Vitamin B12 deficiency is one of the more deceptive health conditions. It develops slowly, often over months or years, and its symptoms, fatigue, brain fog, low mood, tingling in the hands or feet, are so common and non-specific that most people put them down to stress, overwork, or simply getting older.
It is also more widespread than many people expect. Vegetarians and vegans are at particular risk, since B12 is found almost exclusively in animal products, but older adults, people with certain gut conditions, and long-term users of some common medications are also frequently affected.
This guide covers the key signs and symptoms of B12 deficiency, who is most at risk, how it is diagnosed, what your test results mean, and the most effective treatment options.
What is vitamin B12 and why does the body need it?
Vitamin B12 (cobalamin) is a water-soluble vitamin required for the development, function, and myelination of the central nervous system, healthy red blood cell formation, and DNA synthesis [1]. Unlike most water-soluble vitamins, B12 can be stored in the liver for years, which is one reason deficiency tends to develop slowly and is often missed until levels are significantly depleted [1].
· Red blood cell formation: B12 is needed for the normal maturation of red blood cells. Without it, cells become abnormally large and dysfunctional, a condition called megaloblastic anaemia, which reduces the blood's ability to carry oxygen efficiently [1].
· Nervous system health: B12 supports the development and maintenance of myelin, the protective sheath around nerve fibres. Deficiency can cause neurological changes such as numbness and tingling in the hands and feet, and these can occur even without anaemia, which is why early diagnosis matters [1].
· DNA synthesis and cell division: B12 works as a cofactor alongside folate (B9) in the processes that support DNA synthesis and healthy cell division throughout the body [1].
· Mood and brain health: Several studies have found associations between low vitamin B12 status and depression, alongside other neurological effects [1].
B12 works closely with folate (vitamin B9), and deficiency in one often occurs alongside insufficiency in the other, which is why the two are usually tested together.
"Sometimes what feels like stress, burnout, or lack of sleep may actually be a vitamin B12 deficiency. Since symptoms such as tiredness, poor concentration, low mood, and memory changes can be subtle and easily mistaken for everyday stress, the only way to know for sure is through a simple blood test" says Dr Rahmana, GP at Smart Salem.
Vitamin B12 deficiency symptoms: what to look out for
B12 deficiency symptoms tend to develop gradually, which makes them easy to normalise. Many people live with a meaningful deficiency for months or years before it is identified, often because the symptoms are attributed to other, more familiar causes.
B12 is one of eight B vitamins, and while it has its own distinct deficiency profile, it works closely with several others, particularly folate (B9) and B6. Deficiency in one often occurs alongside low levels of another, so it helps to understand how the most clinically significant B vitamin deficiencies differ from B12 itself.
· Folate (B9) deficiency: shares significant symptom overlap with B12 deficiency, including fatigue, brain fog and mouth sores. It is crucially important during pregnancy, since inadequate folate is linked to neural tube defects such as spina bifida [2]. Folate and B12 are usually tested together, because correcting a folate deficiency without identifying an underlying B12 deficiency can allow neurological damage to progress unnoticed [1].
· Vitamin B6 deficiency: vitamin B6 is involved in the production of neurotransmitters, including serotonin, and deficiency can cause anaemia, skin rashes, cracking at the corners of the mouth, a swollen tongue, depression, confusion and reduced immune function [3]. It is more commonly seen in people using certain medications, including some oral contraceptives, which have been associated with lower B6 status [3].
· Vitamin B1 (thiamine) deficiency: most commonly associated with chronic alcohol use, which impairs thiamine absorption and depletes the body's limited stores. In its more severe form it can lead to Wernicke-Korsakoff syndrome, a serious neurological condition causing confusion, loss of muscle coordination and memory problems [4].
1. Fatigue and Weakness
Persistent tiredness that does not improve with rest is one of the most frequently reported symptoms of B12 deficiency. It happens because dysfunctional red blood cells are less efficient at transporting oxygen around the body, leaving muscles and organs relatively starved of the oxygen they need to function properly [1].
