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Vitamin B12 deficiency: signs, causes and when to supplement

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The OneVit Team

The OneVit Team

Updated on

15th October 2026

reading time

8 min

Vitamin B12 deficiency: signs, causes and when to supplement

Tired all the time? Pins and needles you can't explain? Vitamin B12 deficiency is one of the things worth ruling out.

It's more common than many people realise, especially as you get older. And vitamin B12 deficiency symptoms are easy to mistake for stress or a busy few months.

We've looked at the latest UK guidance on the signs, the causes and when a supplement makes sense.

What is vitamin B12 deficiency?

Vitamin B12 deficiency means your body doesn't have enough B12 to work properly. It can come from a lack of B12 in your diet, problems absorbing it, or both1.

B12 is a busy vitamin. Vitamin B12 contributes to normal red blood cell formation. It also contributes to normal functioning of the nervous system2.

Your liver keeps a large store of B123. So a low intake doesn't cause problems overnight. It tends to creep up slowly.

How common is it?

It gets more common with age. NICE estimates it affects around 5% of people aged 65 to 74. That rises to more than 10% of people aged 75 and over1.

You don't need much. Adults aged 19 to 64 need about 1.5 micrograms a day4. The problem is usually getting it or absorbing it.

Vitamin B12 deficiency symptoms to look out for

The most common vitamin B12 deficiency symptoms are unexplained tiredness, brain fog and pins and needles. But they vary a lot from person to person1.

NICE lists these as common symptoms and signs1:

  • tiredness you can't explain
  • trouble concentrating or short-term memory problems (often called brain fog)
  • pins and needles or numbness
  • balance problems, falls or changes to the way you walk
  • blurred vision or other eyesight problems
  • a sore, red or swollen tongue
  • anaemia picked up on a blood test

Low B12 can also be linked with mood changes, including symptoms of depression or anxiety1.

Here's the catch. None of these are unique to B12. Lots of other conditions cause them too1.

And a normal blood test doesn't rule it out. You can have low B12 without anaemia1.

B12 deficiency and your tongue

A sore, smooth or red tongue is one of the more visible signs. Doctors call it glossitis, and it's on NICE's list of common signs1.

Low B12 symptoms in women

The symptoms themselves are the same for women and men. But a few things are specific to women1:

  • your body can need more B12 during pregnancy and breastfeeding
  • anaemia that isn't improving with iron during pregnancy or breastfeeding can be a sign
  • the combined pill can lower B12 test results without causing a true deficiency

If you are pregnant or breastfeeding and think your B12 might be low, speak to your GP or midwife.

What are the 4 stages of B12 deficiency?

B12 deficiency doesn't happen all at once. One well-known model, from the researcher Victor Herbert, splits it into four stages5:

  1. Negative balance: you're taking in less B12 than your body uses.
  2. Depletion: your stores start to run down.
  3. Damaged metabolism: B12 is now too low for normal red blood cell production.
  4. Deficiency anaemia: low B12 shows up as anaemia.

In the first two stages, a standard blood count is usually normal5.

It's a research model, not a diagnostic checklist. Your GP won't stage you this way. But it shows why low B12 can build quietly for a long time before anything shows up.

What causes B12 deficiency?

Most B12 deficiency comes down to two things. Either you're not getting enough, or your body can't absorb it properly1.

Diet

B12 is found naturally in animal foods like meat, fish, milk, cheese and eggs. It isn't found naturally in fruit, vegetables or grains, so vegans may not get enough4.

Reviews of the research have found higher rates of low B12 in vegans than in vegetarians. But vegetarians are at risk too6.

Other diet-related risks include allergies to eggs, milk or fish. Finding it hard to buy or prepare food counts too1.

Absorption problems

To absorb B12, your stomach and gut need to be working well. Some conditions get in the way1:

  • autoimmune gastritis (often called pernicious anaemia)
  • coeliac disease and some other autoimmune conditions
  • some stomach or bowel surgery, including many weight loss operations
  • previous radiotherapy to the abdomen or pelvis

A family history of B12 deficiency or autoimmune conditions also raises your risk1.

Medicines

Some common medicines can lower your B12 over time. The best known is metformin, which is used for type 2 diabetes.

The MHRA now lists low B12 as a common side effect of metformin. The risk goes up with higher doses and longer use7.

Indigestion medicines called proton pump inhibitors (PPIs) and H2 blockers are also linked to low B121.

Do not stop taking a prescribed medicine because of B12 concerns. Speak to your GP or pharmacist first.

Nitrous oxide and age

Recreational nitrous oxide (laughing gas) inactivates B12 in the body. NICE describes recreational use as a significant public health issue1.

Age matters too. It gets harder to absorb B12 as you get older4.

Which foods are rich in B12?

The richest natural sources are animal foods4:

  • meat
  • fish
  • milk and cheese
  • eggs

If you eat these regularly, you should be able to get enough B12 from your diet4.

