Low B12: Early Signs, Symptoms, and What Your Levels Mean

If your B12 is low, you may feel tired in a way rest doesn’t fix, and you may notice pale skin, a sore tongue, or pins and needles in your hands and feet. Memory and mood can change too. Only a blood test can confirm it.^[1][2]

Quick symptom check

Tick any you've noticed regularly.

• I feel tired even after a full night's sleep
• My hands or feet tingle, buzz, or feel numb
• My skin looks paler than usual
• My tongue is sore, red, or unusually smooth
• I'm more forgetful or foggy than I used to be
• I feel short of breath climbing stairs
• My heart races or pounds, even with light effort
• I've lost my appetite without trying

See my resultClear all

Count how many you ticked, then find your range below.

1. 0–2 items

   Your answers don't strongly suggest B12 deficiency, but if these symptoms persist, mention them at your next check-up.

2. 3–5 items

   Several of these are common B12 signals. A blood test is the only way to know for certain.

3. 6 or more items

   This pattern is worth checking soon — especially the numbness or shortness of breath. Talk to a clinician.

See what your B12 levels mean

This is not a medical diagnosis. Talk to a healthcare professional.

On this page

Why vitamin B12 matters

Your body uses vitamin B12 for three jobs that explain almost every symptom on this page. It is needed to make healthy red blood cells, to build and maintain the protective coating around nerves, and to make DNA.^[2]

Your body can’t make B12, so it has to come from food or supplements. When supplies run short, red blood cells come out larger than normal and your hemoglobin drops, which is anemia.^[3] Nerves can also start to misfire, and the mouth and gut can be affected.

That is why low B12 shows up in three groups: signs of anemia, nerve and brain symptoms, and digestive changes.

Early physical signs of low B12

If you are looking for signs of B12 deficiency, below are commonly reported early signs, and you may notice only one or two. The physical signs of low B12 include unusual tiredness and weakness, pale skin, a sore or red tongue, and shortness of breath.^[1][2] They can take years to appear and build slowly, which is why they are easy to blame on stress or a busy schedule.^[2]

Fatigue and weakness

Yes, low B12 can make you tired; weakness and fatigue are on both the NHS and NIH symptom lists.^[1][2] With fewer healthy red blood cells, less oxygen reaches your muscles and brain.^[2][4]

What separates it from ordinary tiredness is that it doesn’t lift with rest. You may sleep a full night and still wake up drained, or find that stairs and errands feel harder than they used to. It often comes with other signs on this list, such as pale skin or breathlessness.

Many things cause fatigue, including thyroid problems, low iron, and poor sleep. A blood test is the way to tell them apart; see what your B12 level actually means.

Pale or yellowish skin

Low B12 can make your skin look paler than usual, because anemia means less red pigment in your blood. Pale skin is on the NIH list of deficiency signs.^[2][5] On its own, pale skin says little; it matters alongside tiredness or breathlessness.

Tongue and mouth changes

Low B12 can affect your tongue and mouth. The NHS lists a sore or red tongue, sometimes with mouth ulcers.^[1] Some develop glossitis, where the tiny bumps on the tongue wear away and it looks unusually smooth.^[2][6]

Low folate or iron can cause the same changes, so a sore tongue doesn’t point to B12 on its own.^[6] It is more telling when it comes with tiredness or tingling.

Shortness of breath

Low B12 can cause shortness of breath, mainly through anemia. With fewer working red blood cells, your heart and lungs work harder to deliver oxygen, so you may feel winded with light effort or notice palpitations.^[1]

Heart and lung problems cause breathlessness too, so new breathlessness always deserves a check. A blood count shows whether anemia is part of the picture; see what your B12 level actually means.

Neurological and mental symptoms

Nerve symptoms can happen before anemia appears, and some people have them without anemia at all.^[2] The main ones are tingling or numbness, muscle weakness, problems with balance, and changes in memory and mood.^[1]

Numbness and tingling

Low B12 can cause tingling in your hands and feet. It can feel like pins and needles, buzzing, or numbness, and it can appear even when your blood count is normal.^[2][5]

What matters is that it persists. A foot that falls asleep after sitting is normal; tingling that keeps returning over weeks is not. Some people later notice clumsiness or unsteadiness when walking.^[1] If that sounds familiar, see what your B12 level actually means.

