Vitamin B12 Test Results Explained

Plain-English guide · Last updated October 17, 2026 · Based on NIH and U.S. government sources

What the vitamin B12 test measures

A vitamin B12 blood test measures the amount of B12 in your blood. It may be ordered alone or as part of a vitamin B test (MedlinePlus). Your body needs B12 to keep nerve and blood cells healthy and to make DNA (NIH Office of Dietary Supplements).

NIH ODS notes your body stores 1,000 to 2,000 times as much B12 as you'd typically eat in a day, so a deficiency can take several years to cause symptoms (NIH Office of Dietary Supplements).

What your B12 level means

According to the NIH ODS health professional fact sheet (NIH Office of Dietary Supplements):

• Blood B12 values lower than 200 or 250 pg/mL are generally considered subnormal (low).
• The cutoff between normal and deficient varies by method and laboratory.
• The most sensitive marker is methylmalonic acid (MMA). It can be used to confirm a deficiency when B12 is between 150 and 399 pg/mL.

MedlinePlus adds that B12 results aren't always clear-cut, because inflammation and some medicines can affect them. If your level is normal but you have symptoms, you may need other tests (MedlinePlus).

Why B12 matters

Low B12 can cause megaloblastic anemia, which can make you feel tired or weak. NIH ODS lists other possible symptoms: pale skin, heart palpitations, loss of appetite, weight loss, numb or tingly hands and feet, balance problems, depression, confusion and poor memory (NIH Office of Dietary Supplements). Some nerve problems from low B12 can be serious, which is why testing and follow-up matter.

Possible reasons for low B12

These are possible causes, not a diagnosis. NIH ODS lists these groups as more likely to run low (NIH Office of Dietary Supplements):

• Older adults, many of whom don't make enough stomach acid to absorb the B12 in food. NIH ODS says people over 50 should get most of their B12 from fortified foods or supplements.
• People with pernicious anemia, an autoimmune disease where the body can't make intrinsic factor, which is needed to absorb B12.
• People with atrophic gastritis, celiac disease or Crohn's disease.
• People who have had stomach or intestinal surgery, for example for weight loss.
• Vegetarians and vegans, because only animal foods contain B12 naturally.
• People taking certain medicines (see below).

Plant-based B12: reliable sources to discuss with your doctor

NIH ODS is clear: plant foods have no B12 unless they are fortified (NIH Office of Dietary Supplements). If you eat mostly or fully plant-based:

• Use fortified foods such as some breakfast cereals and nutritional yeasts. Check the Nutrition Facts label for added B12.
• Consider a B12 supplement. B12 in supplements isn't bound to protein, so it doesn't need stomach acid to be freed, which is why NIH ODS recommends fortified foods or supplements for people over 50. Many B12 supplements are suitable for vegans; check the label.
• Plan for pregnancy and breastfeeding. NIH ODS notes that babies of vegan or strict vegetarian mothers may not get enough B12.

For reference, NIH ODS lists a daily recommended amount of 2.4 mcg for adults, 2.6 mcg during pregnancy and 2.8 mcg while breastfeeding. People with pernicious anemia are usually treated with B12 shots, although very high oral doses might also work; that's a decision for your doctor.

Safety and medicine interactions

NIH ODS says vitamin B12 has not been shown to cause harm, even at high doses (NIH Office of Dietary Supplements). It does list medicines that can lower B12. Tell your doctor and pharmacist about all supplements and medicines you take, and don't stop or change a medicine on your own:

• Metformin (for prediabetes and diabetes) might reduce B12 absorption and lower blood levels. See our A1C guide.
• Stomach acid reducers such as omeprazole (Prilosec), lansoprazole (Prevacid) and cimetidine (Tagamet) can interfere with B12 absorption from food.

When to talk to your doctor

• Your B12 is low or borderline, or your lab suggests an MMA test.
• You have numbness, tingling, balance problems or memory changes.
• You're vegan or vegetarian, over 50, take metformin or acid reducers, or have had stomach surgery.
• You have sudden confusion, weakness or trouble walking: seek urgent care.

Questions to ask at your next appointment

1. Is my B12 level low, borderline or normal, and what cutoff does this lab use?
2. Should I have an MMA test or a complete blood count?
3. Could my medicines or digestion affect my B12?
4. Are fortified foods enough, or do I need a supplement or shots?
5. When should I retest?

Frequently asked questions

Sources

1. .govNIH Office of Dietary SupplementsVitamin B12 Fact Sheet for Consumersods.od.nih.gov (opens in a new tab)
2. .govNIH Office of Dietary SupplementsVitamin B12 Fact Sheet for Health Professionalsods.od.nih.gov (opens in a new tab)
3. .govMedlinePlus (National Library of Medicine)Vitamin B Testmedlineplus.gov (opens in a new tab)
4. .govNCCIH (National Institutes of Health)Using Dietary Supplements Wiselynccih.nih.gov (opens in a new tab)

Medical disclaimer: This article is general education, not medical advice. It does not diagnose, treat, cure or prevent any disease. Always talk with your doctor about your lab results, and talk with your prescriber before starting, stopping or changing any medicine or supplement. If you think you have a medical emergency, call 911. Last updated October 17, 2026.