Vitamin B12 Deficiency: Symptoms, Testing and Treatment Options

In short: Vitamin B12 injections are a prescription-only treatment for people with a confirmed or strongly suspected B12 deficiency, particularly when the body cannot absorb B12 from food, for example in pernicious anaemia (autoimmune gastritis) or after some types of stomach or bowel surgery. Many people with diet-related deficiency can be treated with tablets instead. There is no good evidence that B12 injections boost energy in people whose B12 levels are normal, so a clinical assessment and blood test should come first.

This guide, written by a Ready Health prescribing pharmacist, explains B12 deficiency, how it is diagnosed and how it is treated in line with NICE guideline NG239 (2024).

What is vitamin B12 and why do we need it?

Vitamin B12 (cobalamin) is needed to make healthy red blood cells and to keep the nervous system working properly. It is found naturally in animal foods such as meat, fish, eggs and dairy, and is added to some fortified foods such as certain breakfast cereals and plant milks. To absorb B12, the stomach needs to produce a protein called intrinsic factor.

What are the symptoms of B12 deficiency?

Symptoms often develop slowly and can be vague. NICE lists signs and symptoms including:

• unexplained tiredness
• memory or concentration problems ("brain fog")
• pins and needles, numbness, balance or walking problems
• a sore, red tongue (glossitis)
• problems with eyesight
• anaemia or enlarged red blood cells on a blood test

Deficiency can be present even if a full blood count is normal. Neurological symptoms need prompt assessment, as delays in treatment can lead to lasting nerve damage.

Who is at risk of B12 deficiency?

• Diet: vegan diets without fortified foods or supplements, and very restricted diets
• Health conditions: autoimmune gastritis (pernicious anaemia), coeliac disease, Crohn's disease, other autoimmune conditions
• Surgery: removal of part or all of the stomach, some types of weight loss surgery, or removal of part of the small bowel
• Medicines: long-term metformin, proton pump inhibitors (such as omeprazole), H2-receptor antagonists, colchicine and some epilepsy medicines
• Recreational nitrous oxide use
• Older age, frailty or a poor diet

How is B12 deficiency diagnosed?

A blood test for total B12 or active B12 is the usual first step. NICE recommends testing people who have at least one symptom and at least one risk factor. Blood should be taken before starting any B12 supplement or injection, as treatment can mask the result. If you already take an over-the-counter B12 product, tell the clinician.

Further tests, such as intrinsic factor antibodies, may help identify the cause.

How is B12 deficiency treated?

Treatment depends on the cause:

• Diet-related deficiency: usually B12 tablets plus dietary advice. Injections may be considered if there is a risk of rapid deterioration or tablets are unlikely to be taken reliably.
• Autoimmune gastritis (pernicious anaemia), total gastrectomy or removal of the end of the small bowel: lifelong intramuscular injections, because B12 cannot be absorbed properly from food or standard tablets.
• Other causes of poor absorption or medicine-related deficiency: injections or high-dose tablets, decided case by case.

When injections are needed in the UK, hydroxocobalamin is usually used. Treatment normally starts with a series of loading injections over a couple of weeks, followed by maintenance injections, typically every 2 to 3 months for people who need lifelong treatment. The exact schedule depends on the cause and on whether there are neurological symptoms, and is set by the prescriber in line with national guidance.

NICE advises that symptoms may start to improve within two weeks but can take up to three months, and some may take longer.

Do B12 injections give you more energy if you're not deficient?

There is no good evidence that B12 injections improve energy, mood or weight loss in people who have normal B12 levels. Because they are a prescription-only medicine, they should only be given after a clinician has assessed you and confirmed a clinical need. If you feel persistently tired, it is better to look for the underlying cause, such as iron deficiency, thyroid problems, poor sleep or low mood.

Are B12 injections safe?

When prescribed appropriately they are generally well tolerated. Possible side effects include pain or redness at the injection site, and rarely allergic reactions. Tell your prescriber about any allergies and all medicines you take.

Frequently asked questions

Can I take B12 tablets instead of injections?

Often yes, if the deficiency is diet-related or caused by some medicines. Injections are usually needed when the body cannot absorb B12, such as in pernicious anaemia.

How long do B12 injections take to work?

Some people notice improvement within two weeks, but it can take up to three months, and nerve symptoms may take longer.

Do vegans need B12 injections?

Most vegans can maintain healthy levels with fortified foods or a daily supplement. Injections are only needed if a deficiency is confirmed and other options are not suitable.

Should I have my B12 level checked before treatment?

Yes. A blood test before starting treatment is important because supplements and injections can hide a deficiency and make diagnosis harder.

Key takeaways

• B12 deficiency can cause tiredness, memory problems and nerve symptoms, and should be confirmed with a blood test.
• The right treatment depends on the cause: many people can take tablets, while others need lifelong injections.
• B12 injections are prescription-only and are not proven to boost energy in people without a deficiency.
• Get tested before starting any B12 supplement or injection.

Sources and further reading

• NICE NG239: Vitamin B12 deficiency in over 16s
• NHS: Vitamin B12 or folate deficiency anaemia
• NHS: Treatment for vitamin B12 deficiency

Last reviewed: October 2026 by Shamir Patel, Independent Prescribing Pharmacist, Ready Health. This article is general information and does not replace advice from your own clinician.

If you are in Standish, Wigan or the surrounding area and think you may have low B12, book a blood test and consultation at Ready Health. A clinician will review your symptoms and results and advise on the most appropriate next steps, including whether any treatment is suitable for you.