Vitamin B12
Supports nerves, red blood cells and DNA.
The formula matches the form shown in the 3D model. Other forms of this vitamin can have different formulas.Loading molecular structure…
Cyanocobalamin · Drag to rotate
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One representative form. Hydrogens omitted; bonds simplified.
RCSB PDB · CNCWhere you can find it
- Fish & shellfish
- Meat
- Eggs
- Dairy
- B12-fortified foods
What it does in the body
B12 supports red blood cell formation, DNA synthesis and normal nervous-system function.
Recommended amounts & upper limit
US reference values for healthy adults aged 19–50, excluding pregnancy and breastfeeding. These are daily dietary intakes, not blood-test ranges or prescribed supplement doses. Other ages and life stages have different needs; see the original source.
- Men · RDA
- 2.4 mcg/day
- Women · RDA
- 2.4 mcg/day
- Upper limit · UL
- No established ULLow toxicity potential; absence of a UL does not justify unnecessary high doses.
How to read these levels
RDA means Recommended Dietary Allowance; AI means Adequate Intake, used when an RDA cannot be set. The upper limit (UL) is not a target or an exact toxicity threshold. Going below the RDA on one day does not diagnose deficiency. No established UL does not mean unlimited amounts are safe.
mg = milligrams; mcg = micrograms. 1 mg = 1,000 mcg. IU is vitamin-specific and cannot be converted with one rule for all vitamins.
What happens when the balance changes?
These are possible effects, not a diagnosis or a prediction from one low or high result.
Too little
Can cause megaloblastic anaemia, fatigue, tingling, numbness and balance or memory problems. Neurological injury can occur without anaemia and may become irreversible.
Body areas affected
Blood, brain and peripheral nerves.
Too much
A specific toxicity syndrome from high oral intake is not established. A high blood result is not by itself proof of vitamin toxicity.
Body areas affected
No specific organ toxicity established from intake.
What about blood-test levels?
Serum B12 thresholds vary by laboratory. Clinicians may use methylmalonic acid to clarify borderline results; kidney function affects that test.
Blood-test levels and daily intake use different units and cannot be compared directly. Test methods, reference ranges and clinical context vary. Symptoms alone cannot establish a vitamin deficiency or toxicity; a clinician can decide which tests and treatment are appropriate.
The body connection
Absorption depends on stomach processes and the final part of the small intestine.
Explore small intestine