Do vegans need B12 supplements, and how much?

The objection

“A vegan diet can’t supply B12 without a manufactured pill, and going short of it can damage your nerves for good. Surely a diet that needs a pill to keep you safe is the wrong diet for humans.”

The answer

The objection is right about the pill. Vitamin B12 is made only by microbes, a few kinds of bacteria and archaea, and never by plants or animals [1], so a vegan has to get it from fortified foods or a supplement. The British Dietetic Association (BDA) says that for vegans “the only reliable sources of vitamin B12 are fortified foods and supplements” [2], and the NHS says that because B12 isn’t found naturally in fruit, vegetables and grains, vegans may not get enough of it [3].

In practice supplementation is simple. The BDA, following the Vegan Society, suggests at least 10 micrograms a day or 2,000 micrograms once a week from a supplement, or fortified foods eaten often enough to supply 3 micrograms a day [2][4]. Untreated deficiency can damage nerves, and babies breastfed by vegan mothers who don’t supplement are especially at risk, so it matters most of all in pregnancy and while breastfeeding [4][5].

Needing a supplement doesn’t single the diet out as unnatural, though, because animals can’t make B12 either [1][16]. Cattle and sheep absorb B12 made by microbes in their own digestive system [4][16], and those microbes need cobalt to make it. Forage and many feeds typically hold too little cobalt to meet dairy cows’ needs, so it has to be added to their diet (Siregar et al., 2025) [16]. B12 itself has been used in animal feed worldwide for decades, typically at 3 to 150 micrograms per kilo of feed depending on the species (European Food Safety Authority, 2023) [17], and it is made, like the B12 in vegan supplements, by microbial fermentation [1][17]. The wider argument about supplements and fortified food has its own answer.

The need for a supplement hasn’t stopped dietitians backing the diet. Dietitians Australia says a vegan diet “can provide all the nutrients you need for good health” and that vegans “should eat foods with added vitamin B12 or take a supplement” [19], and the US Academy of Nutrition and Dietetics says appropriately planned vegan diets can be nutritionally adequate for adults [20]. Dietitians in the UK, Ireland and Canada say much the same [2][21][22]. The European Society for Paediatric Gastroenterology, Hepatology and Nutrition (ESPGHAN) is more guarded about children: its 2025 review found the evidence inconclusive on whether a strictly vegan diet supports normal growth, though vegan children’s height and body mass index didn’t differ from their omnivorous peers’ [28].

How much B12 should a vegan take?

A vegan should take at least 10 micrograms of B12 a day or at least 2,000 micrograms once a week, or eat fortified foods two or three times a day that add up to at least 3 micrograms [4]. These are the Vegan Society’s three options, first set out in an open letter in 2001, and the BDA, a professional body for dietitians, repeated them in its 2021 fact sheet [2][4]. The Vegan Society is a campaigning charity that sells its own B12 supplement, which is why the BDA’s backing matters.

Option	Amount	How often
Daily supplement	at least 10 µg	once a day
Weekly supplement	at least 2,000 µg	once a week
Fortified foods	at least 3 µg in total	2 to 3 times a day

The supplement doses are far above the reference intakes, which are about 1.5 micrograms a day for adults aged 19 to 64 in NHS guidance last reviewed in 2020 [3] and 4 micrograms for adults in the European Food Safety Authority’s 2015 figure [29], because the body absorbs a smaller share of B12 the more of it arrives at once. The Vegan Society puts absorption at about 50% of a dose of 1 microgram or less, falling to about 0.5% at 1,000 micrograms or more, so the less often you take it, the bigger each dose has to be [4]. It also advises chewing a tablet or letting it dissolve in the mouth [4].

For pregnancy, Ireland’s Health Service Executive (HSE) tells vegans to take a 10 microgram supplement every day, usually even if their pregnancy multivitamin already contains B12, and to eat B12-fortified foods at least twice a day as well [23]. ESPGHAN said in 2025 that B12 supplements are essential for vegan children and that their diet, growth and nutritional status should be checked regularly [28]. For a child, or in pregnancy and while breastfeeding, check the plan with a doctor or dietitian [15].