Brain fog and memory

Low B12 can cause brain fog. It isn’t a medical term, but the NHS lists problems with memory, understanding, and judgment among the symptoms.^[1]

In daily life that can look like losing your train of thought, forgetting why you walked into a room, or getting confused more easily than before.^[5] In more severe deficiency, confusion and dementia have been described.^[2]

In older adults these changes are easy to put down to age, so ask whether a B12 test makes sense when memory starts to slip.

Low mood and irritability

Low B12 has been linked with low mood, anxiety, and irritability.^[1][5] The evidence is mixed. Some studies find an association between low B12 and depression,^[2] but a 2021 review of 16 trials found no overall effect of B12 supplements on depression in people without a clear deficiency.^[7]

A new or worsening low mood deserves attention whatever the cause. If you are already being treated for depression, don’t stop or change that treatment because of a B12 result; ask your clinician whether testing makes sense.

Symptoms in women and during pregnancy

The symptoms are the same for women as for men. What differs is need and some risks. Recommended intake rises from 2.4 mcg a day for adults to 2.6 mcg during pregnancy and 2.8 mcg while breastfeeding.^[2]

Low B12 in pregnancy is linked with complications and birth defects, and it might cause neural tube defects and developmental delays in the baby.^[8][2] Pernicious anemia is more common in women,^[9] and oral birth control pills can lower B12.^[5] See the dosage table.

Digestive symptoms

Yes, low B12 can cause diarrhea. The NHS lists diarrhea and indigestion among the symptoms.^[1]

Appetite, weight, and bowel changes

Low B12 can reduce your appetite and lead to weight loss you didn’t intend.^[1][5] Nausea, vomiting, and diarrhea are also reported.^[5]

These symptoms have many causes. They point more toward B12 when they come with tiredness, pale skin, or tingling. Unexplained weight loss always deserves a check-up.

What your B12 level actually means

A normal B12 level is one that falls inside the reference range printed on your lab report. MedlinePlus gives a normal range of 299 to 1,054 pg/mL (219 to 773 pmol/L), but each lab sets its own limits.^[10]

The number doesn’t always match how you feel. Some people have symptoms with levels in the normal range, and some have low levels with no symptoms.^[3] Most labs call a result below 200 or 250 pg/mL low, and NIH advises an MMA test when B12 falls between 150 and 399 pg/mL.^[2]

Some medicines can also affect the result, including metformin and acid reducers.^[10] So doctors read B12 alongside your symptoms and other markers. On your report, check the units too: B12 is given in pg/mL or pmol/L, and the numbers look very different.^[10]

Red blood cell size is another clue. Low B12 can make red blood cells larger than normal, which shows up as a high MCV.^[3] A normal MCV doesn’t rule low B12 out, because nerve symptoms can occur without anemia.^[2] The table below shows the markers you are most likely to see on a report.

When serum B12 is not enough

When your B12 result is borderline or doesn’t fit your symptoms, a doctor may order a methylmalonic acid (MMA) test, a homocysteine test, or both. These show whether your cells are actually short of B12.^[11]

MMA test. Your body needs B12 to break down methylmalonic acid, so MMA builds up when B12 is low. It is the most sensitive marker of B12 status, and a level above 0.271 μmol/L suggests deficiency. It also rises with kidney problems and tends to be higher in older adults.^[2] It can be done on blood or urine.^[12]

Homocysteine test. Homocysteine also rises when B12 is low; a level above 15 μmol/L, for example, suggests deficiency.^[2] It is less specific, because low folate or vitamin B6, kidney disease, and an underactive thyroid can raise it too.^[13] That is why folate is usually measured at the same time.^[3]

Neither test is available everywhere, so ask which ones your lab offers.^[11] Some home kits include MMA; see the test kit comparison.

How to check your B12 level at home

You can order a B12 test online without seeing a doctor first. Some send you a finger-prick kit to mail back;^[14] others have blood drawn at a lab or, in some areas, at home.^[15][16] Some also measure folate; none of the three below includes MMA. Review any abnormal result with a clinician.

How B12 deficiency is tested

The test is usually called a vitamin B12 test, a serum B12 test, or a cobalamin test, and it is often ordered with folate as a “B12 and folate” test.^[10][11] It uses a small blood sample from a vein in your arm.

Whether you need to fast depends on what else is being tested and on the lab. MedlinePlus says not to eat or drink for about 6 to 8 hours beforehand,^[10] while Lab Tests Online UK says a standard B12 test needs no fasting.^[11] The difference usually comes down to bundling. B12 is often drawn with other tests that do need fasting, such as homocysteine, which can require 8 to 12 hours.^[13] Follow the instructions your lab gives you.

