Vitamin B12 Deficiency: Symptoms, Testing, Absorption and Who Is Most at Risk

Vitamin B12 deficiency is easy to oversimplify. Someone feels tired, a B12 level is ordered, and the result is either called normal or low. In practice, B12 deserves a little more thought.

Vitamin B12 is required for normal red blood cell production, DNA synthesis and nervous-system function. Deficiency can cause anemia, but neurological symptoms can occur without anemia and without the enlarged red blood cells traditionally associated with B12 deficiency. A normal-looking blood count therefore does not necessarily end the evaluation when the history and symptoms still raise concern.

The reason for a low B12 matters as much as the number itself. Some people are not eating enough B12. Others are eating plenty but cannot release it from food or absorb it normally. Age, metformin, acid-suppressing medications, autoimmune gastritis, pernicious anemia, gastrointestinal disease and certain surgeries can all change B12 status.

For the broader HormoneSynergy® approach to deciding which nutrients deserve testing or supplementation, see What Vitamins Should I Take? An Evidence-Based Guide to Supplements, Nutrient Gaps, Testing & Safety.

What Vitamin B12 Does

Vitamin B12, or cobalamin, is involved in DNA synthesis, normal red blood cell formation and the development and maintenance of the nervous system. It also participates in reactions involving methylmalonic acid and homocysteine, which is why both compounds can become useful laboratory clues when B12 status is uncertain.

The adult Recommended Dietary Allowance is 2.4 mcg per day. Most people eating animal foods obtain adequate amounts because fish, meat, poultry, eggs and dairy products naturally contain B12. Fortified cereals, nutritional yeast and other fortified foods can provide B12 for people who eat little or no animal food.

The body stores a substantial amount of B12, primarily in the liver. Deficiency can therefore develop slowly, sometimes over several years, rather than appearing soon after dietary intake declines.

B12 Absorption Is More Complicated Than Eating Enough

B12 naturally present in food is attached to protein. Stomach acid and digestive enzymes first have to release the vitamin from that food protein. B12 then eventually binds to intrinsic factor, a protein produced by cells in the stomach, and the B12-intrinsic-factor complex is absorbed in the distal small intestine, primarily the terminal ileum.

This process creates several opportunities for absorption to fail.

Reduced stomach acid can make it harder to release B12 from food. Autoimmune damage to stomach cells can reduce both stomach acid and intrinsic factor. Disease or surgery involving the terminal ileum can interfere with the final absorption step.

B12 in fortified foods and supplements is already in a free form, so it does not require stomach acid to separate it from food protein. This is one reason the National Institutes of Health advises adults over 50 to obtain much of their recommended B12 from fortified foods or supplements.

Symptoms Can Extend Well Beyond Anemia

Fatigue and weakness are common presentations, particularly when B12 deficiency has produced megaloblastic anemia. Pallor, palpitations, reduced appetite and a painful or smooth tongue can also occur.

The neurological findings deserve particular attention. B12 deficiency can cause numbness, tingling, altered sensation, balance problems, gait changes and peripheral neuropathy. Cognitive changes, poor memory, depression and other neuropsychiatric symptoms have also been described.

These symptoms are not specific to B12 deficiency. Fatigue has dozens of possible causes, and numbness can arise from diabetes, nerve compression, thyroid disease, medications and other neurological disorders. The symptoms provide a reason to evaluate B12 in the appropriate clinical setting, not proof that a B12 supplement is needed.

Current NICE guidance specifically cautions against ruling out B12 deficiency solely because anemia or macrocytosis is absent. Neurological injury can occur without the classic blood-count abnormalities.

Older Adults Are at Higher Risk

B12 deficiency becomes more common with age. One reason is atrophic gastritis, in which changes in the stomach reduce acid secretion and make it harder to release B12 from food.

The NIH estimates vary considerably depending on the laboratory definition used, but B12 deficiency or low B12 status is clearly more common among older adults than among younger populations. NICE estimates deficiency in roughly 5% of adults ages 65 to 74 and more than 10% of those age 75 and older.

This is also why a normal diet containing animal products does not guarantee normal B12 status later in life. A person may be eating enough B12 but absorbing food-bound B12 less efficiently.

For a broader discussion of nutritional changes in midlife and beyond, see What Vitamins Should Women Over 50 Take? An Evidence-Based Guide.

Metformin and Vitamin B12

Metformin deserves a place in almost every clinical discussion of B12 deficiency because it is so widely used for type 2 diabetes, prediabetes and metabolic disease.