There is little point going far beyond these amounts. The Vegan Society says 10 micrograms spread over a day appears to supply as much as the body can use, and advises against more than 5,000 micrograms a week even though higher amounts have not been shown to be toxic [4]. The NHS says 2 mg (2,000 micrograms) or less a day from supplements is unlikely to cause any harm, though there isn’t enough evidence on what high daily doses do [3]. Health Canada has set no upper limit for B12 because suitable data are lacking, and adds that this does not mean high intakes carry no risk [6].

There is one reason to be wary of very high doses. In the US VITAL cohort of 77,118 people aged 50 to 76 (Brasky et al., 2017), men whose supplements had averaged more than 55 micrograms of B12 a day over ten years had about twice the rate of lung cancer of men who took none (hazard ratio 1.98, 95% CI 1.32 to 2.97) [7]. In absolute numbers, 28 of 1,266 men in that group (2.2%) were diagnosed during follow-up, which averaged six years, against 179 of 14,139 non-users (1.3%) [7]. The link was strongest in current smokers and absent in women, there were too few never-smokers to test it, and the lower dose bands showed no raised risk [7].

An observational study like this can’t show that the supplements caused the cancers, though a 2019 analysis led by researchers at the International Agency for Research on Cancer, of 5,183 people with lung cancer and matched controls from 20 cohorts, found that higher blood B12 went with higher risk (odds ratio 1.15 for each doubling, 95% CI 1.06 to 1.25), and gene variants that raise B12 pointed the same way (Fanidi et al., 2019) [30]. The daily 10 microgram option sits well below the VITAL level. The weekly 2,000 microgram option averages about 290 micrograms a day, which is above it, though each weekly dose is no more than the 2 mg a day that the NHS says is unlikely to cause any harm [3]. In a 2019 trial of 40 vegans and vegetarians with a marginal deficiency, 50 micrograms a day and 2,000 micrograms once a week, both dissolved under the tongue, each brought blood B12 back to adequate levels over 12 weeks, and the authors favoured the smaller daily dose (Del Bo’ et al., 2019) [27].

What happens if you’re low on B12?

A shortage of B12 causes a type of anaemia and can damage the nervous system, and some of that damage can be permanent, which is why the NHS stresses diagnosing and treating it as soon as possible [8]. The NHS lists extreme tiredness, pins and needles, a sore red tongue, mouth ulcers, muscle weakness, problems with vision and memory, and psychological problems ranging from mild depression or anxiety to confusion and dementia [8]. The body’s stores can last around 2 to 5 years, so problems can take a long time to show after a change of diet [9], though the Vegan Society notes that some people run into them within a year [4]. A diet rich in folate, as vegan diets usually are, can hide the anaemia until more serious problems appear, which makes a routine blood count a poor guide [4][10]. Babies are more vulnerable than adults: a breastfeeding mother who is short of B12 can pass the shortage on, and in an infant it can lead to permanent brain damage, or to coma or death if not promptly corrected [4][5].

In EPIC-Oxford, a study of British men whose blood was taken between 1994 and 1997, 52% of 232 vegans had serum B12 below the deficiency cut-off of 118 pmol/L, against 7% of 231 vegetarians and 1 of 226 meat-eaters (Gilsing et al., 2010) [11]. Only 19% of those vegans regularly took a supplement containing B12, over a third of those who did still got less than the UK reference intake of 1.5 micrograms a day, far below the 10 micrograms advised now, and across the study men who took a B12 supplement had no significantly higher blood levels than men who didn’t [4][11]. A 2024 meta-analysis of 17 studies that also measured functional markers of B12 status found the same gap, with vegans’ serum B12 lower and homocysteine higher than meat-eaters’, and in its subgroup analysis vegans who took B12 supplements did better on every marker than vegans who didn’t (Niklewicz et al., 2024) [25].

