Vitamin B12 Deficiency

What it is

Vitamin B12 deficiency is a lack of cobalamin that can cause anemia and nerve damage, and some labs use serum B12 below 200 pg/mL as a cutoff. Vitamin B12 is needed for DNA synthesis, red blood cell formation, and normal neurologic function. Deficiency may develop slowly because the liver stores B12, so symptoms can appear only after months to years of low intake or poor absorption. The most important clinical point is that neurologic injury can occur even without obvious anemia, so numbness, gait problems, memory change, or tongue soreness should not be dismissed if B12 deficiency is possible.

B12 is found mainly in animal-source foods such as meat, fish, eggs, and dairy, and in fortified foods or supplements. This makes deficiency more likely in people with very low intake of animal foods unless they use fortified foods or supplements. In India, this matters because vegetarian dietary patterns are common, and low B12 status is frequently discussed in clinical practice, especially in older adults, people taking metformin, and those with gastrointestinal disorders.

Common causes include:

Cause	Examples
Low intake	Strict vegan diet without fortified foods or supplements, prolonged undernutrition
Poor absorption	Pernicious anemia, autoimmune gastritis, stomach surgery, ileal disease or resection
Medicine-related	Metformin, proton pump inhibitors, H2 blockers
Other	Alcohol misuse, pancreatic disease, bacterial overgrowth, fish tapeworm

How it works

Vitamin B12 from food is released in the stomach, binds to intrinsic factor, and is absorbed in the terminal ileum. If any step fails, deficiency can develop. Pernicious anemia is a classic cause because autoimmune damage reduces intrinsic factor, making absorption difficult even when dietary intake is adequate.

B12 acts as a cofactor in two important human reactions. One helps convert homocysteine to methionine, which supports DNA synthesis and cell division. The other helps convert methylmalonyl-CoA to succinyl-CoA. When B12 is low, homocysteine and methylmalonic acid can rise. That is why these markers are often used when the serum B12 result is borderline or symptoms do not match the basic blood test.

Low B12 can affect:

1. Blood cells: impaired DNA synthesis can lead to megaloblastic or macrocytic anemia.
2. Nervous system: peripheral neuropathy, balance problems, cognitive symptoms, and in severe cases spinal cord involvement.
3. Mucosa and skin: glossitis, mouth ulcers, pallor, and sometimes skin or hair changes.

Diagnosis / how it's measured

Diagnosis is based on symptoms, risk factors, examination, and laboratory testing. No single test is perfect. Serum B12 is widely used first, but a result in the low-normal or borderline range may not exclude clinically important deficiency.

A typical workup may include:

Test	What it helps show
Complete blood count	Anemia, high mean corpuscular volume, other blood changes
Serum vitamin B12	Initial screening test
Methylmalonic acid	Often rises in true tissue-level B12 deficiency
Homocysteine	May rise in B12 deficiency, but also in folate deficiency and other states
Folate and iron studies	Look for overlapping deficiencies
Intrinsic factor antibodies	Supports pernicious anemia if positive

People with B12 deficiency may have fatigue, weakness, shortness of breath, palpitations, numbness or tingling in the hands and feet, poor balance, depression, memory problems, or a sore smooth tongue. Some have macrocytic anemia, but others do not. NHS guidance notes that symptoms can overlap with many other conditions, which is one reason confirmatory testing matters.

Clinicians also look for the cause, not just the low level. A person with long-term metformin use may need monitoring and replacement, while someone with pernicious anemia may need ongoing treatment because the absorption problem usually persists.

Evidence and uses

The main treatment is vitamin B12 replacement and correction of the underlying cause when possible. High-dose oral B12 works for many people, while intramuscular treatment is often used when deficiency is severe, neurologic symptoms are present, or absorption is markedly impaired. Evidence reviews used in the 2024 NICE guideline process found that both oral and intramuscular replacement can be effective in appropriate patients, though regimen choice depends on severity, cause, adherence, and local practice.

Response to treatment is usually tracked clinically and with repeat blood tests. Reticulocyte response can begin within about a week, blood counts often improve over several weeks, and neurologic recovery may take longer and may be incomplete if treatment is delayed. That is why early recognition matters.

Groups commonly considered at higher risk include:

• Older adults
• People with pernicious anemia
• Vegans and some vegetarians without fortified foods or supplements
• People taking metformin long term
• People using acid-suppressing medicines for long periods
• People with Crohn disease, celiac disease, or prior gastric/ileal surgery

When to see a clinician

See a clinician if you have unexplained fatigue, pallor, shortness of breath, numbness, tingling, balance problems, memory change, depression, or a sore tongue, especially if you are vegan, older, or take metformin. Seek prompt medical care if you have progressive weakness, trouble walking, confusion, or severe anemia symptoms.

Do not self-treat persistent symptoms with supplements alone if the cause is unknown. Folate can improve anemia while leaving B12-related nerve injury untreated, so the diagnosis should be clarified before assuming the problem is only folate deficiency. If you already have a diagnosed deficiency, follow-up testing and review of the cause are important because some causes require lifelong treatment.

Limitations and open questions

B12 deficiency is harder to define than many people expect because laboratory cutoffs vary by assay and by country. A serum B12 value that looks borderline may be normal for one person and clinically important for another, especially if neurologic symptoms are present. Methylmalonic acid and homocysteine can improve diagnostic accuracy, but they are not perfect and may be affected by kidney function, folate status, and other factors.

There is also ongoing debate about the best replacement route and schedule for different causes of deficiency. Evidence supports oral therapy for many patients, but severe neurologic disease, poor adherence, or profound malabsorption may still favor injections. Another open question is how aggressively to screen people on long-term metformin or acid-suppressing therapy; practice varies.

Finally, not every low B12 blood result means symptomatic disease, and not every symptom in a person with low-normal B12 is caused by B12 deficiency. Good care depends on combining symptoms, risk factors, blood counts, confirmatory markers when needed, and a search for the underlying cause.