Vitamin B12 and Energy: Fatigue, Deficiency and Who's at Risk

Vitamin B12 and Energy: Fatigue, Deficiency and Who's at Risk

Vitamin B12 helps your body make healthy red blood cells and keep nerves working, so a true deficiency can leave you exhausted, breathless and foggy. But if your B12 level is already normal, extra B12 does not boost energy. The fix for fatigue depends on whether you are actually low.

This guide is for anyone who feels tired all the time and wonders whether B12 is the answer, especially older adults, vegans and people on long-term stomach or diabetes medicines.

Key takeaways

  • Adults need 2.4 mcg of vitamin B12 a day, rising to 2.6 mcg in pregnancy and 2.8 mcg while breastfeeding.
  • B12 deficiency causes fatigue mainly through anemia, a shortage of healthy red blood cells that carry oxygen.
  • B12 supplements do not improve energy or athletic performance in people who are not deficient.
  • Older adults, vegans, people with pernicious anemia, people who have had stomach or bowel surgery, and long-term users of metformin or acid-reducing drugs are at higher risk.
  • Untreated deficiency can cause nerve damage that may become permanent, so numbness or tingling should be checked promptly.

How does vitamin B12 affect energy?

B12 does not act like caffeine. It helps your body make DNA and red blood cells, and without enough of it the bone marrow makes large, poorly working red cells, which reduces oxygen delivery and leaves you tired.

This kind of anemia is called megaloblastic anemia, meaning the red blood cells are oversized and immature. The NIH Office of Dietary Supplements describes it as the hallmark sign of B12 deficiency, along with fatigue, palpitations, pale skin and weight loss. B12 is also needed for the protective coating on nerves, which is why deficiency can affect mood, memory and sensation.

The key point for energy: the NIH fact sheet states that B12 supplementation appears to have no beneficial effect on energy or athletic performance in people without a nutritional deficit. If your level is normal, the cause of your tiredness lies elsewhere, such as sleep, iron, thyroid, mood, or diet patterns like those in our piece on foods that can drain energy and cloud focus.

What are the symptoms of B12 deficiency?

The most common symptoms are extreme tiredness, pale skin, shortness of breath on effort, and a sore red tongue, plus nerve symptoms such as pins and needles. Nerve symptoms can appear even before anemia shows up on a blood count.

According to the NHS, symptoms include:

  • Extreme tiredness and low energy
  • Pins and needles, muscle weakness and vision problems
  • Problems with memory, understanding or judgement
  • Mild depression or anxiety, and in more severe cases confusion
  • A sore, red tongue and mouth ulcers

MedlinePlus adds lightheadedness, loss of appetite, diarrhea or constipation, irritability and shortness of breath during exercise. With long-term deficiency it lists balance problems, numbness and tingling, confusion and, rarely, psychosis or optic nerve damage. Some people have no symptoms at all and are picked up on a routine blood test.

Symptom groupWhat you may noticeWhy it happens
Blood (anemia)Tiredness, pale skin, breathlessness, palpitations, lightheadednessFewer working red blood cells carry less oxygen
NervesPins and needles in hands or feet, weakness, poor balanceB12 is needed to keep nerve coverings healthy
Brain and moodMemory lapses, low mood, confusionNerve and brain cells depend on B12
Mouth and gutSore red tongue, ulcers, appetite loss, bowel changesFast-dividing cells in the mouth and gut are affected

What causes low B12 and who is at risk?

Most deficiency comes from poor absorption, not poor intake. B12 in food is bound to protein and needs stomach acid and a protein called intrinsic factor to be absorbed, so anything that disrupts the stomach or small bowel raises risk.

Intrinsic factor is a protein made in the stomach that carries B12 to the end of the small intestine for absorption. Pernicious anemia is an autoimmune condition in which the immune system attacks the cells that make it. The NHS calls pernicious anemia the most common cause of B12 deficiency in the UK, and the NIH calls it the most common cause of clinically evident deficiency worldwide.

Groups at higher risk, drawn from the NIH and NHS pages:

  • Older adults. Atrophic gastritis, which reduces stomach acid, affects 8 to 9 percent of adults 65 and older, compared with 2 percent of the general population. The NHS reports about 1 in 10 people aged 75 or over and 1 in 20 aged 65 to 74 are affected by B12 or folate deficiency.
  • Vegans and some vegetarians. B12 occurs naturally in animal foods, so a vegan diet needs fortified foods or supplements. Our article on feeling exhausted after switching to plant-based eating covers this in more depth.
  • Babies of vegan mothers who are exclusively breastfed.
  • People with gut conditions or surgery, such as celiac disease or weight-loss surgery like gastric bypass.
  • People on certain medicines, including metformin for diabetes, proton pump inhibitors and H2 blockers for reflux, and some anticonvulsants.
  • People who misuse nitrous oxide. MedlinePlus lists it as a cause of poor B12 status, and our report on the FDA warning about nitrous oxide and nerve damage explains the risk.

How common is it? NIH figures from US survey data show deficiency (below 200 pg/mL) in about 3.6 percent of adults, with insufficiency (below 300 pg/mL) in about 12.5 percent.

