Vitamin B12 (Cobalamin)
Water-soluble vitamin for the nervous system, DNA synthesis, and blood formation
Also known as
Definition
Vitamin B12 (cobalamin) is an essential water-soluble vitamin found exclusively in animal foods. It is indispensable for myelination of nerve fibers, DNA synthesis, and the formation of red blood cells. Vegans, older adults, and individuals with stomach problems (intrinsic factor deficiency) are particularly at risk. Deficiency develops insidiously over months to years, as body stores are large.
| Parameter | Value |
|---|---|
| Unit | pmol/l (pg/ml) |
| Reference Range | 148–664 pmol/l (200–900 pg/ml) |
Agree on your personal target range with your doctor.
Conversion: 1 pg/ml ≈ 0.738 pmol/l. US labs often use pg/ml (normal value 200–900 pg/ml).
↓ What a low value means
A low B12 level indicates inadequate intake (vegan/vegetarian), absorption disorder (intrinsic factor deficiency, atrophic gastritis, metformin use), or increased demand. Consequences: megaloblastic anemia, peripheral neuropathy, cognitive impairment, elevated homocysteine and methylmalonic acid.
↑ What a high value means
Very high B12 values well above the reference range without supplementation may indicate liver disease, myeloproliferative disorders (e.g., polycythemia vera), or solid tumors. With known supplementation, elevated values are generally harmless.
✓ What can influence this value
The B12 level is influenced by intake from animal foods (liver, shellfish, meat, eggs, dairy); on a vegan diet without a B12 source it falls. Absorption disorders such as intrinsic factor deficiency or atrophic gastritis, taking metformin, and long-term PPI therapy can lower it, as can older age. After taking B12 supplements, higher values are to be expected. The assessment is made by the treating doctor.
When to test
B12 is measured, for example, with a vegan or vegetarian diet, metformin use, long-term PPI therapy, older age, and neurological symptoms; with neurological symptoms without delay. How often it is measured is decided by your doctor. Holotranscobalamin (active B12) is a more sensitive early marker than total B12.
Frequently asked questions
How is a B12 value in the lower reference range assessed? +
The laboratory cutoff (> 148 pmol/l) is the lower limit of the reference range. The laboratory's reference range applies; the doctor makes the assessment.
Which forms of vitamin B12 are found in supplements? +
Common forms are methylcobalamin, an active form, the well-researched cyanocobalamin, and adenosylcobalamin. Which form is appropriate is decided by your doctor.
How quickly does the B12 value change? +
With treatment, blood values usually normalize within weeks; neurological symptoms may take months. Conversely, the body can draw on B12 stored in the liver and muscles for years before deficiency becomes clinically visible. Whether and how treatment is given is decided by your doctor.
Is it true that metformin depletes B12? +
Yes. Metformin inhibits vitamin B12 absorption in the ileum through calcium-dependent mechanisms. Studies show that some people taking metformin develop a B12 decline. How often the value is then measured is decided by your doctor.
Vitamin B12 (Cobalamin)
Sources
Last updated: May 28, 2026
This information is for orientation only and does not replace medical advice. Reference ranges can vary by laboratory, method and country.