A single blood level is an imperfect test, though. When 65 of the vegan men gave blood again about six years later, 22 were under the cut-off, and of those, 82% also had low holotranscobalamin, the fraction of B12 that is delivered to cells, but only 32% had both the raised methylmalonic acid and the raised homocysteine that would mark a likely deficiency [11]. A more recent study found no gap where nearly every vegan supplemented. Of 36 vegans and 36 meat-eaters studied by Germany’s Federal Institute for Risk Assessment and published in 2020, 92% of the vegans took B12, and their B12 status was as good as the meat-eaters’ (Weikert et al., 2020) [12]. Both studies are cross-sectional, so they show who was low at one moment and can’t say who went on to become ill.

B12 deficiency isn’t only a vegan problem. NHS guidance last reviewed in 2023 says B12 and folate deficiency affect around 1 in 10 people aged 75 or over and 1 in 20 aged 65 to 74, and that the most common cause of B12 deficiency in the UK is pernicious anaemia, an autoimmune condition that stops the body absorbing B12 from food [8]. Because 10 to 30% of older people may not absorb the B12 in food well, Health Canada’s reference values (2025) advise everyone over 50, whatever they eat, to get most of theirs from fortified foods or a supplement [6]. Some medicines, including metformin and proton pump inhibitors (PPIs), can lower B12 too, as can nitrous oxide, and the NHS says GPs know which medicines do this and will monitor patients if necessary [9]. If you think you have symptoms, see a GP. If you take metformin, a PPI or another long-term medicine, ask your GP whether your B12 should be checked, and don’t change or stop any medicine without speaking to them first.

Can you get B12 from nutritional yeast, spirulina or seaweed?

Fortified nutritional yeast is a reliable source of B12, but spirulina and seaweed are not [4][13]. Not every nutritional yeast has B12 added, so check the label: the NHS lists nutritional yeast flakes “which are fortified with vitamin B12” among vegan sources [13], and MedlinePlus, from the US National Library of Medicine, says that some nutritional yeast products contain it [14]. Spirulina and other blue-green algae supplements often contain large amounts of pseudovitamin B12, which a common laboratory assay counts as B12 but which is inactive in humans (Watanabe et al., 2014) [10].

Seaweed is less settled. The Victorian government’s Better Health Channel in Australia says sea vegetables, mushrooms, tempeh and miso hold only traces of B12, along with a look-alike compound that interferes with how the body absorbs and uses it [15]. The 2014 review, by contrast, reported active B12 in dried purple laver, the seaweed sold as nori, and proposed it as the most suitable plant source for vegetarians, on evidence from laboratory analyses, rat studies and six vegan children [10]. The Vegan Society lists dried nori, spirulina and human gut bacteria among supposed sources that direct studies of vegans found inadequate, and warns that B12 look-alikes in algae can make a blood test read higher than a person’s true status [4]. A small 2024 trial has since tested nori directly: 30 vegetarians who took no B12 supplements were randomised to no nori or to 5 or 8 grams a day, and after four weeks both nori groups had better serum B12, holotranscobalamin and homocysteine than the controls (Huang et al., 2024) [26]. It was open-label, lasted a month and enrolled vegetarians, 80% of whom ate eggs and dairy, so until a longer trial shows nori keeps vegans’ levels up, the BDA’s view stands, and fortified foods and supplements remain the only reliable sources [2].

We can’t rely on our own gut microbes the way cattle do. In people, B12 is taken up through transporters found only in the small intestine, while bacteria further down, in the large intestine, turn about 80% of the B12 that reaches them into analogues with no known vitamin activity (Guetterman et al., 2022) [18]. B12 is found to some extent in soil and plants, but the Vegan Society notes that in over 60 years of vegan experimentation only fortified foods and supplements have proved reliable sources [4].