Vitamin B12 and folate: who is at risk of low levels and what the tests show
Short answer: vitamin B12 and folate are needed to make healthy red cells and to keep nerves working. Low levels are common, treatable and easy to miss, because early symptoms such as tiredness are vague. A blood test shows whether your levels are low.
Who is more likely to have low B12?
People who eat a vegan or vegetarian diet, because B12 comes mainly from animal foods and fortified products
Older adults, who may absorb B12 less well
People with pernicious anaemia, coeliac disease or Crohn's disease, or who have had stomach or bowel surgery
People who take metformin or long-term acid-reducing medicines
People who drink heavily
Symptoms that can go with low B12 or folate
These include tiredness, looking pale, a sore tongue, mouth ulcers, poor concentration and low mood. Low B12 can also cause tingling or numbness in the hands and feet. Tingling or numbness, balance problems or memory changes need prompt medical review, because untreated B12 deficiency can cause lasting nerve damage.
Which test do you need?
Total B12: the standard first test. See vitamin B12.
Active B12: measures the part of B12 your cells can actually use, and can help when total B12 is borderline. See active B12.
Folate: shows whether your folate intake and levels are adequate. See folate.
Why B12 and folate are often checked together
Folic acid can correct the anaemia caused by low B12 while the nerve damage carries on unnoticed. That is why a clinician will want to know your B12 level before treating low folate.
Do not self-treat a low result
B12 deficiency is usually treated with injections or high-dose tablets chosen by a clinician, depending on the cause. If you are planning a pregnancy, the NHS advises taking folic acid, so speak to your GP or midwife.
Frequently asked questions
Do I need to fast for a B12 or folate test?
Usually not, but check the preparation notes on the test page. Tell the clinician if you take supplements.
Can I have too much B12?
Extra B12 from supplements is usually passed out of the body. A very high result when you are not taking supplements can point to another condition, so mention it to a clinician.
Can a full blood count show low B12?
It can hint at it, because large red cells (high MCV) are a clue. But normal blood counts do not rule out low B12, so a specific test is needed.
Related reading
Clinically reviewed by Zak Toon, ACP, on 15 October 2026.
This article is general information based on UK guidance from NHS and NICE. It is not personal medical advice. If you are worried about your health or have symptoms, speak to your GP or call 111.