B12 deficiency: symptoms, causes and fixes

By the DieticiansWorldwide.com editorial team · 17 October 2026 · Clinical nutrition

What vitamin B12 does and why a shortage matters

Vitamin B12 (cobalamin) is a water-soluble vitamin your body needs to make healthy red blood cells, copy DNA and build and maintain the nervous system. The US National Institutes of Health describes it as required for "the development, myelination, and function of the central nervous system" (NIH Office of Dietary Supplements).

Two features make B12 different from most vitamins. First, it occurs naturally only in foods of animal origin. Plants do not make it, although some foods such as breakfast cereals and nutritional yeast are fortified with it. Second, absorbing it from food is a multi-step process. Stomach acid releases B12 from protein in food, it then binds to intrinsic factor (a protein made by cells in the stomach lining), and the pair is absorbed at the far end of the small intestine. A problem at any of those steps can cause deficiency, even in someone who eats plenty of meat and dairy.

That is why B12 deficiency is not just a "vegan problem". In clinic, dietitians see it in older adults with stomach inflammation, in people taking common medicines for diabetes or reflux, after weight-loss surgery and in people who use nitrous oxide recreationally.

How much B12 you need

Daily requirements are tiny, measured in micrograms (µg, one millionth of a gram), and different authorities set different figures because they use different methods.

• UK: the NHS says adults aged 19 to 64 "need about 1.5 micrograms a day of vitamin B12" (NHS, B vitamins and folic acid).
• US and Canada: the Recommended Dietary Allowance ranges from 2.4 to 2.8 µg for adults, with the higher end for pregnancy and breastfeeding (NIH ODS).
• European Union: in 2015 the European Food Safety Authority set an Adequate Intake of 4 µg a day for adults, 4.5 µg in pregnancy and 5 µg during breastfeeding, based on blood markers of B12 status (EFSA NDA Panel, 2015).

None of these numbers is "right" and the others "wrong"; they reflect different definitions. In practice, an omnivorous diet usually exceeds all three. The risk lies in diets that exclude animal foods without a reliable fortified source, and in absorption problems, where no amount of food will be enough.

Symptoms: how you feel when B12 is low

Low B12 tends to creep up slowly. The NIH notes that symptoms "can take several years to appear" (NIH ODS). That slow onset makes it easy to blame the tiredness on work, age or stress.

The NHS lists general anaemia symptoms such as feeling weak or tired, shortness of breath, headaches, palpitations, loss of appetite and "a sore or red tongue, sometimes with mouth ulcers". Specific to B12, it adds numbness, muscle weakness, pins and needles, problems with balance and coordination, incontinence, and "psychological problems, which can range from mild depression or anxiety, to confusion and dementia" (NHS, symptoms of B12 or folate deficiency anaemia).

NICE's 2024 guideline groups the common symptoms and signs that should prompt a test:

• abnormal blood count results, such as anaemia or large red cells (macrocytosis);
• cognitive difficulties, including poor concentration or short-term memory loss, sometimes called "brain fog";
• eyesight problems linked to the optic nerve, such as blurred vision;
• glossitis (a smooth, sore tongue);
• nerve or mobility problems: balance issues and falls, an unsteady gait, pins and needles or numbness;
• unexplained fatigue (NICE NG239).

Two points in that guideline matter for anyone reading their own blood results. NICE tells clinicians not to rule out B12 deficiency "based solely on the absence of either anaemia or macrocytosis", and to be aware that deficiency can be associated with depression, anxiety or psychosis. In other words, a normal full blood count does not settle the question. The NIH makes the same point: neurological symptoms "can occur without anemia".

The four stages of B12 deficiency

People often ask about the "four stages" of deficiency. The model comes from a 1994 paper by the haematologist Victor Herbert, which described what happens when B12 intake stops (Herbert, American Journal of Clinical Nutrition, 1994):

1. Serum depletion. The B12 carried on the delivery protein transcobalamin (measured as "active B12" or holotranscobalamin) falls first, within days of absorption stopping.
2. Cell depletion. Stores inside cells, including red blood cells, start to run down.
3. Biochemical deficiency. Chemical reactions that need B12 slow down, so homocysteine and methylmalonic acid (MMA) build up in the blood.
4. Clinical deficiency. Anaemia and other symptoms appear.

The model explains why a total B12 test can look normal while active B12 or MMA is already abnormal. It is a framework rather than a strict timetable, and nerve symptoms do not always wait for stage four.

Who is at risk

NICE lists common risk factors that, combined with at least one symptom, should lead to testing (NICE NG239). They fall into five groups.

