Vitamin B12 deficiency means your body has too little vitamin B12 (cobalamin), either because your diet does not supply enough or, more often, because your gut cannot absorb it. It typically causes tiredness, a sore red tongue, pins and needles and memory problems, and it can lead to anemia and nerve damage. It is confirmed with blood tests and treated with B12 injections or tablets, sometimes for life, depending on the cause.
Source: Cleveland Clinic, NHS, MedlinePlus
Source: NHS, American Family Physician (2025), NHS
Contents
- What is vitamin B12 deficiency?
- How common is it?
- Vitamin B12 deficiency symptoms
- Causes of vitamin B12 deficiency
- Who is at risk?
- How vitamin B12 deficiency is diagnosed
- Vitamin B12 deficiency treatment
- Complications and outlook
- Prevention and when to see a doctor
- Vitamin B12 deficiency: frequently asked questions
What is vitamin B12 deficiency?
Vitamin B12 is a nutrient your body needs to make red blood cells and DNA. Vitamin B12 deficiency, also called cobalamin deficiency, happens when you either do not get enough of it from food or cannot absorb what you eat. Left untreated, it can cause physical, neurological and psychological problems, but it is a treatable condition.
Source: Cleveland Clinic
Absorbing B12 is a multi-step process. Stomach acid first releases B12 from the protein in food. The freed vitamin then binds to intrinsic factor, a protein made by cells in the stomach lining, and only this combined form is absorbed further down the digestive tract, in the last part of the small intestine (the ileum). Anything that reduces stomach acid, destroys the cells that make intrinsic factor, or damages the ileum can therefore lead to vitamin B12 deficiency even when the diet is adequate.
Source: Cleveland Clinic, American Family Physician (2017)
How much B12 do you need?
Recommended amounts differ slightly between countries. The NHS says adults aged 19 to 64 need about 1.5 micrograms (mcg) a day, while Cleveland Clinic, following US recommendations, gives about 2.4 mcg a day for adults, with higher needs during pregnancy and breastfeeding. Either way, the quantities are tiny, and a normal diet containing animal products usually provides enough.
Source: NHS, Cleveland Clinic
Because the body keeps a store of B12 that lasts roughly 2 to 5 years, symptoms of vitamin B12 deficiency can take a long time to appear after a change in diet or the start of an absorption problem. This slow onset is one reason vitamin B12 deficiency is often missed.
Source: NHS
How common is it?
Estimates vary depending on the population studied and the cut-off used to define a low level. A 2017 review in American Family Physician reported deficiency in about 6% of people younger than 60 in the US and UK and nearly 20% of those older than 60, with much higher rates in parts of Latin America, Africa and India. A 2025 review in the same journal, using a stricter definition, put the figure at about 2% to 3% of US adults.
Source: American Family Physician (2017), American Family Physician (2025)
Age is a strong factor. According to the NHS, about 1 in 10 people aged 75 or over and about 1 in 20 people aged 65 to 74 are affected by B12 or folate deficiency anemia.
Source: NHS
Vitamin B12 deficiency symptoms
Symptoms of vitamin B12 deficiency usually develop gradually, and some people have none at all.
Source: Cleveland Clinic
General and anemia-related symptoms
- Extreme tiredness, weakness and low energy
- Shortness of breath, rapid breathing or palpitations
- Headaches
- Pale skin
- A sore, red or swollen tongue, sometimes with mouth ulcers
- Loss of appetite, weight loss, indigestion, nausea or diarrhea
Source: NHS, Cleveland Clinic, MedlinePlus
Many of these come from anemia: without enough B12, the bone marrow makes abnormally large red blood cells that do not work properly. If you want to compare these with other forms of anemia, our guide to anemia symptoms explains the wider picture.
Source: NHS
Nerve and brain symptoms
- Pins and needles or numbness, often in the hands and feet
- Muscle weakness and problems with balance, coordination or walking
- Vision problems
- Memory, understanding and judgment problems
- Incontinence
Source: NHS, Cleveland Clinic
Psychological symptoms
Mood can be affected too, ranging from mild depression or anxiety and irritability through to confusion and, in severe cases, dementia or psychosis.
