Vitamin B12

What Is Vitamin B12?

Vitamin B12, also called cobalamin, is a water-soluble vitamin that your body needs for some of its most essential jobs — building healthy red blood cells, keeping your nerves working properly, and helping create DNA. Your body cannot make B12 on its own, so you have to get it from the food you eat or from supplements.

B12 is found naturally in animal-based foods like meat, fish, eggs, and dairy. Your stomach produces a special protein called intrinsic factor that grabs onto B12 and helps your intestines absorb it. If anything goes wrong with that process, your levels can drop even if your diet seems perfectly fine.

What Does It Measure?

A vitamin B12 blood test measures how much B12 is circulating in your bloodstream. This gives your doctor a snapshot of whether your body has enough of this vitamin to carry out its daily work — particularly making red blood cells and maintaining your nervous system.

In some cases, your doctor may also order a methylmalonic acid (MMA) test or a homocysteine test alongside B12. These additional markers can help confirm a true deficiency, since blood B12 levels can sometimes look normal even when your cells are not getting enough.

Normal Ranges

| Group | Range | Unit | |---|---|---| | Adults | 200 – 900 pg/mL | pg/mL | | Borderline low | 200 – 300 pg/mL | pg/mL | | Deficient | Below 200 pg/mL | pg/mL | | Children (1–18 years) | 200 – 900 pg/mL | pg/mL | | Older adults (65+) | 300 – 900 pg/mL | pg/mL | | Pregnant individuals | 200 – 900 pg/mL | pg/mL |

Note: Some labs report results in pmol/L. To convert pg/mL to pmol/L, multiply by 0.738. For example, 200 pg/mL is roughly 148 pmol/L.

What Does a High Level Mean?

A high B12 level is less commonly discussed, but values significantly above the normal range can sometimes signal an underlying issue. Taking B12 supplements or getting B12 injections is the most common and harmless reason for elevated levels.

Possible causes of high B12:

• B12 supplements or injections (the most frequent cause — and usually nothing to worry about)
• Liver disease, since the liver stores large amounts of B12 and releases it when damaged
• Certain blood cancers like chronic myelogenous leukemia or polycythemia vera
• Kidney disease that reduces B12 clearance

Common symptoms:

• High B12 from supplements typically causes no symptoms at all
• If caused by liver or blood disorders, symptoms relate to that underlying condition rather than the B12 itself

Recommended next steps:

• If you are taking high-dose supplements, your doctor may simply note the result and move on
• If you are not supplementing and your level is unexpectedly high, your doctor may order liver function tests or a complete blood count to investigate further

What Does a Low Level Mean?

B12 deficiency is surprisingly common, affecting an estimated 6 percent of adults under 60 and nearly 20 percent of those over 60 in the United States. Because B12 is so important for your blood and nervous system, low levels can cause wide-ranging symptoms.

Possible causes of low B12:

• A diet low in animal products (vegetarians and especially vegans are at higher risk)
• Pernicious anemia, an autoimmune condition where the body attacks the cells that make intrinsic factor
• Digestive conditions like celiac disease, Crohn's disease, or chronic gastritis
• Previous stomach or intestinal surgery (including gastric bypass)
• Long-term use of certain medications, particularly metformin and proton pump inhibitors like omeprazole
• Heavy alcohol use
• Older age, since stomach acid production decreases over time and you need acid to free B12 from food

Common symptoms of low B12:

• Fatigue and weakness
• Pale or slightly yellowish skin
• Tingling or numbness in the hands and feet
• Difficulty with balance and walking
• A sore, swollen, or unusually smooth tongue
• Difficulty concentrating or "brain fog"
• Mood changes, including depression or irritability
• Shortness of breath or dizziness

Recommended next steps:

• Your doctor may recommend B12 supplements — either high-dose oral tablets (typically 1,000 to 2,000 mcg daily) or B12 injections
• If pernicious anemia is suspected, you may need lifelong B12 injections since your body cannot absorb it through the gut
• Recheck your levels after 2 to 3 months of treatment
• A complete blood count (CBC) may be ordered to check for anemia

When Should You Get Tested?

Consider asking your doctor about a B12 test if you:

• Follow a vegetarian or vegan diet
• Are over 60 years old
• Have unexplained fatigue, numbness, or tingling in your extremities
• Have been diagnosed with a digestive disorder
• Take metformin for diabetes or proton pump inhibitors for acid reflux
• Have had stomach or intestinal surgery
• Are pregnant or breastfeeding (your baby depends on your B12 supply)
• Have a family history of pernicious anemia
• Drink alcohol heavily

How to Improve Your Levels

Food sources: The richest sources of B12 are clams, liver, trout, salmon, tuna, beef, milk, yogurt, cheese, and eggs. Fortified foods like breakfast cereals, nutritional yeast, and plant-based milks can also provide B12, which is especially important for vegetarians and vegans.

Supplements: B12 supplements come in several forms — cyanocobalamin and methylcobalamin are the most common. For most people, a daily supplement of 250 to 1,000 mcg is sufficient for maintenance. If you have trouble absorbing B12, sublingual (under-the-tongue) tablets or injections may work better than standard pills.

Address underlying causes: If a medication is contributing to your low B12, talk to your doctor about adjusting your treatment plan. If you have a digestive condition, managing it effectively can improve absorption over time.

Regular monitoring: If you are at ongoing risk for deficiency (vegan diet, pernicious anemia, or malabsorption), plan to have your levels checked regularly — typically once a year.

Frequently Asked Questions

Q: Can low B12 cause permanent nerve damage?

If left untreated for a long time, severe B12 deficiency can cause nerve damage that may not fully reverse even after treatment. This is why catching it early matters. The tingling, numbness, and balance problems caused by B12 deficiency usually improve with treatment, but the longer they persist before treatment begins, the harder they can be to resolve completely.

Q: I eat meat regularly — can I still be deficient?

Yes. Even if your diet includes plenty of B12-rich foods, conditions like pernicious anemia, chronic gastritis, or long-term use of acid-reducing medications can prevent your body from absorbing it properly. If you have symptoms of deficiency, it is worth getting tested regardless of your diet.

Q: Are B12 injections better than pills?

For most people, high-dose oral supplements work just as well as injections. However, if you have pernicious anemia or severe absorption problems, injections bypass the gut entirely and deliver B12 directly into your bloodstream, making them the more reliable option in those situations.

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This content is for educational purposes only and is not medical advice. Always consult your healthcare provider about your lab results.