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Water-soluble vitamin / supplement

Vitamin B12: uses, side effects & precautions

Also known as Cyanocobalamin, Hydroxocobalamin, Methylcobalamin, Cobalamin, Cytamen.

This information is for general guidance only and does not constitute medical advice. Always consult a qualified healthcare professional for personal medical concerns. In an emergency, call your local emergency number.

What it is used for

  • Treating and preventing vitamin B12 deficiency
  • Treating a type of anaemia caused by low B12 (including pernicious anaemia)
  • Supporting people who absorb B12 poorly, such as after some stomach or bowel surgery
  • Supporting those on diets low in animal products, such as strict vegetarians or vegans

How it works

Vitamin B12 is needed to make healthy red blood cells, keep nerves working properly and help the body use folate. When the body cannot get or absorb enough, it leads to anaemia and nerve problems, which replacing B12 corrects.

How it is taken

Available as tablets, dissolvable or sublingual tablets, oral drops, and as an injection given by a healthcare professional. People who cannot absorb B12 through the gut (for example with pernicious anaemia) often need injections. Take or receive it exactly as your doctor, pharmacist or the label directs.

No doses here on purpose. The right strength and schedule depend on you — only a doctor or pharmacist should set them. Always follow your prescription and the label.

Common side effects

  • Usually well tolerated
  • Mild diarrhoea
  • Itching or mild rash
  • Pain or redness at the injection site (with injections)

Serious effects — get help

  • Signs of a serious allergic reaction: swelling of the face, lips or throat, difficulty breathing, severe rash
  • Rarely, low potassium and changes in heart rhythm when treating severe deficiency
  • Worsening of certain eye nerve conditions in rare cases

If a reaction is severe — for example trouble breathing, swelling of the face or throat, or collapse — go to the nearest emergency room now or call your local emergency number. Do not wait for an online consultation.

Interactions to watch

  • Long-term stomach acid reducers (proton pump inhibitors and H2 blockers) which can lower absorption
  • The diabetes medicine metformin, which can reduce B12 levels over time
  • Heavy or long-term alcohol use, which affects absorption

Who should take care

  • Tell your clinician about any allergy to cobalt or vitamin B12 products
  • Severe deficiency should be assessed by a clinician, as folate alone can mask it while nerve damage continues
  • Discuss long-term needs if you follow a vegan diet or have had gut surgery
  • Pregnancy and breastfeeding needs should be guided by your clinician

Frequently asked questions

Do I need injections or are tablets enough?
It depends on the cause. If your body cannot absorb B12 from the gut (for example pernicious anaemia), injections are usually needed. If your diet is low in B12 but absorption is normal, tablets may be enough. Your clinician will advise.
Who is most at risk of B12 deficiency?
Vegans and strict vegetarians, older adults, people on long-term acid-reducing medicines or metformin, and those who have had stomach or bowel surgery are at higher risk.
Can B12 give me more energy?
If you are deficient, correcting it can reduce tiredness caused by the deficiency. There is little evidence that extra B12 boosts energy if your levels are already normal.
Is it safe to take a lot of B12?
B12 is water-soluble and excess is usually passed in urine, so it has low toxicity. Still, take only the amount your clinician or the label recommends.

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