2. Tingling or Numbness in the Hands and Feet
One of the more distinctive symptoms of B12 deficiency, peripheral tingling or numbness reflects the role B12 plays in maintaining the nerves' protective myelin sheath [1]. It often begins in the fingertips or toes and can progress if the deficiency is not corrected, sometimes alongside balance problems and limb weakness in more advanced cases.
3. Brain Fog and Memory Difficulties
Difficulty concentrating, poor memory, mental sluggishness and a general sense of cognitive slowing are commonly reported alongside B12 deficiency. These symptoms can be subtle at first and easy to attribute to stress or poor sleep, which is part of why deficiency so often goes unrecognised.
4. Low Mood and Depression
B12 is involved in the pathways that regulate mood, and low B12 status has been associated with higher rates of depression in a number of studies [1]. Some people find their mood improves once their levels are corrected, although B12 deficiency is only one of many possible contributors to low mood, and it is worth ruling in or out with a simple blood test rather than assuming.
5. Pale or Yellowish Skin
Pale skin is a recognised effect of B12 deficiency, largely because the blood is carrying less oxygen than it should [1]. In megaloblastic anaemia, the breakdown of abnormal red blood cells can also release bilirubin, which can occasionally give the skin a mild yellow or jaundiced tinge.
6. Glossitis and Mouth Sores
Glossitis, an inflammation of the tongue that leaves it looking swollen, smooth and sometimes sore, is a recognised sign of B12 deficiency [1]. Mouth ulcers and soreness at the corners of the mouth can also occur.
7. Shortness of Breath and Heart Palpitations
Palpitations are a recognised effect of B12 deficiency [1], and in more significant cases the reduced oxygen-carrying capacity of the blood can also cause breathlessness, particularly during physical activity. These symptoms reflect the strain that anaemia places on the cardiovascular system.
Who is most at risk of vitamin B12 deficiency?
While anyone can develop B12 deficiency, certain groups are at significantly higher risk because of dietary patterns, biology, or medication use.
· Vegetarians and vegans: B12 is found almost exclusively in animal products such as meat, fish, dairy and eggs, so people following plant-based diets who do not supplement consistently are at high risk of deficiency. This is the most common dietary cause of B12 deficiency worldwide, though fortified foods and supplements can substantially reduce the risk [1].
· Older adults: the stomach's production of intrinsic factor, a protein required for B12 absorption, tends to decline with age, and conditions such as atrophic gastritis become more common. Adults over 50 are at meaningfully higher risk of B12 malabsorption, even with an otherwise adequate diet [1].
· People with gut conditions: conditions including coeliac disease, Crohn's disease, atrophic gastritis, and a history of gastric or bariatric surgery can all impair B12 absorption. In these cases the issue is not dietary intake but the gut's reduced ability to extract and absorb B12 [1].
· Long-term medication users: metformin, used to treat type 2 diabetes, and proton pump inhibitors, used for acid reflux, can both reduce B12 absorption when taken over extended periods. People on these medications long term may benefit from having their B12 levels monitored [1].
· Pregnant and breastfeeding women: B12 requirements increase during pregnancy and lactation. Deficiency in a breastfeeding mother can mean her infant receives inadequate B12 through breast milk, which can have serious developmental consequences for the baby [1].
· People with pernicious anaemia: an autoimmune condition in which the body attacks the cells that produce intrinsic factor, pernicious anaemia prevents B12 from being absorbed at all, regardless of dietary intake. It is the most common cause of clinically significant B12 deficiency worldwide and is more common in women, and it requires treatment by injection rather than oral supplementation alone [1].
How Is Vitamin B12 Deficiency Diagnosed?
B12 deficiency is diagnosed through a blood test measuring serum B12 levels, reported in picograms per millilitre (pg/mL) or picomoles per litre (pmol/L).
