Vitamin B12 blood test: what it measures and what your result means

Medically reviewed by Dr Lebeth Jacob · 21 October 2026

Quick answer: Vitamin B12 helps your body make healthy red blood cells and keeps your nerves working properly. You get it from animal foods such as meat, fish, eggs and dairy. A B12 blood test shows whether you have enough. Low B12 is common and can cause tiredness, pins and needles and memory problems. It's very treatable, but it's important not to leave it, because nerve damage can become permanent.

What is vitamin B12?

B12 is a vitamin your body can't make for itself, so it has to come from food or supplements. It's found naturally in animal foods, and is added to some breakfast cereals, plant milks and yeast extracts.

To absorb B12, your stomach makes a protein called intrinsic factor. If you don't make enough, or your gut doesn't absorb well, you can become low in B12 even with a good diet.

Some tests measure total B12. Others measure active B12, the part your cells can use, which can give a clearer answer when total B12 is borderline.

Why is B12 tested?

  • You feel tired, weak or breathless, or look pale
  • You have pins and needles, numbness, or problems with balance
  • You have memory problems, low mood or difficulty concentrating
  • You have a sore, red tongue or mouth ulcers
  • You eat a vegan or mostly plant-based diet
  • You take metformin or a stomach-acid medicine such as omeprazole long term

What does a low B12 result mean?

Low B12 means your body isn't getting or absorbing enough. Common reasons are:

  • Pernicious anaemia: an autoimmune condition that stops you absorbing B12. It's the most common cause in the UK.
  • A vegan diet without fortified foods or a supplement
  • Gut conditions such as coeliac disease or Crohn's disease, or past stomach surgery
  • Some medicines, including metformin and long-term stomach-acid medicines
  • Heavy drinking

A GP may test for intrinsic factor antibodies to check for pernicious anaemia. B12 deficiency is often treated with injections at first, then injections or tablets depending on the cause.

What does a high B12 result mean?

A high B12 is usually caused by supplements or B12 injections, and isn't harmful. If it's high and you don't take B12, a GP may look for other causes, such as liver conditions or some blood conditions.

What can affect your result?

  • B12 supplements, multivitamins, energy drinks and injections raise your level. Tell the doctor reviewing your result if you take them.
  • Pregnancy and the contraceptive pill can lower total B12 without a true deficiency
  • Low folate often goes with low B12, so they're usually tested together

When to see a GP

Your results report shows the laboratory's normal range and flags any result outside it. Speak to your GP if:

  • Your B12 is flagged low, even if you feel well
  • You have pins and needles, numbness, balance problems or memory changes (see them promptly)
  • You're pregnant or planning a pregnancy
  • Your B12 is flagged high and you don't take supplements

Get urgent medical help if you develop new problems walking, sudden weakness in your legs, or severe confusion.

How we test vitamin B12

You can have vitamin B12 tested with a blood sample taken at one of our 250+ partner clinics, or with a finger-prick kit at home. Samples are analysed by a UKAS-accredited laboratory. Results are usually ready in 3–5 working days, with a doctor's comments on every result.

Tests that include vitamin B12

Prices for in-clinic tests include your appointment. Compare every test with vitamin B12 in the biomarker explorer

Frequently asked questions

Can low B12 be a sign of cancer?

Rarely. Low B12 is usually caused by pernicious anaemia, diet, medicines or gut conditions. People with long-standing pernicious anaemia have a slightly higher risk of stomach cancer, so a GP will follow up any new symptoms such as indigestion that doesn't go away or difficulty swallowing.

Does a high B12 mean cancer?

Usually not. The most common reason is taking supplements. If your B12 is high without supplements, your GP may check for other causes.

Is it safe to take 1,000 micrograms of B12 a day?

For most people, yes. The NHS says taking 2 milligrams (2,000 micrograms) or less of B12 a day in supplements is unlikely to cause harm. Adults need about 1.5 micrograms a day from food, so high-dose supplements are only needed if you're low or can't absorb B12 well.

How long does it take to recover from B12 deficiency?

Tiredness often improves within a few weeks of starting treatment. Nerve symptoms such as pins and needles can take months to improve. Treating it early gives the best chance of full recovery.

What foods contain B12?

Meat, fish, shellfish, eggs, milk, cheese and yoghurt. If you're vegan, fortified cereals, plant milks and yeast extracts can help, but most vegans need a supplement.

What is the difference between total and active B12?

Total B12 measures all the B12 in your blood. Active B12 measures the part your cells can actually take up. Active B12 can be more useful when a total B12 result is borderline.