Signs of Low Vitamin B12: What Is Documented and What Is Not

An egg and a piece of cheese, two everyday animal foods that contain vitamin B12

The short answer

The documented signs of low vitamin B12 are megaloblastic anaemia and neurological changes such as numbness, tingling, balance problems, memory changes and a sore, smooth tongue. Nerve signs can appear without anaemia, a high folate intake can hide the blood picture, and risk groups include vegans, older adults, metformin users and people with pernicious anaemia. Fatigue alone is too vague to point to B12.¹

What does vitamin B12 do in the body?

Vitamin B12 is a cofactor for two enzymes, methionine synthase and methylmalonyl-CoA mutase. When B12 runs low, these reactions stall, homocysteine and methylmalonic acid accumulate, and both blood cell formation and nerve function come under pressure.

The EU has authorised health claims for vitamin B12 in fixed wording, among them these three. Vitamin B12 contributes to normal energy-yielding metabolism. Vitamin B12 contributes to normal functioning of the nervous system. Vitamin B12 contributes to normal red blood cell formation. These are functions of the nutrient, not promises about any product.

Deficiency shows itself clinically in two places: the blood and the nervous system.¹¹ Clinical B12 deficiency with classic haematological and neurological manifestations is relatively uncommon, but subclinical deficiency affects between 2.5% and 26% of the general population depending on the definition used.² Overt deficiency occurs in approximately 2% to 3% of adults in the United States.¹

Close-up of a salmon fillet, a food naturally rich in vitamin B12

What are the first signs of low vitamin B12?

The first signs are usually nonspecific: fatigue, brain fog and low mood. Nerve signs such as tingling, numbness or unsteady walking are more telling, and the blood change doctors look for is megaloblastic anaemia. Because these overlap with many conditions, they point to B12 only when a risk factor is also present.¹

Symptoms may include fatigue, brain fog, depression, peripheral neuropathy and ataxia.¹ Subclinical deficiency, usually defined as a total serum B12 below 200 pmol/L, often presents asymptomatically or with subtle generic symptoms that are easily ascribed to other causes.⁴

Sign	System	How specific to B12?
Macrocytic anaemia	Blood	Fairly specific together with low B12, but folate deficiency causes it too
Sore, smooth tongue (glossitis)	Mucosa	Moderately specific
Numbness and tingling in feet or hands	Nerves	Typical of B12 neuropathy but also seen in diabetes
Balance problems and ataxia	Nerves	Typical of spinal cord involvement
Memory and mood changes	Nerves	Common but nonspecific
Fatigue	General	Nonspecific, points to B12 only with a risk factor

Can low B12 damage nerves without causing anaemia?

Yes. Neurological signs can appear before, alongside or without anaemia, and they may become irreversible if diagnosis and treatment are delayed. That is why a normal blood count does not rule out B12 deficiency.¹

Vitamin B12 deficiency leads to neurological deterioration and megaloblastic anaemia, and if left untreated, to death.⁴ Missing the diagnosis can result in severe complications including degeneration of the spinal cord and pancytopaenia.⁹

Why can folate hide a B12 deficiency?

With a high folic acid intake, the blood count of someone who is low in B12 can look normal while the nerve signs are still there. The blood picture looks normal, so the diagnosis is masked at exactly the moment it matters most.⁷

Clinicians learned this in the 1940s and 1950s: in people with pernicious anaemia who took folic acid above 5 mg per day, the B12 deficiency was missed because the blood picture no longer showed it.⁷ Modern data reinforce the concern: cognitive test scores are lower and blood homocysteine and methylmalonic acid concentrations are higher in people with low B12 and elevated folate than in those with low B12 and nonelevated folate.⁷ The practical point is that a normal full blood count in someone with nerve symptoms and high folate intake does not exclude B12 deficiency.

Which signs are too vague to point to B12?

Fatigue, brain fog, poor concentration and low mood on their own are too vague to point to B12. Testing makes sense when at least one risk factor and one clinical feature are present together; universal screening is not recommended.¹

Subclinical deficiency presents asymptomatically or with subtle generic symptoms that are often mistakenly ascribed to unrelated disorders.⁴ The evidence-based position is conditional testing: a risk factor such as a plant-based diet, metformin use, long-term acid suppression, gastric surgery or older age, combined with a compatible sign such as neuropathy, macrocytosis or glossitis.

Who is most likely to run low on B12?

Risk falls into four main groups: people with low intake, older adults with reduced absorption, medication users, and people with pernicious anaemia or stomach surgery. Cobalamin is poorly represented in plants, so deficiency can arise from diet alone.¹¹

Plant-based diets

Vegans and strict vegetarians are at risk because cobalamin is poorly represented in plants.¹¹ A 2024 meta-analysis found no difference between vegans and vegetarians in holotranscobalamin or methylmalonic acid, but vegans had lower serum B12 and higher total homocysteine than vegetarians.⁸ Fortified foods or a supplement are the usual way to cover B12 on a plant-based diet; this does not mean every vegan is deficient.