How is B12 deficiency tested?

A simple blood test for serum B12 is the starting point, often with a full blood count. If the result is borderline, a second marker called methylmalonic acid helps confirm whether your cells are really short of B12.

Per the NIH fact sheet, most labs treat serum B12 below 200 or 250 pg/mL as low, and below 300 pg/mL as insufficient. For results between 150 and 399 pg/mL, experts suggest checking methylmalonic acid (MMA), a substance that builds up when B12 is lacking; an MMA above 0.271 micromol/L suggests deficiency. MMA also rises with kidney disease, so results need interpretation. MedlinePlus notes doctors may also test for antibodies to intrinsic factor and parietal cells to look for pernicious anemia.

If you track your health numbers, our guide to tracking the wellness measures that matter can help you keep lab results and symptoms side by side for appointments.

Do B12 supplements help with tiredness?

They help if you are deficient, often noticeably, but they do not raise energy in people with normal levels. The form and route depend on the cause of the deficiency.

For diet-related deficiency, tablets usually work. When absorption is the problem, as in pernicious anemia, MedlinePlus says B12 shots are usually given about once a month, sometimes more often at first, and some people need them for life. High-dose oral supplements are an option for some people, because a small share of a large dose is absorbed without intrinsic factor. The NIH estimates absorption at about 50 percent for doses under 1 to 2 mcg, about 2 percent at 500 mcg, and about 1.3 percent at 1,000 mcg.

No upper limit has been set for B12 because it has low potential for toxicity. That does not make megadoses useful, and taking B12 before testing can mask the result. For general guidance on whether pills or food make more sense, see our article on finding the right balance between food and supplements, and for routine questions, our vitamin timing guide.

Good food sources of B12

From the NIH fact sheet, per 3 oz cooked serving: beef liver about 70.7 mcg, clams about 17 mcg, oysters about 14.9 mcg and Atlantic salmon about 2.6 mcg. Fortified nutritional yeast and fortified cereals are the main plant-based options, so check the label.

Common mistakes with B12 and fatigue

  • Assuming B12 is the cause without testing. Tiredness has many causes, and normal B12 means the search should continue.
  • Starting supplements right before a blood test. This can hide a deficiency and its cause.
  • Treating only the anemia. Folic acid can improve the blood count while nerve damage from low B12 continues, so both need checking.
  • Ignoring medicines. Years of metformin or acid-reducing drugs can lower B12 quietly.
  • Stopping injections early when the cause is pernicious anemia, which usually needs ongoing treatment.

A checklist before you talk to your doctor about tiredness

  1. Write down how long you have felt tired and whether it is getting worse.
  2. Note any pins and needles, balance problems, memory changes or a sore tongue.
  3. List your diet pattern, especially if you eat little or no animal food.
  4. List all medicines, including metformin, reflux tablets and anticonvulsants, and how long you have taken them.
  5. Note any past stomach or bowel surgery and conditions such as celiac disease.
  6. Ask for a blood count and B12 level, and ask whether MMA or antibody tests make sense.

When to see a doctor

See your doctor if tiredness lasts more than a few weeks without an obvious reason, and sooner if you have any of these red flags:

  • Numbness, tingling or pins and needles in hands or feet
  • Unsteady walking, weakness or frequent falls
  • Memory problems, confusion or a marked change in mood
  • Breathlessness, chest pain or a racing heart, especially on mild effort
  • Vision changes
  • Pregnancy or breastfeeding on a vegan diet without a reliable B12 source

The NHS stresses early diagnosis because some nerve problems can become permanent if treatment is delayed. Seek urgent care for chest pain, severe breathlessness or sudden confusion.

FAQ

How long does it take for B12 to improve energy?

If you are deficient, many people notice better energy within weeks of starting treatment, as new healthy red blood cells form. Nerve symptoms often take longer and may not fully recover if damage was long-standing. If you are not deficient, B12 is unlikely to change your energy at all, so keep looking for the real cause.

Are B12 injections better than tablets?

It depends on the cause. Injections bypass the gut, so they are standard when absorption fails, such as in pernicious anemia or after some stomach surgery. For diet-related deficiency, tablets usually work. Some people with absorption problems can use high-dose oral B12 under medical supervision. Your doctor will choose based on your tests.

Can you take too much vitamin B12?

No upper limit has been set because B12 has low potential for toxicity, according to the NIH. Still, very high doses offer no extra benefit if your level is normal, and taking them before testing can hide a deficiency. Tell your doctor about any B12 products you use.

Does metformin cause B12 deficiency?

Metformin can reduce B12 absorption and lower blood levels, especially with long-term use. That does not mean you should stop it. Ask your doctor whether periodic B12 testing makes sense for you, particularly if you develop tingling in your feet, which can be mistaken for diabetic nerve damage.

What to do next

  • If you are tired and in a higher-risk group, ask for a B12 test before starting supplements.
  • If you are vegan, choose a reliable source: fortified foods or a B12 supplement.
  • Review long-term medicines with your pharmacist for effects on B12.
  • If you are diagnosed, follow the full treatment plan and repeat tests as advised.

This article is for general information and is not medical advice.

Sources

Last reviewed: October 2026.

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