Haematology
Vitamin B12
Hydroxocobalamin / cyanocobalamin — Replaces vitamin B12 in deficiency — by injection in pernicious anaemia and absorption problems.
Education and reference only. This is a plain-language class overview — it deliberately contains no doses. Always check the current Summary of Product Characteristics (SmPC), the BNF and your local formulary before prescribing or administering any medicine.
Quick answer
What is Vitamin B12?
Vitamin B12 is essential for healthy blood and nerves. Supplements treat B12-deficiency anaemia and the nerve problems of deficiency, given by injection when the body cannot absorb it from food.
- How it works: B12 is needed to make red blood cells and to maintain the protective sheath around nerves.
- In practice: In practice the route depends on the cause.
What it is
Vitamin B12 is essential for healthy blood and nerves. Supplements treat B12-deficiency anaemia and the nerve problems of deficiency, given by injection when the body cannot absorb it from food.
How it works
B12 is needed to make red blood cells and to maintain the protective sheath around nerves. In pernicious anaemia and some gut conditions the body cannot absorb dietary B12, so it is given by injection to bypass the absorption problem; in dietary deficiency, oral replacement may suffice.
In practice
In practice the route depends on the cause. Where deficiency is due to a problem absorbing B12 — most often pernicious anaemia (an autoimmune loss of the stomach factor needed to absorb it) or after certain gut surgery — replacement is given by injection, because tablets will not be absorbed; injections are given as a loading course and then as long-term maintenance. Where deficiency is purely dietary (for example a strict vegan diet), oral replacement can work. Two practical points matter: neurological symptoms (numbness, tingling, balance and memory problems) can occur and may not fully reverse if treatment is delayed, so deficiency is treated promptly; and if someone is both folate- and B12-deficient, the B12 is started first or alongside, never folate alone, to avoid worsening the nerve problem.
Examples
Practical use
How to take it & use it well
- If your deficiency is due to an absorption problem such as pernicious anaemia, you will usually need B12 by injection given by a nurse, because tablets are not absorbed properly in this situation.
- If your deficiency is purely dietary, for example a vegan diet, oral tablets may be enough - take them as advised, often once daily.
- Maintenance injections are typically given at regular intervals and are often needed lifelong, so keep all your appointments even when you feel well.
- Take the treatment promptly, as delaying can mean some nerve-related symptoms do not fully reverse.
- If both B12 and folate are low, your clinician will usually treat B12 first or at the same time to avoid worsening nerve problems.
Common uses
- Pernicious anaemia and absorption-related B12 deficiency (injection)
- Dietary B12 deficiency (e.g. vegan diet)
- B12-related nerve symptoms
Monitoring
- Full blood count and B12 (and folate) levels
- Neurological symptoms and their response
- Ongoing need for maintenance injections
Weighing it up
Advantages & disadvantages
Advantages
- It corrects B12-deficiency anaemia and can relieve tiredness, breathlessness and other symptoms.
- Started early, it can prevent or limit nerve damage that might otherwise become permanent.
- Injections reliably bypass gut absorption problems, making them very effective for pernicious anaemia.
- For dietary deficiency, oral tablets offer a simple and convenient option.
Disadvantages
- When absorption is the problem, tablets do not work and regular injections are needed instead.
- Treatment for pernicious anaemia is usually lifelong and means ongoing appointments.
- Some nerve damage may not fully recover if treatment is delayed.
- Injections can cause temporary soreness or bruising at the injection site.
Key safety principles
What to watch for
- Treat promptly — neurological damage from deficiency may not fully reverse if delayed.
- If both B12 and folate are low, do not give folate alone — start or include B12 to avoid worsening nerve damage.
- Absorption-related deficiency needs injections (tablets will not be absorbed); maintenance is usually long-term.
Key interactions
What to avoid or check alongside
- Long-term use of acid-reducing medicines (such as proton pump inhibitors) and the diabetes medicine metformin can lower B12 levels over time.
- Giving folic acid without enough B12 can mask the anaemia while nerve damage continues, so the two need to be balanced.
- Heavy alcohol use can affect B12 status and absorption.
- Few medicines directly clash with B12 treatment itself, but tell your clinician about everything you take so monitoring is appropriate.
Patient & carer advice
- If your deficiency is from absorption (e.g. pernicious anaemia), you will need regular injections, often for life
- Report numbness, tingling, balance or memory problems
- A vegan or very limited diet can cause it — ask about supplements
Use with
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Answers
Vitamin B12: frequently asked questions
Why do I need B12 injections instead of tablets?
If your body cannot absorb B12 from the gut, as in pernicious anaemia, tablets will not work because they rely on that same absorption. Injections deliver B12 directly and reliably.
Will I need B12 treatment for life?
If the cause is a lasting absorption problem such as pernicious anaemia, yes - usually lifelong injections. Dietary deficiency may only need treatment until your levels and diet improve.
Can a vegan diet cause B12 deficiency?
Yes, because B12 comes mainly from animal foods. People on a vegan diet often need a supplement, which can usually be taken as tablets if absorption is normal.
Will my symptoms go away once I start treatment?
Tiredness and anaemia often improve, but nerve-related symptoms may only partly recover, especially if treatment was delayed. Starting promptly gives the best chance of full recovery.
Why does B12 need treating before folic acid sometimes?
Treating folate alone when B12 is low can improve the blood count while nerve damage from low B12 continues. So clinicians treat B12 first or together with folate.
Authoritative sources
Always verify against the source
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