Older adults

B12 deficiency can affect all ages but most particularly elderly individuals.² Older age is a recognised risk factor alongside malabsorption and limited dietary intake.¹

Metformin and acid blockers

Metformin lowers serum B12 by a pooled 57 pmol/L after 6 weeks to 3 months of use, and by 63.70 pM overall depending on dose and duration, a 14.68% greater decrease from baseline.⁵⁶ Annual B12 assessment is recommended for diabetic patients taking metformin.⁶ Proton pump inhibitor use is more frequent among B12-deficient individuals than non-deficient individuals (OR 1.49).¹²

Pernicious anaemia and surgery

Pernicious anaemia is the most common cause of B12 deficiency anaemia in the world and is an autoimmune disease comprising autoimmune chronic atrophic gastritis and cobalamin deficiency.¹⁰ Malabsorptive conditions in general, such as surgery on the stomach or small intestine, are listed as a risk factor.¹

A person walking along a forest path holding a railing, seen from behind

Which blood test shows low B12?

No single test is reliable on its own. Serum B12 is the screening marker; methylmalonic acid or homocysteine confirm it, and a full blood count shows the anaemia. A Delphi panel of 42 experts agreed that serum B12 is useful for screening and that methylmalonic acid or homocysteine can support the diagnosis.³

Serum B12 is diagnostic for deficiency if below 180 pg/mL. Borderline levels of 180 to 350 pg/mL warrant a methylmalonic acid measurement, which is diagnostic if elevated.¹ Severe functional B12 deficiency has been documented with normal and even high serum B12, so serum B12 has limited diagnostic value as a stand-alone marker.⁴ For a deeper discussion, see our article on which blood markers are worth checking before you supplement.

Test	What it measures	Role in diagnosis
Total serum B12	Total circulating cobalamin	Screening marker; below 180 pg/mL is diagnostic, 180 to 350 pg/mL is borderline
Holotranscobalamin (holoTC)	B12 bound to transcobalamin, the transport protein	A recognised status marker; low values point to low B12
Methylmalonic acid (MMA)	A metabolite that builds up when B12 is short	Confirms deficiency when raised in a borderline case
Homocysteine	An amino acid that builds up when B12 is short	Supports the diagnosis; it is not specific to B12
Full blood count	Red blood cells, including megaloblastic anaemia	Can look normal with nerve signs, or when folate intake is high

When should you see a doctor rather than self-supplement?

See a doctor when you have a risk factor plus a clinical feature, or any nerve symptoms such as numbness, tingling or balance problems. A diagnosed deficiency is treated by a doctor,.¹

Testing should be considered in patients with at least one risk factor for and one clinical feature of B12 deficiency; universal screening is not recommended.¹ Persistently elevated B12 levels above 1000 pg/mL on two measurements are associated with solid tumours, haematologic malignancy and increased risk of cardiovascular death, so more is not automatically better.¹ Our Vitamin B-Complex with 500 µg methylcobalamin per capsule is intended for general daily intake, not for treating a diagnosed deficiency.

How much B12 do adults need?

The EU label reference intake for vitamin B12 is 2.5 µg per day for adults. This is the reference value used on food supplement labels, including ours, where 500 µg of methylcobalamin per capsule equals 20000% of the label RI.

For older adults the question is often absorption rather than intake. We cover this in detail in which nutrients absorb worse as you get older. If you are choosing a B12 form, our comparison of methylcobalamin versus cyanocobalamin explains the differences.

Frequently Asked Questions

Can you have B12 deficiency with a normal blood count?

Yes. Nerve signs can occur without anaemia, and a high folate intake can leave the blood count looking normal. Severe functional B12 deficiency has even been documented with normal serum B12 levels.⁴

How quickly do nerve signs become permanent?

The sources give no fixed timeline. A 2024 expert consensus states that the symptoms can become irreversible when diagnosis comes late, which is why nerve signs are a reason to see a doctor soon.³

Does metformin really lower B12?

Yes. Pooled analyses show lower serum B12 with metformin use, by 57 pmol/L in trials and 63.70 pM overall, with annual B12 assessment recommended for users.⁵⁶

Are vegans and vegetarians equally at risk?

Not quite. A 2024 meta-analysis found no difference in holotranscobalamin or methylmalonic acid, but vegans had lower serum B12 and higher homocysteine than vegetarians. Both groups have a reason to plan their B12 intake.⁸

Is a high serum B12 ever a warning sign?

It can be. Persistently elevated B12 above 1000 pg/mL on two measurements is associated with solid tumours, haematologic malignancy and increased cardiovascular death risk, and should be medically evaluated.¹

The Bottom Line

The signs of low B12 fall into two documented categories: megaloblastic anaemia and neurological changes such as neuropathy, ataxia and cognitive changes. Nerve signs can appear without anaemia, and folate can mask the blood picture. Serum B12 below 180 pg/mL is diagnostic; borderline values of 180 to 350 pg/mL need methylmalonic acid confirmation. Vegans, older adults, metformin and PPI users, and people with pernicious anaemia or gastric surgery carry the highest risk. With a risk factor and a clinical feature, ask a doctor for testing rather than guessing with supplements.¹³