Diet

A diet low in B12 without regular supplements: vegan diets, diets that exclude meat for religious reasons, allergies to eggs, milk or fish, difficulty buying or preparing food (for example in dementia or frailty), low income, and restricted eating, including eating disorders. A 2014 review of 40 studies found deficiency rates of up to 86.5% in adults and older people following vegetarian diets, with the highest rates in vegans who did not take supplements (Pawlak and colleagues, European Journal of Clinical Nutrition, 2014). The range was wide, from 0% to 86.5%, because studies used different tests and populations, but the direction was consistent.

If you are supporting a child or adult with very limited eating, B12 is one of the nutrients worth checking; our guide to ARFID covers that situation in more detail.

Stomach and gut conditions

Autoimmune gastritis, often called pernicious anaemia, destroys the stomach cells that make acid and intrinsic factor, so B12 cannot be absorbed from food or ordinary supplements. The NIH says atrophic gastritis affects about 2% of the general population but 8% to 9% of adults aged 65 and over. Coeliac disease and other autoimmune conditions such as thyroid disease and type 1 diabetes also raise the risk.

Medicines

NICE names metformin, proton pump inhibitors (such as omeprazole), H2-receptor antagonists, colchicine, phenobarbital, pregabalin, primidone and topiramate. Metformin is the big one, because so many people take it. In June 2022 the UK medicines regulator said B12 deficiency "may affect up to 1 in 10 people who take it" and that the risk "increases with increasing metformin dose and treatment duration" (MHRA Drug Safety Update, 2022). The MHRA's advice is to keep taking metformin as long as it is tolerated, and to test B12 if deficiency is suspected.

Surgery and radiotherapy

Total gastrectomy, removal of the terminal ileum, many bariatric operations including gastric bypass and sleeve gastrectomy, and previous abdominal or pelvic radiotherapy. NICE says deficiency is "highly likely" after total gastrectomy or complete terminal ileal resection if the person is not receiving B12 replacement.

Recreational nitrous oxide

NICE lists recreational nitrous oxide use as a risk factor and recommends different first tests (homocysteine or MMA) when it is the suspected cause, because total B12 can look normal.

Testing: which blood test and what the numbers mean

Under the 2024 NICE guideline, the first test can be either total B12 (serum cobalamin) or active B12 (holotranscobalamin). Active B12 should be used as the first test in pregnancy (NICE NG239).

Laboratories with validated local cut-offs may use their own. The NIH notes that many US labs treat values below 200 or 250 pg/mL (1 pg/mL equals 1 ng/L) as low, and that MMA can confirm deficiency when B12 falls between 150 and 399 pg/mL (NIH ODS).

Several things can distort the result. NICE warns that over-the-counter B12 tablets, injections or patches can raise blood levels "without fully treating a deficiency", that the combined contraceptive pill can lower total B12 without true deficiency, and that people of Black ethnicity may have a higher reference range. Blood should be taken before any B12 is started. If a test is indeterminate and symptoms are serious, or the person is pregnant, has had relevant surgery or probably has autoimmune gastritis, NICE says to start treatment while waiting for the MMA result.

Treatment: tablets versus injections

The right route depends on why the deficiency happened. This is the heart of the "B12 injections vs tablets" debate, and the 2024 guideline gives clear rules (NICE NG239).

When injections are the standard

NICE recommends lifelong intramuscular B12 for people whose deficiency is caused, or probably caused, by autoimmune gastritis, and for people who have had a total gastrectomy or complete terminal ileal resection. In these situations the absorption route is permanently broken. For other malabsorption causes, such as coeliac disease, partial gastrectomy or some bariatric surgery, NICE says to consider injections instead of tablets, and if tablets are used, to prescribe "at least 1 mg a day".

In the UK, the NHS describes a loading phase of hydroxocobalamin injections "every other day" at first, followed by an injection every 2 to 3 months for life when the cause is not dietary (NHS, treatment). People with neurological symptoms are referred to a haematologist and may need injections every 2 months.

When tablets work well

For diet-related deficiency, NICE says to consider oral B12, with at least 1 mg a day in pregnancy or breastfeeding. For deficiency caused by medicines or nitrous oxide, tablets or injections are both options, chosen by clinical judgement and preference. Injections may be preferred if someone has neurological or blood problems that could worsen quickly, or if they are unlikely to take tablets reliably, for example because of frailty, cognitive impairment or homelessness.

The best evidence comparing the routes is a 2018 Cochrane review of three randomised trials with 153 participants. In two trials using 1,000 µg a day by mouth, there was "no clinically relevant difference" in blood B12 levels compared with injections over three to four months, and oral treatment cost less (Wang and colleagues, Cochrane Database of Systematic Reviews, 2018). The authors rated the evidence as low quality because the trials were small, and none measured symptoms, so oral treatment is reasonable for many people but not a proven substitute in every case.

