What is Vitamin B12?
Vitamin B12 (cobalamin) is a water-soluble vitamin the body needs to make red blood cells, maintain the nervous system, and synthesize DNA. It is found in animal foods and requires stomach acid and a protein called intrinsic factor to be absorbed. Deficiency is common in older adults, vegans, and people on acid-reducing or diabetes medications.
Why it matters
B12 deficiency causes anemia and can lead to irreversible nerve damage if prolonged. Because symptoms are gradual and nonspecific, it is often missed until advanced.
What it measures
Serum vitamin B12. Because it can be normal despite deficiency, methylmalonic acid (MMA) and homocysteine are sometimes added to confirm.
Reference & optimal ranges
Reference ranges vary by lab, assay, age, and sex. The ranges below reflect commonly published adult intervals and are for education only always interpret results with the range printed on your own lab report and a clinician.
High B12 is usually harmless (excess is excreted) but can occasionally flag liver or blood disorders when not from supplements.
Common symptoms
- Usually none
Potential causes
- Supplementation
- Liver disease
- Certain blood disorders (rare)
Low B12 causes anemia and neurological symptoms that can become permanent if untreated.
Common symptoms
- Fatigue and weakness
- Numbness or tingling in hands/feet
- Balance problems
- Memory issues, brain fog
- Sore tongue, mouth ulcers
- Pale skin
Potential causes
- Vegan/vegetarian diet
- Pernicious anemia (autoimmune)
- Older age, low stomach acid
- Metformin or acid-reducing drugs
- Malabsorption (celiac, Crohn's)
How to improve your Vitamin B12
Lifestyle
Review medications
Long-term metformin and acid reducers deplete B12; discuss monitoring with your clinician.
Nutrition
B12-rich foods
Meat, fish, eggs, and dairy are rich sources; fortified foods and supplements are essential for vegans.
Supplement or inject if deficient
Oral B12 works for most; injections are used for absorption problems like pernicious anemia.
Exercise
General activity
No direct link, but energy improves once deficiency is corrected.
Sleep
General health
Fatigue from deficiency often resolves with treatment.
Frequently asked questions
Scientific references
- Vitamin B12 Fact Sheet for Health Professionals NIH Office of Dietary Supplements (2024)
- Vitamin B12 Test MedlinePlus, U.S. National Library of Medicine
Educational information, not medical advice. This page is for general education and does not replace diagnosis or treatment by a licensed clinician. Do not start, stop, or change any medication, supplement, or treatment based on this content. Reference and optimal ranges vary between laboratories interpret your results with the range on your own report and a qualified professional.