Vitamin B12 deficiency symptoms creep up slowly, because the liver holds several years’ worth of the vitamin. By the time you feel tired, breathless or notice pins and needles, the stores may have been running low for a long time. Unlike many vitamin worries, this one matters: left untreated, the nerve damage it causes can become permanent.
In short
| What to know | The benchmark |
|---|---|
| Daily need for adults | About 1.5 micrograms a day (NHS), easily covered by meat, fish, eggs and dairy |
| Most common signs | Tiredness, breathlessness, a sore red tongue, mouth ulcers |
| Signs that need a GP soon | Pins and needles, numbness, problems with balance, memory or mood |
| Main causes in the UK | Pernicious anaemia (an autoimmune condition), a vegan diet without supplements, some long-term medicines |
| Blood test (total B12) | Below 180 ng/L: deficiency likely. 180 to 350 ng/L: unclear. Above 350 ng/L: unlikely (NICE, March 2024) |
| Treatment | Tablets or injections decided by your GP, depending on the cause |
What vitamin B12 does
Vitamin B12 is needed to make red blood cells and to keep the nervous system healthy. Without it, the bone marrow produces fewer, larger red blood cells that carry oxygen badly: this is a form of anaemia, sometimes called megaloblastic anaemia.
It also protects the sheath around your nerves. That is why a lack of B12 can cause nerve symptoms, sometimes even before any anaemia shows on a blood count.
The body needs very little, about 1.5 micrograms a day for adults aged 19 to 64 according to the NHS, and stores a lot in the liver. A deficiency therefore usually takes years to develop, which is also why it is easy to miss.
The symptoms of vitamin B12 deficiency
The NHS groups the symptoms in two families. They usually develop gradually and can worsen if the deficiency goes untreated.
Symptoms linked to anaemia
- feeling weak or tired, often for weeks;
- shortness of breath or rapid breathing on effort;
- loss of appetite;
- a sore, red or smooth tongue, sometimes with mouth ulcers;
- problems with vision.
Some of these signs can appear in people who are low in B12 but have not yet developed anaemia, so a normal blood count does not rule it out. Tiredness on its own has many other causes, including low vitamin D in winter, covered in our guide to vitamin D deficiency symptoms.
Symptoms linked to the nerves and brain
- pins and needles or numbness, often in the hands and feet;
- muscle weakness;
- problems with balance and coordination, unsteady walking;
- problems with memory, understanding and judgement;
- changes in mood, from low mood or anxiety to confusion.
These neurological signs are the ones that make B12 deficiency urgent. Many symptoms improve with treatment, but the NHS warns that some can become irreversible, and the longer the deficiency lasts, the higher the risk.
The most common causes
Pernicious anaemia
In the UK, the most common cause of severe B12 deficiency is pernicious anaemia. It is an autoimmune condition: the immune system attacks the stomach cells that make intrinsic factor, the protein needed to absorb B12 from food. It mainly affects older people and is more frequent in those who already have another autoimmune condition, such as an underactive thyroid.
Because the problem is absorption, eating more B12 does not fix it. Treatment is usually by injection, for life.
Diet
B12 is found almost only in foods of animal origin: meat, fish, eggs, milk and cheese. Some breakfast cereals, plant drinks and yeast extracts are fortified. People who follow a vegan diet without fortified foods or a supplement are at real risk; vegetarians who eat eggs and dairy much less so.
Medicines and other causes
Some long-term medicines reduce B12 absorption, in particular metformin (used for type 2 diabetes) and proton pump inhibitors, the stomach acid medicines taken for reflux. If you take one of them for years, ask your GP or pharmacist whether your B12 should be checked. Never stop them on your own.
Other causes include surgery on the stomach or small bowel, coeliac disease or Crohn’s disease. Recreational nitrous oxide (“laughing gas”, balloons) inactivates vitamin B12 and can cause serious nerve damage even when the blood level looks normal.
The blood test and what your result means
Your GP will usually check your full blood count and your B12 level, sometimes folate at the same time, because folate deficiency gives similar anaemia.
Since March 2024, NICE guidance (NG239) uses these thresholds for total B12:
| Total B12 | What it suggests |
|---|---|
| Below 180 ng/L | Deficiency likely |
| 180 to 350 ng/L | Unclear result: deficiency possible |
| Above 350 ng/L | Deficiency unlikely |
Some laboratories measure active B12 instead, the fraction your cells can actually use: below 25 pmol/L, deficiency is likely; between 25 and 70 pmol/L, the result is unclear; above 70 pmol/L, it is unlikely. The reference range printed by your own laboratory still applies.
These numbers are only part of the picture. NICE stresses that symptoms count as much as the result, and that treatment should not wait for test results when someone has anaemia or nerve symptoms. When pernicious anaemia is suspected, a test for antibodies against intrinsic factor may be added.
How it is treated
Treatment depends on the cause, and your GP decides it:
- when the problem is absorption (pernicious anaemia, some bowel conditions), B12 is usually given by injection, several in the first weeks, then every two to three months, often for life;
- when the cause is diet, B12 tablets taken every day are often enough, along with fortified foods.
Tiredness and anaemia usually improve within weeks. Nerve symptoms take longer, often several months, and may not fully recover, which is why early treatment matters.
Over-the-counter B12 supplements are a sensible precaution for vegans. They are not a substitute for a GP visit if you have symptoms: taking them before a blood test can mask the result and delay finding the cause.
When to see a GP
Book an appointment if you have:
- tiredness or breathlessness that lasts several weeks;
- a sore red tongue or recurring mouth ulcers;
- you follow a vegan diet without supplements, or take metformin or a stomach acid medicine long term, and feel unwell.
Ask for an urgent appointment, the same day if possible, if you have pins and needles, numbness, problems with balance or walking, or new problems with memory or mood. Seek help quickly too after using nitrous oxide if you notice tingling, numbness or weakness in the legs.
Call 999 if you have sudden weakness, chest pain, or severe breathlessness at rest.