Cutoffs vary somewhat by laboratory and method, but most define subnormal values as below 200 to 250 pg/mL (148 to 185 pmol/L) [1]. Levels in the low-normal range, broadly 200 to 300 pg/mL, do not necessarily rule deficiency out: symptoms can still be present, and functional deficiency at the cellular level can occur even when circulating B12 looks adequate. Experts generally recommend that, when serum B12 falls between roughly 150 and 400 pg/mL, additional markers, most often methylmalonic acid (MMA) and homocysteine, are checked to confirm whether a true deficiency is present [1]. An MMA level above about 0.271 micromol/L, or a homocysteine level above around 15 micromol/L, both point toward vitamin B12 deficiency, although homocysteine is a less specific marker since it is also affected by kidney function and folate status [1].
In the ICD-10 classification system used in medical records and referral letters, vitamin B12 deficiency without anaemia is generally coded under E53.8 (deficiency of other specified B group vitamins), while the various forms of B12 deficiency anaemia, including pernicious anaemia, dietary deficiency and other subtypes, fall under the D51 code series [5].
If you suspect B12 deficiency, particularly if you fall into one of the higher-risk groups above, it is worth requesting testing rather than waiting for symptoms to worsen. A comprehensive blood panel that tests B12 and folate together, alongside other relevant markers, is usually the most efficient way to identify which B vitamin deficiency, or combination of deficiencies, is involved. Smart Salem's premium health screening and wider range of preventative health screenings can help build that picture in a single appointment.
Vitamin B12 Deficiency Treatment: Diet, Supplements and Injections
The right treatment for B12 deficiency depends on the underlying cause. If the issue is dietary, oral supplementation and dietary changes may be enough. If absorption itself is impaired, as with pernicious anaemia or significant gut conditions, injections bypass the digestive system entirely and are often the more reliable option.
Diet
The richest dietary sources of vitamin B12 include:
· Meat and offal: beef, lamb and liver are among the most concentrated sources [1].
· Fish and seafood: salmon, tuna, sardines, clams and mussels all provide significant amounts [1].
· Dairy products: milk, cheese and yoghurt offer meaningful B12, making them important for vegetarians who include dairy [1].
· Eggs: contain B12, mainly in the yolk, though in smaller amounts than meat or fish [1].
· Fortified foods: plant-based milks, breakfast cereals and nutritional yeast are often fortified with B12, which makes them particularly relevant for vegans [1].
For people with absorption issues, improving diet alone is unlikely to resolve deficiency, however much B12 is consumed. The problem in these cases is not intake but the gut's ability to absorb what is eaten.
Oral Supplements
For people with a dietary deficiency and intact absorption, oral B12 supplements are effective and widely available. Doses used therapeutically are typically much higher than the standard recommended intake, which allows a small percentage to be absorbed through passive diffusion even without intrinsic factor [1].
Methylcobalamin and cyanocobalamin are the two most common forms used in supplements; methylcobalamin is an active form that does not require conversion by the body, and some clinicians prefer it, though both are effective for most people [1]. Sublingual (under the tongue) B12 tablets are also available, and current evidence suggests they work about as well as standard oral tablets [1].
Injections
For people with pernicious anaemia, significant gut absorption issues, or more severe deficiency with neurological symptoms, intramuscular B12 injections are typically the most reliable route, since they deliver B12 directly into the bloodstream and bypass the gut entirely [1]. Some research suggests that very high-dose oral supplementation can be similarly effective to injections in normalising B12 levels, though this evidence is still considered limited, and a clinician is best placed to advise which route suits a given cause of deficiency [1].
Injections are typically given as a course of loading doses followed by maintenance doses every one to three months, depending on the underlying cause and severity of the deficiency. Neurological symptoms caused by B12 deficiency can take months to improve with treatment, and in cases where deficiency has been severe or prolonged, improvement may be partial rather than complete [1]. This is one of the clearest arguments for early testing and treatment, rather than waiting to see if symptoms settle on their own.
How Smart Salem Supports B12 Testing and Diagnosis
B12 deficiency is one of the conditions most worth testing for proactively: the symptoms are easy to miss, the deficiency develops over years, and the consequences of a late diagnosis, particularly neurological damage, can be significant and only partially reversible.
