Vitamin B12 (cobalamin) is an essential water-soluble vitamin vital for red blood cell formation, neurological function, and DNA synthesis. The recommended daily dose varies by age, physiological state, and individual health circumstances. In the UK, adults aged 19–64 years require 1.5 micrograms (µg) daily, whilst breastfeeding mothers need 2.0 µg. Children's requirements range from 0.3 µg in infancy to 1.5 µg in adolescence. Treatment doses for deficiency are substantially higher, typically involving intramuscular injections of 1,000 µg or oral supplementation of 50–2,000 µg daily, depending on the underlying cause and severity. Understanding appropriate dosing ensures optimal health outcomes and prevents complications associated with deficiency.

What Is the Recommended Daily Dose of Vitamin B12?

Vitamin B12 (cobalamin) is an essential water-soluble vitamin that plays a crucial role in red blood cell formation, neurological function, and DNA synthesis. The recommended daily intake varies according to age, physiological state, and individual health circumstances.

For adults aged 19 to 64 years, the UK Department of Health recommends a daily intake of 1.5 micrograms (µg) of vitamin B12. This reference nutrient intake (RNI) represents the amount sufficient to meet the needs of approximately 97% of the population. Pregnant women require 1.5 µg daily (the same as non-pregnant adults), whilst breastfeeding mothers need 2.0 µg to support both their own requirements and those of their infant through breast milk.

Children's requirements are lower and increase with age: infants aged 0-6 months have a safe intake of about 0.3 µg daily, those aged 7–12 months need 0.4 µg daily, children aged 1–3 years require 0.5 µg, those aged 4–6 years need 0.8 µg, and children aged 7–10 years should receive 1.0 µg daily. Adolescents aged 11–14 years require 1.2 µg, increasing to the adult dose of 1.5 µg from age 15 onwards.

These recommendations assume normal absorption and dietary intake from animal-based foods such as meat, fish, dairy products, and eggs. Vitamin B12 is not naturally present in reliable amounts in plant foods (some seaweeds and fermented foods contain B12 analogues that are not bioactive in humans), which means vegans and some vegetarians need to obtain their intake through fortified foods or supplements. It's worth noting that the body can store vitamin B12 in the liver for several years, which provides some protection against short-term dietary inadequacy. However, maintaining consistent daily intake remains important for optimal health, particularly for vulnerable groups who may have increased requirements or impaired absorption.

Vitamin B12 Dosage for Deficiency Treatment

When vitamin B12 deficiency is diagnosed, treatment doses are substantially higher than the daily maintenance requirements. The approach depends on the severity of deficiency, underlying cause, and presence of neurological symptoms. NICE guidance distinguishes between dietary deficiency and deficiency caused by malabsorption conditions such as pernicious anaemia.

For pernicious anaemia or malabsorption disorders, the standard treatment in the UK involves intramuscular (IM) injections of hydroxocobalamin, the preferred form of B12. The typical regimen consists of 1 mg (1,000 µg) administered three times weekly for two weeks. For maintenance therapy without neurological involvement, 1 mg is given every 3 months. If neurological symptoms are present, treatment is more intensive: 1 mg on alternate days until no further improvement occurs, then maintenance doses of 1 mg every 2 months.

For dietary deficiency without neurological involvement, oral supplementation may be appropriate. Treatment doses typically range from 50 to 150 µg daily of cyanocobalamin, preferably taken between meals, or 1,000 µg daily for more rapid repletion, continued until deficiency is corrected. Absorption is limited to approximately 1.5 µg per dose through the intrinsic factor pathway, though passive diffusion allows absorption of about 1% of higher doses.

Oral therapy is generally unsuitable for pernicious anaemia or conditions affecting the terminal ileum (where B12 is absorbed), as these patients lack intrinsic factor or have damaged absorption sites. However, high-dose oral therapy (1,000–2,000 µg daily) may be effective in some cases through passive diffusion, with appropriate monitoring.

It's important to investigate the cause of deficiency with tests such as anti-intrinsic factor antibodies and to check folate status. Folate deficiency should be corrected, but B12 replacement should be started first to avoid worsening neurological damage. Treatment response should be monitored through clinical improvement, reticulocyte count (which typically rises within a week), and full blood count after 8 weeks. Patients should be advised that neurological recovery may take several months and, in some cases, may be incomplete if treatment is delayed.