Source: NHS, MedlinePlus, Cleveland Clinic
Nerve and brain symptoms of vitamin B12 deficiency can occur in people who have not yet developed anemia, so a normal blood count does not rule the condition out. Fatigue on its own is also very non-specific; the NHS lists iron deficiency, diabetes, sleep apnea and an overactive thyroid among the many medical causes of tiredness, and our article on thyroid symptoms covers how thyroid problems present.
Causes of vitamin B12 deficiency
Most cases of vitamin B12 deficiency are caused by poor absorption rather than a lack of B12 in food.
Source: MedlinePlus, NHS
Pernicious anemia
Pernicious anemia is the most common cause of vitamin B12 deficiency in the UK. It is an autoimmune condition: the immune system attacks the stomach cells that produce intrinsic factor, so B12 cannot be absorbed. It is more common in women around the age of 60, in people with a family history of it and in people with other autoimmune conditions such as Addison’s disease or vitiligo.
Source: NHS
Diet
Dietary deficiency is less common, but people who rarely eat meat, fish or dairy can become deficient, and vegans are at risk if they do not take B12 supplements or eat fortified foods. Restrictive or long-term poor diets, poor nutrition during pregnancy and exclusive breastfeeding by a mother on a vegan or strict vegetarian diet can also lead to low levels.
Source: NHS, NHS, MedlinePlus, American Family Physician (2017)
Stomach and bowel conditions
Atrophic gastritis, Crohn’s disease, celiac disease, removal of part of the ileum and, rarely, fish tapeworm infection can all reduce absorption. When no clear cause is found, the 2025 American Family Physician review suggests testing for H. pylori along with antibodies linked to pernicious anemia; our guide to H. pylori infection explains how that test works.
Source: Cleveland Clinic, American Family Physician (2017), MedlinePlus, American Family Physician (2025)
Surgery
Operations on the stomach or small intestine can impair B12 absorption. A gastrectomy (removal of part of the stomach) increases the risk, and gastrointestinal procedures such as gastric bypass are a recognized cause.
Source: NHS, Cleveland Clinic, MedlinePlus
Medicines and other substances
Several common medicines can lower B12 levels over time, including metformin (for diabetes), proton pump inhibitors (PPIs) and H2 blockers (for acid reflux and ulcers), and some anticonvulsants. Heavy alcohol use and nitrous oxide misuse are also causes of vitamin B12 deficiency.
Source: NHS, NHS, Cleveland Clinic, MedlinePlus
Functional and genetic causes
Occasionally, blood levels look normal but a problem with the proteins that carry B12 between cells causes a “functional” vitamin B12 deficiency, which can still damage the spinal cord. Rare inherited conditions such as transcobalamin II deficiency also exist.
Source: NHS, Cleveland Clinic
Who is at risk?
Groups at higher risk of vitamin B12 deficiency include adults over 75, vegans and strict vegetarians without supplements, people with digestive disorders or previous stomach or bowel surgery, long-term users of metformin, PPIs or H2 blockers, people with autoimmune conditions such as Sjögren’s syndrome, and heavy drinkers.
Source: Cleveland Clinic, American Family Physician (2017), NHS
Medicine-related risk builds with time. The 2017 American Family Physician review lists metformin use for more than 4 months and PPI or H2-blocker use for more than 12 months as risk factors, and cites a trial in which the number needed to harm with metformin was 14 for B12 deficiency over 4.3 years. The 2025 review describes a randomized trial in which low or borderline B12 occurred in 19.1% of people taking metformin compared with 9.5% on placebo, and suggests periodic B12 checks with long-term use. If you take metformin for type 2 diabetes, ask your doctor whether a test is due rather than stopping the medicine yourself.
Source: American Family Physician (2017), American Family Physician (2025)
Weight-loss operations are a particular concern. The 2017 review recommends that people who have had bariatric surgery take 1 mg of oral vitamin B12 daily indefinitely. If you are planning a procedure such as a gastric sleeve, lifelong vitamin follow-up should be part of the plan.
Source: American Family Physician (2017)
How vitamin B12 deficiency is diagnosed
A doctor can often diagnose vitamin B12 deficiency from your symptoms and blood test results. Symptoms matter because some people have symptoms with normal vitamin levels, while others have low levels without symptoms. An examination may include checking reflexes and sensation.