Long-term metformin therapy can reduce B12 absorption and lower blood B12 concentrations. A 2025 American Family Physician review cited randomized evidence showing substantially more low or borderline B12 levels among people receiving metformin than placebo.

This does not mean everyone taking metformin needs high-dose B12 supplementation. It does mean B12 status deserves consideration when metformin has been used long term, particularly when anemia, neuropathy, cognitive symptoms or other compatible findings develop.

This is especially relevant because diabetic neuropathy and B12-related neuropathy can overlap clinically. Automatically attributing numbness or tingling to diabetes can miss a correctable contributor.

Proton-Pump Inhibitors and Other Acid-Suppressing Medications

Proton-pump inhibitors such as omeprazole and other medications that suppress gastric acid can reduce the ability to release B12 from food protein. Long-term use has been associated with an increased likelihood of B12 deficiency.

The relationship is not a reason to stop an appropriately prescribed medication. It is a reason to include medication history when unexplained B12 deficiency or compatible symptoms are being evaluated.

H2-receptor blockers can raise similar concerns because they also reduce gastric acid production.

Pernicious Anemia and Autoimmune Gastritis

Pernicious anemia is not simply another term for low B12. It is a consequence of autoimmune disease affecting the stomach and the intrinsic-factor system required for normal B12 absorption.

When B12 deficiency has no obvious dietary or medication explanation, autoimmune gastritis deserves consideration. Testing may include antibodies against intrinsic factor and, depending on the clinical circumstances, parietal-cell antibodies and additional evaluation for autoimmune gastritis.

A negative intrinsic-factor antibody test does not completely exclude autoimmune gastritis. NICE recommends further evaluation when clinical suspicion remains high.

Finding the cause matters because an irreversible absorption problem changes the long-term treatment plan. Someone who briefly ate a B12-poor diet and someone who can no longer produce adequate intrinsic factor do not have the same problem simply because their serum B12 results look similar.

Gastrointestinal Disease and Surgery

Celiac disease and Crohn's disease can interfere with B12 status, particularly when disease involves the distal small intestine. Extensive terminal ileal disease is especially relevant because that is where the B12-intrinsic-factor complex is absorbed.

Gastric bypass, gastrectomy and other gastrointestinal operations can also interfere with B12 absorption. The mechanism varies with the operation. Some procedures reduce stomach acid or intrinsic factor, while others alter the intestinal anatomy required for normal absorption.

Total gastrectomy and complete terminal ileal resection create particularly strong reasons for lifelong B12 replacement because essential parts of the normal absorption pathway are no longer present.

Vegetarian and Vegan Diets

Vitamin B12 occurs naturally in animal foods. Unfortified plant foods are not reliable sources.

A well-designed vegan diet can be nutritionally excellent, but B12 requires deliberate attention. Fortified foods or a B12 supplement are generally necessary. Vegetarians who consume eggs and dairy may obtain more B12 naturally, although intake can still be insufficient depending on the diet.

This is one circumstance where waiting for symptoms before thinking about B12 makes little sense. Prevention is easier than correcting a longstanding deficiency after neurological symptoms appear.

Nitrous Oxide Is an Overlooked Cause

Recreational nitrous oxide exposure has become an increasingly recognized cause of neurological B12 problems. Nitrous oxide can inactivate vitamin B12 and produce a functional deficiency even when a conventional serum B12 measurement is not dramatically low.

When nitrous oxide exposure is suspected, NICE recommends using methylmalonic acid or homocysteine rather than relying only on serum B12 as the initial biochemical assessment.

This is particularly important in someone presenting with new numbness, weakness, balance problems or gait changes after repeated nitrous oxide exposure.

How Vitamin B12 Deficiency Is Tested

Serum or plasma B12 is usually the first test. The challenge is that the cutoff for deficiency is not identical across laboratories or guidelines.

NIH notes that values below roughly 200 to 250 pg/mL are commonly considered subnormal. A 2025 American Family Physician review uses less than 180 pg/mL as clearly deficient and considers 180 to 350 pg/mL borderline enough to justify additional testing when the clinical picture fits.

The precise threshold is less important than avoiding the assumption that every result inside a laboratory reference interval excludes functional deficiency.

A complete blood count is also useful because B12 deficiency may produce anemia and an elevated mean corpuscular volume, or MCV. The absence of either finding does not rule it out.

Why Methylmalonic Acid Can Be Helpful

Methylmalonic acid, usually abbreviated MMA, rises when cellular B12 availability becomes inadequate. It is particularly useful when serum B12 is borderline or when the laboratory result does not fit the clinical picture.

NIH considers MMA the most sensitive biochemical marker commonly used to assess B12 status. An elevated MMA alongside a low or borderline B12 strengthens the case for true deficiency.