How long results take depends on the lab. Because labs use different units and reference ranges, compare your result only with the range printed on your own report. If your result is borderline, your doctor may add an MMA or homocysteine test. Tell them about any supplements or medicines you take, since some affect results.^[10] To test at home instead, compare the kits above.

Who is at risk and what causes it

Low B12 has two basic causes: you don’t take in enough, or your body can’t absorb what you take in.^[9]

Not enough B12 in your diet. B12 occurs naturally in animal foods such as meat, fish, eggs, and dairy.^[2] A vegan diet without fortified foods or supplements is a recognized risk.^[9] Body stores of B12 can last around 2 to 5 years, so a deficiency can appear long after a diet change.^[9]

Your body can’t absorb it. The leading cause in the UK is pernicious anemia, explained below.^[9] Others are gut conditions, surgery, and some medicines. Older adults are at higher risk, often because atrophic gastritis reduces absorption.^[2]

The dosage table shows general intake figures; a treatment dose comes from your clinician.

Intrinsic factor and pernicious anemia

Intrinsic factor is a protein made in your stomach. It binds to B12 from food so the B12 can be absorbed further down, in the last part of the small intestine.^[9] People with pernicious anemia can absorb neither the B12 in food nor the free B12 in most supplements, which is why it causes more severe deficiency.^[2]

Pernicious anemia is an autoimmune condition in which the immune system attacks the stomach cells that make intrinsic factor. It is more common in people with a family history of it and in people with another autoimmune condition, such as vitiligo or Addison’s disease.^[9]

Because the problem is absorption, eating more B12 doesn’t fix it. Treatment works around the gut, usually with injections, although high-dose tablets are an option for some people. See pills vs injections.

Malabsorption and other causes

Several other conditions and medicines can reduce how much B12 you absorb:^[9][5]

• Stomach surgery, including gastrectomy and some weight-loss operations
• Gastritis, a long-term inflammation of the stomach lining
• Crohn’s disease and celiac disease, which affect the small intestine
• Long-term use of metformin or of acid reducers such as proton pump inhibitors and H2 blockers
• Heavy alcohol use, and nitrous oxide

If any of these apply, ask your clinician whether regular B12 checks make sense. Don’t stop a prescribed medicine on your own.

Treatment and prevention

Treatment follows the cause and runs in three steps: food, tablets, and injections.^[2]

If diet is the issue, B12-rich or fortified foods and a daily supplement fill the gap. If your body can’t absorb B12, diet alone won’t work. Treatment usually starts with injections, because they bypass absorption, though high oral doses might also work.^[2][17] Your clinician sets the dose, form, and schedule based on your test results and symptoms.

To prevent it on a plant-based diet, use fortified foods or a regular supplement. The supplement comparison sets the main forms side by side.

Pills vs injections

Whether pills can replace injections depends on why your B12 is low. When the cause is diet, daily tablets are usually enough.^[17]

When the cause is absorption, as with pernicious anemia, injections are the traditional treatment. But high-dose tablets of 1,000 to 2,000 mcg a day raised B12 about as well as injections in trials, although the evidence is low quality.^[18][19]

Injections work faster, so they are often used for severe deficiency or nerve symptoms.^[19][17] The dose and route are your clinician’s call. Compare forms in the supplement table.

Sublingual and other dose forms

Sublingual B12 is a tablet or lozenge that dissolves under your tongue. Evidence suggests it works no better than a regular tablet you swallow, and no form of B12 has been shown to absorb better than another.^[2]

B12 also comes in multivitamins, as a prescription nasal gel, and as injections.^[2] For most people, the best form is the one they will take consistently.

The table below lists daily needs by group and general supplement ranges. The two numbers measure different things, so don’t compare them directly.

How long it takes to correct

B12 deficiency is often described in four stages. Blood levels fall, then the body’s stores run down, then markers such as MMA and homocysteine rise, and finally anemia appears.^[20]

During treatment, the NHS describes a blood test around 7 to 10 days after starting and another after approximately 8 weeks.^[17] Many problems improve, but nerve damage can sometimes be permanent.^[8] The dosage table shows general amounts.

FAQ

When to see a doctor

See a doctor if you have symptoms of low B12. The NHS advises getting diagnosed and treated as soon as possible, because some problems can become permanent if left untreated.^[1]

Don’t wait if numbness is spreading, if you have trouble walking or keeping your balance, if your memory or thinking is changing, or if you are breathless with a racing heart. Don’t start, stop, or change a medicine because of this page.

This page is general information, not a diagnosis or medical advice. Read our medical disclaimer.