Source: NHS, MedlinePlus
Blood tests
- Full blood count: checks hemoglobin and whether red blood cells are larger than normal.
- Serum B12 and folate levels: both are usually measured together.
- Methylmalonic acid (MMA) and homocysteine: these build up when B12 is lacking and help when the B12 level is borderline.
- Reticulocyte count: measures young red blood cells.
Source: NHS, MedlinePlus, Cleveland Clinic
Interpreting the numbers
Laboratories use different reference ranges, and sources do not agree on a single cut-off. The 2025 American Family Physician review treats a total B12 level below 180 pg/mL as diagnostic and recommends measuring MMA when the level is 180 to 350 pg/mL; a raised MMA confirms the diagnosis. The 2017 review gave a lower diagnostic threshold of 150 pg/mL (111 pmol/L). B12 results can also be falsely high in alcohol use disorder, liver disease or cancer, so your doctor will read the result alongside your symptoms.
Source: American Family Physician (2025), American Family Physician (2017)
Finding the cause
Because the cause of vitamin B12 deficiency decides the treatment, further tests may follow. These can include antibody tests for pernicious anemia (anti-intrinsic factor antibodies have a positive predictive value of about 95%), testing for H. pylori, an upper endoscopy and, rarely, a bone marrow biopsy. Pernicious anemia tests are not always conclusive.
Source: NHS, American Family Physician (2017), American Family Physician (2025), MedlinePlus
You may be referred to a hematologist if the cause is unclear, you are pregnant or your nervous system seems affected; to a gastroenterologist if an absorption problem is suspected; or to a dietitian if diet is the likely cause.
Source: NHS
Testing everyone is not recommended. The 2025 review advises testing people with at least one risk factor and at least one clinical feature of vitamin B12 deficiency.
Source: American Family Physician (2025), American Family Physician (2017)
Vitamin B12 deficiency treatment
Most cases of vitamin B12 deficiency are easily treated with injections or tablets, and the right approach depends on the cause and how severe the deficiency is. Treatment may last until levels recover or may be lifelong.
Source: NHS, Cleveland Clinic
Injections
In the UK, treatment usually starts with hydroxocobalamin injections every other day for a short period. Hydroxocobalamin is preferred over cyanocobalamin there because it stays in the body longer. After that first phase:
- If the cause is not dietary, such as pernicious anemia, injections are usually given every 2 to 3 months for life.
- If the cause is dietary, options are daily B12 tablets taken between meals or an injection twice a year.
- If there are nerve symptoms such as numbness or tingling, a hematologist may advise injections every 2 months and decide how long to continue.
Source: NHS
Tablets and other forms
High-dose oral B12 is increasingly used. The 2025 American Family Physician review states that 1,000 mcg taken daily by mouth is noninferior to injections, including in pernicious anemia and after gastric bypass, while injections should be considered for severe deficiency or nerve involvement. B12 is also available as a nasal spray or gel.
Source: American Family Physician (2025), American Family Physician (2017), Cleveland Clinic, MedlinePlus
The table summarizes typical approaches; your own plan should come from your doctor.
| Situation | Typical approach | Duration |
|---|---|---|
| Start of treatment (UK) | Hydroxocobalamin injections every other day | Short loading phase |
| Pernicious anemia or other absorption problem | Injection every 2–3 months, or high-dose daily tablets where the doctor agrees | Usually lifelong |
| Diet-related deficiency | Daily tablets between meals or twice-yearly injection, plus diet changes | Until levels and diet recover; lifelong for many vegans |
| Nerve symptoms present | Hematologist review; injections, e.g. every 2 months | Set by the specialist |
| After bariatric surgery | 1 mg oral B12 daily | Indefinitely |
Source: NHS, American Family Physician (2025), American Family Physician (2017)
Why folic acid is not given alone
Folate deficiency often causes the same type of anemia and is treated with folic acid tablets, usually for about 4 months. However, doctors check B12 before starting folic acid, because folic acid can improve the blood picture enough to hide vitamin B12 deficiency while nerve damage continues. When both are low, B12 is replaced first.