MMA is not perfect. Kidney dysfunction can raise MMA independently of B12 status, and values tend to increase with age. Renal function therefore has to be considered before interpreting an elevated result as proof of B12 deficiency.

What About Homocysteine?

Homocysteine can also rise when B12 is inadequate because B12 participates in the conversion of homocysteine to methionine.

It is less specific than MMA. Folate deficiency, kidney dysfunction and several other factors can increase homocysteine, so an elevated result does not identify B12 as the cause by itself.

Homocysteine can still add useful information when interpreted alongside B12, folate, MMA, kidney function and the clinical history.

For more on B12 physiology, methylation and neurologic function, see Vitamin B12 Benefits for Longevity, Brain Health, Energy & Methylation.

Should B12 Be Tested Before Starting a Supplement?

When deficiency is being investigated, obtaining laboratory testing before beginning high-dose B12 is generally preferable. Supplements can increase serum B12 and make subsequent testing harder to interpret.

There is an important exception. When significant neurological symptoms or severe megaloblastic anemia make B12 deficiency strongly likely, treatment should not be unnecessarily delayed while waiting for confirmatory testing. NICE specifically addresses this situation because prolonged neurological deficiency can become harder to reverse.

Someone already taking B12 should tell the clinician the form, dose and frequency before the laboratory results are interpreted.

Oral B12 or Injections?

Intramuscular B12 injections were once treated as the default for nearly every significant deficiency. High-dose oral therapy works surprisingly well for many patients.

Randomized comparisons have found that large oral doses can normalize B12 similarly to injections in many people. Oral therapy is therefore a reasonable treatment for many dietary and medication-related deficiencies and for some people with impaired absorption.

Injections remain important. Severe deficiency, substantial neurological manifestations, adherence concerns and certain irreversible malabsorption disorders can make intramuscular therapy the more appropriate choice. NICE recommends lifelong intramuscular replacement when autoimmune gastritis is the cause or suspected cause, and after total gastrectomy or complete terminal ileal resection.

Treatment should be matched to the clinical problem rather than to the assumption that one route is inherently better for everyone.

Methylcobalamin, Cyanocobalamin and the “Best” Form of B12

Supplement labels now make the form of B12 sound almost as important as the vitamin itself. Methylcobalamin is frequently marketed as biologically superior because it is one of the metabolically active forms of B12.

The available evidence does not support declaring methylcobalamin universally superior. NIH notes that cyanocobalamin, methylcobalamin, adenosylcobalamin and hydroxycobalamin are all used as B12 sources and that available evidence has not demonstrated meaningful differences in supplemental absorption among these forms.

Methylcobalamin remains a perfectly reasonable form, particularly in practitioner formulations that combine B12 with methylfolate. The choice of form is usually less important than identifying whether B12 is needed, using an adequate dose and correcting the underlying reason for deficiency.

HormoneSynergy® offers RetzlerRx® Methyl B-12 Synergy Plus, a high-potency methylcobalamin and folate formulation intended for targeted supplementation rather than routine use simply because B12 is associated with energy.

B12 Is Not an Energy Booster When B12 Status Is Already Adequate

Fatigue is one of the reasons people buy B12 supplements, and B12 deficiency can certainly cause fatigue. The second half of that statement is frequently lost in supplement marketing.

Giving extra B12 to someone who already has adequate B12 status has not been shown to increase energy or athletic performance. Correcting a deficiency can improve deficiency-related symptoms; pushing an adequate nutrient level higher is not the same intervention.

This is why persistent fatigue deserves evaluation rather than an automatic B12 recommendation.

How HormoneSynergy® Approaches B12

We begin with the reason B12 has become a question. A vegan diet, metformin use, long-term acid suppression, gastrointestinal surgery, unexplained macrocytosis, neuropathy and low serum B12 each lead to somewhat different next steps.

When testing is appropriate, serum B12 and a complete blood count provide a useful starting point. MMA can clarify a borderline result. Homocysteine and folate may add context in selected cases. When deficiency is confirmed without an obvious explanation, the evaluation should move toward the cause rather than ending with a bottle of B12.

The purpose is to distinguish inadequate intake from impaired absorption and temporary causes from conditions likely to require lifelong replacement.

Our broader What Vitamins Should I Take? guide explains how we apply the same approach to vitamin D, magnesium, iron, calcium, multivitamins and other common supplements. Individual vitamin and mineral options are available in the HormoneSynergy® Vitamins & Minerals collection.