B12 foods for vegetarians and vegans

Plant foods don't naturally contain B12. Fortified foods are the main way to fill that gap1:

  • some breakfast cereals
  • yeast extract spreads
  • fortified drinks, such as some plant milks

Not every product is fortified, so check the label.

Food comes first. Think of supplements as backup, not a replacement for a varied diet.

Why a blood test comes before a supplement

If you think you have vitamin B12 deficiency, see your GP before you start a supplement. Here's why.

NICE advises testing people who have at least one common symptom and at least one risk factor1. A single blood test can be enough, though some people need more tests1.

Taking B12 before the test can raise your levels without fully treating a deficiency1. That makes the result harder to read.

The cause matters too. B12 deficiency from autoimmune gastritis, for example, needs lifelong injections from your GP1.

Watch out for high-dose folic acid

Taking more than 1mg of folic acid a day can mask the symptoms of B12 deficiency. If it's missed, the deficiency can eventually damage your nervous system4.

If you have numbness, balance problems or changes to your eyesight, do not wait. Speak to your GP.

When does a B12 supplement make sense?

A B12 supplement makes most sense when the gap is in your diet, not your gut. That usually means you:

  • follow a vegan diet
  • are vegetarian and eat few eggs or dairy foods
  • rarely eat B12-fortified foods

A review of 18 studies found vegetarians can run low on B12 whatever their age or diet type. It concluded they should take steps to keep their intake up, including regular supplements6.

What B12 does in your body

Vitamin B12 contributes to the reduction of tiredness and fatigue. It also contributes to normal energy-yielding metabolism2.

On top of that, vitamin B12 contributes to normal psychological function. And it contributes to the normal function of the immune system2.

Which type of vitamin B12 tablets?

Not all B12 supplements are equal. NICE advises choosing one that contains cyanocobalamin, methylcobalamin or adenosylcobalamin1.

OneVit Vitamin B12 contains 1,000µg of methylcobalamin per daily tablet.

How much is safe?

Taking 2mg (2,000µg) or less of B12 a day in supplements is unlikely to cause any harm4. Do not take more than the recommended daily dose on the label.

If you are pregnant, breastfeeding or taking any medicines, speak to your GP or pharmacist before taking a B12 supplement.

How long does it take to recover from B12 deficiency?

With treatment from your GP, symptoms often start to improve within two weeks. But it can take up to three months1.

It can take much longer for symptoms to go completely. Some people feel a little worse at first before they get better1.

If your symptoms are not improving, are getting worse or come back, speak to your GP1.

Frequently asked questions

What are the first signs of B12 deficiency? Unexplained tiredness, brain fog and pins and needles are among the most common. But symptoms vary a lot, and many other conditions cause them too. A blood test is the only way to know.

Can B12 deficiency cause hair loss? Hair loss isn't on NICE's list of common symptoms of B12 deficiency. It has lots of possible causes, so it's worth speaking to your GP.

Is pernicious anaemia the same as B12 deficiency? Not quite. The term is often used for autoimmune gastritis, one possible cause of B12 deficiency. It stops your stomach absorbing B12 properly, so it needs lifelong treatment from your GP.

Can you get enough B12 on a vegan diet? B12 isn't found naturally in plant foods. So vegans need fortified foods, such as some cereals and plant milks, or a B12 supplement.

How long do B12 tablets take to work? For a diagnosed deficiency, symptoms often improve within two weeks to three months of treatment. If you're covering a diet gap, you're topping up your intake. So you may not notice a change.

Can you take too much B12? Taking 2mg or less a day in supplements is unlikely to cause any harm. Do not take more than the recommended daily dose on the label.

Can I take B12 and folic acid together? Yes, many supplements contain both. But folic acid doses above 1mg a day can hide the signs of B12 deficiency. If you are taking high-dose folic acid, speak to your GP or pharmacist.

Is B12 deficiency serious? It can be if it's not spotted. Untreated, it can eventually damage your nervous system. The good news is that treatment works for most people.

This article is for informational purposes only and does not constitute medical advice. If you are concerned about vitamin B12 deficiency, consult a GP or qualified healthcare professional.

References

  1. National Institute for Health and Care Excellence. Vitamin B12 deficiency in over 16s: diagnosis and management (NG239). 2024. NICE

  2. Department of Health and Social Care. Great Britain nutrition and health claims (NHC) register. GOV.UK

  3. MSD Manual Professional Version. Vitamin B12 deficiency. MSD Manuals

  4. NHS. B vitamins and folic acid. NHS

  5. Herbert V. Staging vitamin B-12 (cobalamin) status in vegetarians. Am J Clin Nutr. 1994;59(5 Suppl):1213S-1222S. American Journal of Clinical Nutrition

  6. Pawlak R, Parrott SJ, Raj S, Cullum-Dugan D, Lucus D. How prevalent is vitamin B12 deficiency among vegetarians? Nutr Rev. 2013;71(2):110-117. PubMed

  7. Medicines and Healthcare products Regulatory Agency. Metformin and reduced vitamin B12 levels: new advice for monitoring patients at risk. Drug Safety Update. 2022;15(11). GOV.UK

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