Source: NHS, American Family Physician (2017)
Checking that treatment works
A blood test about 7 to 10 days after starting treatment checks hemoglobin and reticulocytes, and a further test at around 8 weeks confirms the response. Long-standing deficiency or an ongoing underlying condition may need B12 for life.
Source: NHS, Cleveland Clinic
Complications and outlook
Untreated vitamin B12 deficiency can cause problems with the nervous system that are sometimes permanent, as well as temporary infertility, heart conditions, and complications in pregnancy including birth defects. Adults with severe anemia may develop heart failure.
Source: NHS
Nerve-related complications can include peripheral neuropathy, degeneration of the spinal cord (subacute combined degeneration), trouble walking, bowel or bladder incontinence, erectile dysfunction, memory loss, and paranoia or delusions.
Source: Cleveland Clinic, American Family Physician (2017)
The outlook depends on how early the problem is found and how low levels have fallen. Most people respond well to treatment, and some complications improve, but nerve damage can be permanent if treatment is delayed or the damage is severe. This is why the NHS stresses diagnosing and treating it as early as possible.
Source: MedlinePlus, NHS, Cleveland Clinic
Prevention and when to see a doctor
For most people, a varied diet prevents dietary vitamin B12 deficiency. Good sources include meat, fish, eggs, milk and cheese, plus fortified foods such as some breakfast cereals, plant-based milks, soy products, yeast extracts and nutritional yeast. Fruit, vegetables and grains do not naturally contain B12, so check nutrition labels if you rely on fortified products.
Source: NHS, Cleveland Clinic, NHS
- Vegans: take a B12 supplement or eat fortified foods regularly; many will need tablets for life.
- Adults over 50: the 2017 review advises using fortified foods or supplements, as absorption from food declines with age.
- After weight-loss surgery: follow your surgical team’s vitamin plan.
- Long-term metformin or acid-reducing medicines: ask about periodic B12 checks; long-term PPI use should be reviewed from time to time.
- Alcohol: limit how much you drink, and keep digestive conditions under control.
Source: NHS, American Family Physician (2017), American Family Physician (2025), Cleveland Clinic
On safety, the UK Department of Health and Social Care advises that taking 2 mg or less of B12 a day in supplements is unlikely to cause harm, although evidence on high daily doses is limited. If you are also concerned about other common nutrient gaps, our guide to vitamin D deficiency covers who is at risk and how to test.
Source: NHS
When to see a doctor
See a doctor if you have symptoms that could mean vitamin B12 deficiency, especially pins and needles, numbness, balance problems, memory changes or a sore red tongue, or if you belong to a risk group and feel persistently tired. Prompt diagnosis matters because the risk of permanent problems rises the longer the deficiency goes untreated. Do not start high-dose folic acid on your own without having your B12 checked.
Vitamin B12 deficiency: frequently asked questions
What are the first signs of vitamin B12 deficiency?
Early signs are often vague: tiredness, low energy, a sore or red tongue and mouth ulcers. Pins and needles, memory problems or low mood may follow, sometimes before any anemia appears.
How long does it take to recover from vitamin B12 deficiency?
Blood results are rechecked about 7 to 10 days after starting treatment and again around 8 weeks. Mild cases can respond quickly, but nerve symptoms may take longer and sometimes do not fully resolve.
Source: NHS, Cleveland Clinic, MedlinePlus
Are B12 tablets as good as injections?
For many people, yes. Reviews in American Family Physician report that 1 to 2 mg of oral B12 daily works as well as injections, but injections are preferred for severe deficiency or nerve involvement, and UK practice often starts with injections.
Source: American Family Physician (2025), American Family Physician (2017), NHS
Can vitamin B12 deficiency be permanent?
Vitamin B12 deficiency itself can almost always be corrected, but nerve damage may be permanent if treatment is delayed. Some causes, such as pernicious anemia, need lifelong treatment.
Source: NHS, MedlinePlus
Does metformin cause vitamin B12 deficiency?
Long-term metformin use is linked to lower B12 levels, and trials show more deficiency in people taking it than in those on placebo. Periodic B12 checks are suggested; do not stop metformin without medical advice.
Source: American Family Physician (2025), NHS
Disclaimer: this article is for general health education and does not replace advice from a qualified doctor who can assess your individual case. Last updated: October 2026.