Choosing a supplement

If you buy B12 yourself, NICE advises choosing a product that contains cyanocobalamin, methylcobalamin or adenosylcobalamin, because "some supplements do not contain enough, or the right type, of vitamin B12 to be effective". The NIH reports that B12-only supplements typically contain 500 to 1,000 µg, B-complex products 50 to 500 µg and multivitamins 5 to 25 µg. The NHS says taking "2mg or less a day of vitamin B12 in supplements is unlikely to cause any harm", and the US has set no upper limit because B12 is considered safe even at high doses. Our separate guide to vitamin B12 supplements compares forms in more detail, and the supplements hub explains how products are regulated.

How fast does treatment work?

NICE tells clinicians to explain that symptoms "could start to improve within 2 weeks, but this may take up to 3 months", that it can take much longer for symptoms to disappear completely, and that symptoms can sometimes get worse before they improve. Follow-up is recommended at 3 months, or at 1 month in pregnancy or breastfeeding. If symptoms are still interfering with daily life on tablets, the next step is the maximum licensed oral dose or a switch to injections.

So the honest answer to "what is the fastest way to fix B12 deficiency?" is: get the cause identified and the right treatment started without delay. For anyone with suspected anaemia and neurological symptoms, NICE says treatment should not wait for test results at all.

Raising B12 through food

Food matters most for prevention and for people whose deficiency is purely dietary. The NIH lists these amounts per serving, taken from the USDA FoodData Central database (NIH ODS):

Two practical lessons follow. If you eat fish, meat, eggs and dairy regularly, you almost certainly meet the UK, US and EU intakes. If you are vegan, "natural" plant sources such as tempeh, seaweed or mushrooms are not reliable; fortified foods or a supplement are needed. You can check how your overall pattern compares with dietary guidelines using our free healthy eating check.

Food will not fix deficiency caused by autoimmune gastritis or surgery, because the problem is absorption, not intake. That is the most common misunderstanding we see: people with pernicious anaemia trying to eat their way out of it.

How strict does the diet need to be?

For prevention, not very, as long as you have one dependable source every day. A vegan who has fortified plant milk, fortified cereal and nutritional yeast most days, or takes a supplement, does not need to plan every meal around B12. Someone who has stopped eating meat and dairy and relies on whole plant foods alone does need to act.

For treatment, diet is a support, not the cure. NICE stresses that a vegetarian or vegan diet "may not be the cause, or the only cause" of someone's deficiency, so a clinician should look for medicines, gut conditions and other factors before assuming diet is to blame.

Generic diet sheet or an individual plan?

A handout listing "B12-rich foods" is fine for a healthy omnivore. It is not enough when deficiency sits alongside other problems: diabetes treated with metformin, life after bariatric surgery, coeliac disease, pregnancy on a vegan diet, an older relative who is eating poorly, or a restricted eating pattern. Each of those needs an assessment of what you eat, which medicines you take and what your blood results show, and often coordination with your GP.

A registered dietitian can review your intake, recommend fortified foods that suit your culture and budget, check for related shortfalls such as iron and folate, and make sure supplements are not hiding a result your doctor needs. Our guide to the difference between a dietitian and a nutritionist explains which titles are protected, and our United Kingdom country guide covers how to find one.

If B12 is one part of a wider plan, for example lowering blood pressure with the DASH diet or cutting salt with a low-sodium diet, a dietitian can fit it all together rather than giving you several separate sheets. The same applies if you are losing weight with a dietitian-led weight-loss plan after bariatric surgery.

Book a session with a dietitian

B12, memory and dementia

Because low B12 can cause confusion and memory problems, people sometimes hope that supplements will protect the brain. The NIH says randomised trials "have shown no improvement in cognitive function" among older adults with or without dementia given B12 alone or with folic acid and vitamin B6. That does not make B12 irrelevant: correcting a true deficiency is essential, and NICE lists cognitive difficulties as a reason to test. But in people with normal levels, extra B12 is not a dementia treatment. Our article on the MIND diet looks at the eating patterns that have been studied for brain health.

The practical takeaway

• If you have symptoms such as fatigue, pins and needles, balance problems, a sore tongue or memory changes, and a risk factor (a vegan diet, metformin, acid-suppressing medicines, stomach surgery, autoimmune disease or nitrous oxide use), ask your GP for a B12 test before starting supplements.
• Expect treatment to match the cause: lifelong injections for autoimmune gastritis or major gut surgery; tablets of at least 1 mg a day are an option for many other causes.
• If you are vegan, use fortified foods or a supplement every day; plant foods alone are not a reliable source.
• If you take metformin long term, keep taking it, and ask whether periodic B12 checks are right for you.
• For a plan that fits your diet, medicines and health conditions, book a session with a registered dietitian.