What does vitamin B12 do?
The short answer
Vitamin B12 keeps nerves healthy, helps make red blood cells and is needed to copy DNA. The body cannot make it, and it is found almost only in animal foods, so vegetarians, older adults and people taking metformin for a long time are at higher risk of running low.
Choose your depth
Vitamin B12 keeps nerves healthy, helps make red blood cells and is needed to copy DNA. The body cannot make it, and it is found almost only in animal foods, so vegetarians, older adults and people taking metformin for a long time are at higher risk of running low.
Choose Detailed for the fuller picture, or Scientific for mechanisms and study data.
Vitamin B12 works alongside folate in some of the most fundamental processes in the body.
Its main jobs
- Nerves. It helps maintain myelin, the insulating coat around nerve fibres.
- Blood. Without enough B12, red blood cells are made too large and too few, a type of anaemia.
- DNA. It is needed for every cell that divides.
Who is at risk of low B12
- Vegetarians and vegans, because B12 comes mainly from milk, curd, eggs, fish and meat
- Older adults, who absorb it less well
- People taking metformin or acid-reducing medicines for long periods
- People with conditions of the stomach or small intestine
Signs of deficiency
Tiredness, pale skin, breathlessness, tingling in the hands and feet, poor memory or low mood. These signs are not specific, so a blood test is the way to know.
Testing and treatment
A simple blood test measures B12. Deficiency is treated with tablets or injections, depending on the cause and severity.
Cobalamin is a cofactor for methionine synthase, converting homocysteine to methionine with 5-methyltetrahydrofolate as methyl donor, and for methylmalonyl-CoA mutase. Deficiency raises homocysteine and methylmalonic acid, the latter being a more specific functional marker.
Absorption requires gastric acid, intrinsic factor and an intact terminal ileum. Metformin is thought to impair calcium-dependent ileal absorption. In the DPPOS analysis (Aroda et al., 2016), metformin use was associated with an odds ratio of 1.13 per year for B12 deficiency, supporting periodic testing.
What we know
- B12 deficiency can cause anaemia and nerve damage.
- Vegetarian diets and long-term metformin raise the risk.
- Deficiency is easily tested and treated.
What remains uncertain
- The best screening interval for people on metformin.
- Whether supplementing people with normal levels offers any benefit.
Related ingredients
Sources
Last updated 15 October 2026.
This is educational content and does not replace advice from a qualified doctor. If you have symptoms or take medicines, please speak to your doctor before making changes.
