Vitamin B12 Deficiency: Fatigue, Nerve Symptoms, and Who Should Be Tested

Vitamin B12 deficiency is one of the more common and correctable nutritional shortfalls in adults, particularly in older people and those on certain medications. If you have been feeling persistently tired or have noticed tingling in your hands or feet, a simple blood test ordered by your doctor can tell you whether B12 is a contributing factor.

What Vitamin B12 Does in the Body

According to the NIH Office of Dietary Supplements (ODS), vitamin B12 serves three core physiological roles:

• It is required for healthy red blood cell formation.
• It is required for the development, myelination, and function of the central nervous system.
• It is required for DNA synthesis.

These are well-established functions backed by strong evidence. When B12 levels fall low enough to impair these processes, the consequences can show up in the blood, the nervous system, and overall energy levels.

Recognizing the Symptoms

Because B12 is involved in red blood cell production, a significant deficiency can lead to megaloblastic anemia, a condition in which red blood cells are abnormally large and cannot carry oxygen efficiently. The NIH ODS rates the evidence that adequate B12 helps prevent megaloblastic anemia as strong.

Common symptoms associated with deficiency include:

• Persistent fatigue and weakness
• Numbness or tingling in the hands, legs, or feet
• Difficulty walking or balance problems
• Inflammation of the tongue (glossitis)
• Pale or jaundiced skin
• Mood changes, including irritability or depression
• Memory difficulties or cognitive slowing

It is important to note that these symptoms overlap with many other conditions. Only a clinician can interpret your blood work in the context of your full health history and determine whether low B12 is the cause.

The Adult Target and Safety Profile

The NIH ODS sets the Recommended Dietary Allowance for vitamin B12 at 2.4 micrograms per day for adults. No Tolerable Upper Intake Level has been established because of its low potential for toxicity. The body does not store large amounts of excess B12, and absorption becomes self-limiting at high oral doses, meaning the gut absorbs proportionally less as the dose increases.

B12 is generally considered safe even at doses well above the daily target. However, higher doses are not the same as more effective doses for people who are already sufficient. Talk to your doctor or pharmacist before starting any supplement, particularly at high doses, to make sure it fits your individual situation.

Vitamin B12 supplements are not intended to diagnose, treat, cure, or prevent any disease, and these statements have not been evaluated by the FDA.

Who Is Most at Risk for Deficiency

Certain groups are significantly more likely to have low B12 levels and should discuss testing with their health care provider.

Older Adults

Absorption of B12 from food depends on adequate stomach acid and a protein called intrinsic factor. As people age, stomach acid production often declines, reducing the ability to release B12 from food. Older adults can still absorb the crystalline form of B12 found in fortified foods and supplements, which does not require stomach acid for absorption.

People Following Plant-Based Diets

B12 is found almost exclusively in animal-sourced foods such as meat, fish, poultry, eggs, and dairy. People who follow vegan or strict vegetarian diets have very limited dietary sources and are at elevated risk of deficiency without supplementation or fortified foods.

People With Certain Gastrointestinal Conditions

Conditions that affect the stomach or small intestine, including pernicious anemia, atrophic gastritis, Crohn's disease, and celiac disease, can impair B12 absorption. Anyone with a diagnosed GI condition should ask their doctor whether B12 monitoring is appropriate.

People Taking Specific Medications

The NIH ODS identifies several medications that can interfere with B12 status:

• Proton pump inhibitors such as omeprazole (Prilosec) and lansoprazole (Prevacid) can interfere with B12 absorption from food by slowing gastric acid release and may lead to deficiency over time.
• Histamine H2-receptor antagonists such as cimetidine (Tagamet) and ranitidine (Zantac) can similarly reduce B12 absorption from food.
• Metformin, used for prediabetes and type 2 diabetes, might reduce B12 absorption and has been shown to significantly lower serum B12 concentrations in some people.

If you take any of these medications regularly, talk to your doctor about whether periodic B12 monitoring makes sense for you. Do not adjust your medication or add a supplement without consulting a health care provider first.

What the Evidence Does and Does Not Support

It is easy to find broad claims about B12 online. The NIH ODS evidence ratings give a more grounded picture:

• B12 is required for red blood cell formation, nervous system function, and DNA synthesis: strong evidence.
• Adequate B12 helps prevent megaloblastic anemia: strong evidence.
• B12 supplementation reduces the risk of cardiovascular disease or stroke: insufficient evidence.
• B12 supplementation improves cognitive function or slows dementia or Alzheimer's disease: insufficient evidence.
• B12 supplementation boosts energy, athletic performance, or endurance in people who are not deficient: insufficient evidence.
• B12 intake or blood level affects cancer risk: insufficient evidence.

The pattern here matters. Correcting a true deficiency can resolve the symptoms that deficiency causes. But for people who already have adequate levels, supplementing further does not appear to deliver the extra benefits that are often marketed alongside B12 products.

Getting Tested and Next Steps

A serum B12 test is a routine blood draw that your primary care doctor can order. If your levels are low or borderline, your clinician can recommend the appropriate form and dose of supplementation based on the underlying cause. For example, someone with pernicious anemia may need injections, while someone with a dietary gap may do well with an oral supplement.

Because individual needs vary, CareDoctor recommends consulting a clinician or pharmacist for personal dosing guidance rather than self-selecting a dose based on general population figures. The 2.4 microgram daily target from the NIH ODS is a population reference value, not a one-size-fits-all prescription.

Sources

• NIH Office of Dietary Supplements, Vitamin B12 Health Professional Fact Sheet: https://ods.od.nih.gov/factsheets/VitaminB12-HealthProfessional/
• NIH Office of Dietary Supplements, Vitamin B12 Consumer Fact Sheet: https://ods.od.nih.gov/factsheets/VitaminB12-Consumer/