Quick Answer
The most recognisable symptoms of vitamin B12 deficiency are extreme tiredness, pins and needles in the hands and feet, a sore smooth red tongue, mouth ulcers, brain fog, poor balance and pale or slightly yellow skin.
Symptoms build slowly over months or years because the liver holds a large store, so people often adapt without noticing how bad they feel.
Neurological symptoms matter most. Numbness, tingling and unsteadiness need prompt attention because prolonged deficiency can cause nerve damage that does not fully reverse.
Why B12 runs low
B12 is only found naturally in animal foods, and absorbing it is unusually complicated. It must be released from food by stomach acid, then bound to a protein called intrinsic factor made by the stomach lining, then absorbed at the very end of the small intestine.
Break any link in that chain and intake stops mattering. This is why deficiency is common in people who eat plenty of meat, not just in vegans.
The main causes are a diet without animal products, pernicious anaemia (an autoimmune attack on intrinsic factor), long term use of acid-reducing medication such as proton pump inhibitors, metformin, gastric surgery, coeliac disease, Crohn's disease and simple age-related decline in stomach acid.
The full symptom picture
- Fatigue and breathlessness from the large, poorly functioning red blood cells that deficiency produces.
- Pins and needles or numbness, usually starting symmetrically in the feet or fingertips.
- A sore, beefy red, smooth tongue, known as glossitis, sometimes with recurring mouth ulcers.
- Brain fog, poor concentration and memory lapses that feel out of character.
- Unsteadiness or clumsiness, particularly noticeable walking in the dark or on uneven ground.
- Pale skin with a faint yellow tinge, caused by red cells breaking down prematurely.
- Mood changes, including irritability and depressive symptoms.
- Palpitations as the heart compensates for reduced oxygen carrying capacity.
- Disturbed vision in rare, advanced cases where the optic nerve is affected.
Not everyone gets the anaemia. A meaningful proportion of people present with neurological symptoms while their blood count still looks normal, which is exactly why a normal full blood count does not rule deficiency out.
How long it takes to develop
A healthy adult liver stores enough B12 to last two to five years. Someone who switches to a strict vegan diet without supplementing therefore feels fine for a long time, then declines gradually as the reserve empties.
That slow onset is the trap. Because there is no sharp moment when things change, people put the tiredness down to work, age or stress and never connect it to diet.
Getting diagnosed
The first test is usually serum B12. Below about 200 pg/mL (148 pmol/L) is generally considered deficient, and results between 200 and 300 are a grey zone.
For borderline results, doctors often add methylmalonic acid or homocysteine. Both rise when B12 is functionally low, so they catch cases that serum B12 alone misses.
If deficiency is confirmed and diet is not the obvious explanation, testing for intrinsic factor antibodies helps identify pernicious anaemia, which needs lifelong treatment rather than a diet change.
Treatment
Dietary deficiency responds well to oral supplements. High dose tablets work even without intrinsic factor because a small percentage is absorbed by simple diffusion, which is why 1000 microgram doses are commonly used.
Absorption problems and pernicious anaemia are usually treated with intramuscular injections, often a loading series followed by maintenance every two or three months. Some clinicians now use high dose oral therapy instead, and evidence suggests it can work, but this is a decision for your doctor.
Blood counts typically start improving within a week or two. Neurological symptoms improve more slowly, over months, and damage present for a long time may not resolve completely.
Who should be checked
- Vegans and long term vegetarians who do not supplement.
- Adults over 60, in whom absorption declines.
- Anyone taking a proton pump inhibitor or metformin long term.
- People who have had stomach or bowel surgery, including bariatric procedures.
- Those with coeliac disease, Crohn's disease or other malabsorption conditions.
- Anyone with unexplained tingling, balance problems or anaemia.
Preventing it in the first place
If you eat meat, fish, eggs or dairy regularly, your intake is almost certainly fine and any deficiency points at absorption. If you do not, supplementation is not optional. Around 10 micrograms daily, or 2000 micrograms weekly, is the usual vegan recommendation, and fortified foods can contribute.
One important caution: folic acid supplements can correct the anaemia of B12 deficiency while the nerve damage quietly continues. This is why doctors check both rather than treating anaemia blindly with folate.
When to seek help urgently
Numbness, tingling that is spreading, difficulty walking, confusion or vision changes warrant a prompt appointment rather than a wait-and-see approach. Nerve damage becomes less reversible the longer it persists.
Severe anaemia with breathlessness at rest, chest pain or fainting is an emergency and needs same day assessment.
The bottom line
B12 deficiency hides behind tiredness and brain fog until nerves get involved, and by then the fix takes longer. A sore red tongue with tingling hands in someone who is vegan, over sixty, or on long term acid suppression is a strong clue.
It is a cheap blood test and a cheap treatment. The only expensive part is leaving it too long.
Frequently asked questions
How long does it take to recover from B12 deficiency?
Energy and blood counts usually improve within two to four weeks of treatment. Nerve symptoms such as tingling take much longer, often three to six months, and anything present for over a year may not resolve fully.
Can stress cause B12 deficiency?
Not directly. Stress can worsen fatigue and mask the problem, and it often coincides with a poorer diet, but the actual cause is almost always low intake or impaired absorption.
Do B12 injections work better than tablets?
They bypass the gut entirely, which matters if absorption is the problem. For dietary deficiency, high dose oral tablets work perfectly well and are far more convenient.
Can you have low B12 with normal blood counts?
Yes, and it is common. Neurological symptoms can appear before the red cells change size, which is why methylmalonic acid or homocysteine testing is used when symptoms and serum B12 disagree.
Is it possible to take too much B12?
There is no established upper limit because excess is excreted in urine. Very high doses are used therapeutically without harm, though they can distort test results if you get retested soon after.
