B12 deficiency means the body lacks enough vitamin B12 to make healthy red blood cells and keep nerves working properly. It develops slowly, and the common symptoms, tiredness, breathlessness, pins and needles, a sore tongue and low mood, are vague and easy to dismiss. Prolonged deficiency can cause nerve damage that is hard to reverse, so it is worth catching early.
The key message is simple: the symptoms are treatable and the test is straightforward, so if you recognise the signs, it is worth getting checked rather than putting persistent tiredness or tingling down to a busy life. The earlier it is picked up, the more easily and fully it is put right.
What vitamin B12 does
Vitamin B12 helps the body make red blood cells and keep the nervous system working properly, and it plays a part in releasing energy from food. The body cannot make it, so it has to come from the diet, mainly from animal foods such as meat, fish, eggs and dairy, or from fortified foods and supplements. NHS guidance puts the adult requirement at around 1.5 micrograms a day, a small amount that most people eating animal produce reach easily. The body stores B12 in the liver, which is why a shortfall can take months or years to show up, and also why symptoms can build gradually rather than appearing overnight. That slow onset is a double-edged thing: it gives some buffer against a short-term dip in intake, but it also means that by the time symptoms are obvious, a deficiency may have been developing for a long while, which is worth bearing in mind if you are in a higher-risk group.
Signs and symptoms
Because B12 affects blood and nerves, the symptoms the NHS lists are wide-ranging and can include:
- Tiredness, weakness and breathlessness.
- Pins and needles, numbness or problems with balance.
- A sore, red tongue or mouth ulcers.
- Low mood, irritability or problems with memory and concentration.
These overlap with many other conditions, which is exactly why a test matters rather than guesswork. Nerve-related symptoms in particular should not be left, as they are the ones most likely to become lasting if deficiency continues.
What causes it
There are three broad reasons B12 runs low. The first is diet: people who eat little or no animal produce, including some vegans and vegetarians, can fall short without fortified foods or supplements. The second, and most common cause in the UK, is an absorption problem. Pernicious anaemia, an autoimmune condition, stops the gut absorbing B12, and other gut conditions or surgery can do the same. The third is medicines: some long-term treatments, including certain drugs for acid reflux and diabetes, can reduce absorption over time; our guide to supplements that interact with medicines covers this overlap. Age also makes absorption less efficient.
How it is diagnosed and treated
Diagnosis usually starts with a blood test to measure B12 levels, often alongside a full blood count and folate, since these are related. If deficiency is confirmed, further tests may look for the cause, such as checking for the antibodies linked to pernicious anaemia. Identifying the cause matters because it shapes the treatment: someone who is short on B12 through diet is managed differently from someone who cannot absorb it at all.
It is also common for GPs to check related deficiencies at the same time, because the fatigue picture overlaps. Vitamin D deficiency and iron deficiency can occur alongside low B12, particularly in people with restricted diets or gut conditions, and vitamin D and B12 deficiency together can make tiredness and low mood noticeably worse than either alone. One blood panel usually covers all three.
Treatment depends on the cause, and NICE guidance sets out the approach. Where the problem is absorption, B12 is usually given as injections, often frequently at first and then at regular intervals to maintain levels. Where the cause is dietary, high-dose tablets or dietary changes may be enough. Many people feel better within weeks, though nerve symptoms can take longer to improve and, if long-standing, may not fully resolve. Treatment is often long-term or lifelong when the underlying cause is ongoing, such as pernicious anaemia.
Which vitamin deficiency causes hair loss?
Hair shedding is a common reason people search for vitamin deficiencies, and the honest answer is that no single vitamin is usually to blame. Iron deficiency is the nutritional cause most consistently linked to diffuse hair shedding, particularly in women, and low B12 and vitamin D have been associated with it in some studies, though the evidence is weaker. When a deficiency is the trigger, the shedding usually follows the pattern of telogen effluvium, a temporary all-over thinning that recovers once the cause is corrected. If hair loss is your main concern, a blood test to check iron, B12 and vitamin D beats buying hair vitamins, because supplements only help when a deficiency actually exists.
Who is most at risk
Some groups are more likely to run low and may benefit from being alert to it. People following a vegan or largely plant-based diet without fortified foods or supplements are at higher risk, since B12 comes mainly from animal produce. Older adults are more prone because absorption becomes less efficient with age. Those with autoimmune or digestive conditions, or who have had gut surgery, and people on certain long-term medicines, also warrant attention. Being in one of these groups does not mean you are deficient, but it does make the symptoms worth taking seriously if they appear.
Can you prevent it?
Where the cause is dietary, prevention is straightforward: include B12-containing foods, or use fortified foods and a food supplement for vitamin B12 if you avoid animal produce; our guide to whether you need supplements puts this in context. Where the cause is an absorption problem, diet alone will not fix it, which is why diagnosis matters rather than simply eating more of the right foods. If you are in a higher-risk group, it is reasonable to raise B12 with a healthcare professional during routine reviews so it can be monitored sensibly rather than left to chance.
When to see a clinician
If you have ongoing tiredness, tingling, mouth soreness or mood and memory changes, it is worth speaking to a healthcare professional and asking whether a blood test is appropriate. Self-treating with supplements can mask the picture without addressing an underlying absorption problem, so a proper assessment is the better route. It is easy to attribute these symptoms to stress or a busy life and press on, but B12 deficiency is one of the more satisfying things to catch, because the treatment is well established and effective, and many people feel markedly better once levels are restored. Caught and treated, it is very manageable, and the main risk comes from leaving it too long rather than from the condition itself.
