA has not observed any harmful effect. Even so, follow the dose given on the product or prescribed.

Should a vegetarian take B12?

Often, yes. Eggs and dairy provide it, but in modest amounts. Sante.fr recommends that vegetarians and flexitarians supplement or use fortified foods. Review your situation with your doctor.

Which food is highest in vitamin B12?

Liver: 52.6 µg per 100 g of cooked calf’s liver according to Ciqual. Next come oysters (28.6 µg), mackerel (19 µg) and mussels (17.6 µg).

Does metformin lower B12?

Yes, it is a common side effect according to the UK medicines agency, affecting up to 1 patient in 10. Periodic testing is advised for patients at risk. Don’t stop your treatment: talk to your doctor.

B12 tablet or injection?

The tablet is enough in most cases, according to the 2018 Cochrane review. Injection is reserved for people who absorb B12 poorly through the gut, as in pernicious anaemia.

This article is for information only and does not replace the advice of your doctor or a dietitian.

Our sources

• Anses. Avis et rapport d’expertise collective relatifs à l’actualisation des références nutritionnelles françaises en vitamines et minéraux (saisine 2018-SA-0238). March 2021.
• Anses. Table de composition nutritionnelle des aliments Ciqual. 2025.
• Santé publique France, Manger Bouger. Peut-on être végétarien ou végétalien sans risque pour sa santé ?
• Service public d’information en santé (sante.fr). Doit-on prendre des compléments de vitamine B12 lorsqu’on est flexitarien ? 2026.
• ANSM. Vitamine B12 injectable et buvable : préférez les comprimés aux ampoules pour l’usage par voie orale. September 2024.
• MHRA (Medicines and Healthcare products Regulatory Agency). Metformin and reduced vitamin B12 levels: new advice for monitoring patients at risk. Drug Safety Update, June 2022.
• Wang H, Li L, Qin LL, Song Y, Vidal-Alaball J, Liu TH. Oral vitamin B12 versus intramuscular vitamin B12 for vitamin B12 deficiency. Cochrane Database of Systematic Reviews, 2018. PMID 29543316.
• Mazur M, Ndokaj A, Salerno C et al. Efficacy of sublingual and oral vitamin B12 versus intramuscular administration: insights from a systematic review and meta-analysis. Frontiers in Pharmacology, 2025. PMID 41487531.
• Kakarlapudi Y, Kondabolu SK, Tehseen Z et al. Effect of Metformin on Vitamin B12 Deficiency in Patients With Type 2 Diabetes Mellitus and Factors Associated With It: A Meta-Analysis. Cureus, 2022. PMID 36628003.