01 / Reference ranges
What counts as normal
Normal vs optimal
There is no established optimal above the deficiency range. In the 200 to 300 pg/mL gray zone, clinicians confirm with methylmalonic acid (MMA), which rises when cells are actually short of B12; a normal MMA argues against true deficiency regardless of the borderline serum number.
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02 / Why it matters
What the number tells you
B12 deficiency causes anemia and, more importantly, nerve damage that can become permanent: numbness, balance problems, memory complaints. Per the NIH Office of Dietary Supplements, most laboratories define subnormal as lower than 200 or 250 pg/mL (148 or 185 pmol/L), and low-marginal status (200 to 300 pg/mL) is far more common than classic deficiency. Because absorption, not intake, drives most deficiency, older adults, people on metformin or acid blockers, and anyone after gastric surgery are the groups that run low, along with vegans (B12 comes almost entirely from animal foods).
03 / In practice
Testing and what moves it
When it is tested
- With anemia (especially with large red cells), unexplained neuropathy, memory changes, or a risk factor: vegan diet, metformin, long-term acid suppression, gastric surgery, age over 65.
- The RDA for adults is 2.4 mcg/day; fortified foods and supplements count, and oral supplementation works even in absorption problems at high doses.
What raises it
- Supplements and injections. NICE guideline NG239 (2024): an over-the-counter B12 preparation "may increase total or active B12 concentrations without fully treating a deficiency," so a level taken while supplementing mostly reflects the supplement.
What lowers it
- Low absorption: pernicious anemia, gastric or ileal surgery, celiac and Crohn disease, metformin, proton-pump inhibitors and H2 blockers, age-related low stomach acid.
- Vegan diets without fortified foods or supplements.
04 / Related
Read next
Frequently asked questions
What is a normal B12 level?
Labs vary. The NIH ODS says most laboratories define subnormal as lower than 200 or 250 pg/mL, and calls 200 to 300 pg/mL a common low-marginal zone; NICE's 2024 guideline treats 180 to 350 ng/L (the same number as pg/mL) as indeterminate. Borderline results are confirmed with a methylmalonic acid test rather than treated on the serum number alone.
Does metformin really lower B12?
Yes. The NIH ODS says metformin "might reduce the absorption of vitamin B12 and significantly reduce serum vitamin B12 concentrations," and the ADA's 2026 Standards of Care (recommendation 3.10) say to consider periodic B12 assessment in people on long-term metformin, especially with anemia or peripheral neuropathy.
Should vegans supplement B12?
Yes. The NIH ODS says natural food sources of B12 are limited to animal foods, so vegans need fortified foods or a supplement. The adult RDA is 2.4 mcg a day, there is no tolerable upper intake level, and ODS says the vitamin is generally considered safe even at high doses because the body does not store excess amounts.