Smart Salem is a DHA-approved diagnostic centre offering comprehensive health screening that includes B12, folate, and other relevant markers in a single appointment, with results interpreted by qualified clinicians. If you fall into a higher-risk group, whether that is following a vegetarian or vegan diet, being an older adult, taking a long-term medication known to affect B12 absorption, or simply recognising the symptoms described in this article, it is worth getting tested rather than waiting for things to worsen. You can explore Smart Salem's premium health screening, browse the full range of preventative health screenings, or view Smart Salem's wider health and wellness services to find the right starting point.
Vitamin B12 Deficiency FAQs
Can B12 deficiency cause permanent damage if left untreated?
Potentially, yes. Neurological symptoms such as numbness and tingling can develop even without anaemia, and if deficiency goes uncorrected for a long time, some nerve damage may not fully reverse with treatment. This is why early diagnosis and intervention matter [1].
How long does it take for B12 deficiency symptoms to improve with treatment?
It varies. Some symptoms, particularly fatigue, can start improving within weeks of starting treatment, while neurological symptoms often take longer, sometimes several months, and may only partially improve if the deficiency was severe or long-standing before it was caught.
Is B12 deficiency more common in women than men?
Overall rates of B12 deficiency in the general population are broadly similar between men and women. However, pernicious anaemia, one specific autoimmune cause of B12 deficiency, is more common in women [1].
Can stress deplete B12 levels?
There is no strong evidence that stress directly depletes the body's B12 stores. That said, prolonged stress can affect appetite and eating patterns, which may indirectly reduce intake in some people, so it is still worth checking levels if you are also experiencing the symptoms described above.
What is the difference between B12 deficiency and pernicious anaemia?
B12 deficiency is the broader condition, and it can have several different causes. Pernicious anaemia is one specific cause: an autoimmune condition where the body attacks the cells needed to absorb B12, meaning deficiency will persist regardless of diet unless treated, usually with injections [1].
Can I get enough B12 from fortified foods if I am vegan?
Yes, for many people. Fortified foods such as certain plant-based milks, breakfast cereals and nutritional yeast, together with a B12 supplement, can substantially reduce the risk of deficiency on a vegan diet [1]. Regular testing is still a sensible way to confirm your levels are where they should be.
Should children or teenagers be tested for B12 deficiency?
Not routinely, but testing may be worth considering if a child or teenager follows a strict vegan diet, has a relevant gut condition, or shows symptoms such as persistent fatigue or developmental concerns. A doctor can advise on whether testing is appropriate for a specific child.
Does B12 deficiency affect fertility or pregnancy?
B12 deficiency has been linked to infertility, and in pregnancy it can affect the developing baby, potentially contributing to neural tube defects, developmental delays and anaemia in the infant if the mother's deficiency is significant [1]. This is one reason B12 status is worth checking as part of preconception or early pregnancy care.
When should I get tested for B12 deficiency?
It is worth getting tested if you are in one of the higher-risk groups described in this article, whether that is diet, age, a gut condition, long-term medication use, or pregnancy, or if you are experiencing symptoms such as persistent fatigue, tingling, brain fog or low mood that have not improved. Testing is straightforward and gives a clear answer rather than leaving you to guess.
Sources
1 National Institutes of Health, Office of Dietary Supplements – Vitamin B12: Fact Sheet for Health Professionals – https://ods.od.nih.gov/factsheets/VitaminB12-HealthProfessional/
2 National Institutes of Health, Office of Dietary Supplements – Folate: Fact Sheet for Health Professionals – https://ods.od.nih.gov/factsheets/Folate-HealthProfessional/
3 National Institutes of Health, Office of Dietary Supplements – Vitamin B6: Fact Sheet for Health Professionals – https://ods.od.nih.gov/factsheets/VitaminB6-HealthProfessional/
4 National Institutes of Health, Office of Dietary Supplements – Thiamin: Fact Sheet for Health Professionals – https://ods.od.nih.gov/factsheets/Thiamin-HealthProfessional/
5 CDC National Center for Health Statistics – ICD-10-CM Browser Tool – https://icd10cmtool.cdc.gov/