The HormoneSynergy® Perspective

Vitamin B12 deficiency is worth finding because it is treatable and because prolonged neurological deficiency can have lasting consequences. It is also easy to overdiagnose when every nonspecific symptom is attributed to B12 or to underdiagnose when a normal blood count is allowed to close the discussion.

Serum B12 is useful, but it is not always the entire answer. MMA can clarify borderline results. Medication history can reveal metformin or acid suppression. Diet can identify inadequate intake. Gastrointestinal disease, surgery or autoimmune gastritis can explain why adequate dietary B12 is not being absorbed.

Once deficiency is established, replacing B12 is usually straightforward. Understanding why it became deficient is the part that determines whether treatment is temporary, long term or lifelong.

Additional evidence-based articles on nutrition, healthy aging, hormones, metabolic health, preventive cardiology, body composition and cognitive health are available in the HormoneSynergy® Longevity Medicine Resource Library.

Frequently Asked Questions

What are the symptoms of vitamin B12 deficiency?

Symptoms can include fatigue, weakness, anemia, numbness or tingling, balance problems, gait changes, memory or concentration difficulties, depression and a sore or smooth tongue. B12 deficiency can produce neurological symptoms even when anemia or macrocytosis is absent.

What vitamin B12 level is considered deficient?

Laboratory cutoffs vary. NIH notes that values below approximately 200 to 250 pg/mL are commonly considered subnormal, while some clinical guidance uses less than 180 pg/mL as clearly deficient. Borderline results should be interpreted with symptoms, risk factors and, when appropriate, methylmalonic acid testing.

What is methylmalonic acid, and why is it tested?

Methylmalonic acid, or MMA, is a metabolite that rises when B12 availability is inadequate. It is particularly useful when serum B12 is borderline or does not fit the clinical picture. Kidney dysfunction can also raise MMA, so renal function must be considered when interpreting the result.

Can you have B12 deficiency with a normal blood count?

Yes. Neurological B12 deficiency can occur without anemia or macrocytosis. A normal complete blood count should not by itself exclude B12 deficiency when compatible symptoms and risk factors are present.

Does metformin cause vitamin B12 deficiency?

Long-term metformin use can reduce B12 absorption and lower serum B12 levels. Periodic assessment may be appropriate in long-term users, particularly when anemia, neuropathy or other symptoms compatible with deficiency develop.

Can acid reflux medications lower B12?

Long-term proton-pump inhibitors and other gastric acid-suppressing medications can reduce the release of B12 from food and are associated with an increased risk of B12 deficiency in some people.

Who is most at risk for vitamin B12 deficiency?

Higher-risk groups include older adults, vegans and some vegetarians, people taking metformin or long-term acid-suppressing medications, people with autoimmune gastritis or pernicious anemia, individuals with celiac or Crohn's disease, and people who have undergone certain stomach or intestinal surgeries. Recreational nitrous oxide use can also cause functional B12 deficiency.

Are B12 injections better than pills?

Not for everyone. High-dose oral B12 can effectively correct deficiency in many people. Injections may be preferred for severe deficiency, significant neurological symptoms and certain forms of irreversible malabsorption. Treatment should be chosen according to the cause and severity of the deficiency.

Is methylcobalamin better than cyanocobalamin?

Current evidence has not established that one supplemental form of B12 is universally superior. Methylcobalamin is a biologically active form and is widely used, but cyanocobalamin, hydroxycobalamin and other forms can also effectively provide B12.

Does vitamin B12 give you more energy?

Correcting B12 deficiency can improve fatigue caused by the deficiency. Additional B12 has not been shown to increase energy or athletic performance in people who already have adequate B12 status.

References

• National Institutes of Health Office of Dietary Supplements. Vitamin B12 Fact Sheet for Health Professionals.
• National Institutes of Health Office of Dietary Supplements. Vitamin B12 Fact Sheet for Consumers.
• Patel H, McGuirk R. Vitamin B12 Deficiency: Common Questions and Answers. American Family Physician. 2025;112(3):294-300.
• National Institute for Health and Care Excellence. Vitamin B12 Deficiency in Over 16s: Diagnosis and Management. NICE Guideline NG239. Published March 6, 2024.
• Wang H, Li L, Qin LL, Song Y, Vidal-Alaball J, Liu TH. Oral vitamin B12 versus intramuscular vitamin B12 for vitamin B12 deficiency. Cochrane Database of Systematic Reviews. 2018.

HormoneSynergy® provides this material for educational purposes. It is not intended to diagnose, treat, cure or prevent disease and does not replace individualized medical care. Nutritional supplements can interact with medications and may not be appropriate for everyone. New or progressive neurological symptoms warrant medical evaluation.