Who Needs Higher Doses of Vitamin B12?

Several groups require higher vitamin B12 doses due to increased needs, impaired absorption, or specific health conditions. Identifying these individuals is essential for preventing deficiency and its associated complications.

Older adults frequently need higher vitamin B12 intake because gastric acid production decreases with age, reducing the release of B12 from food proteins. Additionally, atrophic gastritis—affecting 10–30% of people over 50—impairs intrinsic factor production. Many older adults benefit from supplementation or consumption of fortified foods, as crystalline B12 in supplements and fortified products doesn't require stomach acid for absorption. Clinical assessment and testing may be appropriate for those at risk.

Individuals with gastrointestinal conditions often require higher doses or injectable B12. This includes people with:

• Pernicious anaemia (autoimmune destruction of intrinsic factor-producing cells), typically requiring lifelong IM injections

• Crohn's disease or surgical resection affecting the terminal ileum

• Coeliac disease causing malabsorption (may improve with gluten-free diet)

• Chronic pancreatitis reducing enzyme availability needed for B12 release

Vegans and strict vegetarians need reliable B12 sources through fortified foods or supplements, as plant-based diets contain negligible amounts of active B12. A daily supplement of 10 µg or a weekly dose of 2,000 µg is often recommended. Individuals taking certain medications may also need higher doses, including those on metformin (commonly prescribed for type 2 diabetes), proton pump inhibitors, or H2-receptor antagonists for extended periods. The MHRA advises checking B12 levels in patients on metformin who have symptoms of deficiency or risk factors, and considering periodic monitoring.

Pregnant and breastfeeding women, particularly those following plant-based diets, require adequate B12 intake to support foetal development and prevent infant deficiency. While folic acid is the key preventive measure for neural tube defects, ensuring adequate B12 status is also important for overall maternal and foetal health.

People exposed to nitrous oxide (medical or recreational use) may develop functional B12 deficiency, as the gas inactivates B12-dependent enzymes, potentially requiring supplementation.

How to Take Vitamin B12 Supplements Safely

Vitamin B12 supplements are generally considered very safe, even at doses far exceeding the recommended daily intake, because it is water-soluble and excess amounts are typically excreted in urine. However, following best practices ensures optimal absorption and minimises potential issues.

Forms of B12 supplements include cyanocobalamin and methylcobalamin tablets, sublingual preparations, sprays, and injectable hydroxocobalamin. Cyanocobalamin is the most common and cost-effective form in over-the-counter supplements, whilst hydroxocobalamin is preferred for injections in the UK as it has a longer duration of action. For most people taking oral supplements, standard tablets are sufficient, though sublingual forms are marketed as having superior absorption—evidence for this advantage is limited for individuals with normal absorption capacity.

Timing and absorption considerations: B12 can be taken with or without food. For dietary deficiency, NICE CKS suggests taking oral cyanocobalamin between meals. The body can only absorb approximately 1.5 µg per dose through the intrinsic factor mechanism, which is why higher doses rely on passive diffusion for additional absorption.

Important safety considerations include:

• Inform your GP about all supplements you are taking

• Avoid self-treating suspected deficiency without medical assessment, as supplementation can mask underlying pernicious anaemia whilst allowing neurological damage to progress

• Do not start high-dose folic acid supplements before B12 deficiency is excluded, as this can worsen neurological damage

• Check for interactions if taking medications such as metformin, chloramphenicol, or proton pump inhibitors

• Monitor response through appropriate blood tests if treating deficiency

Pregnant women, individuals with Leber's disease (a rare hereditary eye condition), and those with cobalt allergy should consult healthcare professionals before supplementing. Quality matters: choose supplements from reputable manufacturers that follow Good Manufacturing Practice standards.

If you experience any suspected adverse reactions to vitamin B12 products, report them via the MHRA Yellow Card scheme (yellowcard.mhra.gov.uk).

Signs You May Need a Different B12 Dose

Recognising when your current vitamin B12 dose may be inadequate—or occasionally excessive—is important for maintaining optimal health and preventing complications. Several indicators suggest the need for dose adjustment.

Persistent or worsening symptoms despite supplementation warrant medical review. These include:

• Neurological symptoms: tingling or numbness in hands and feet (paraesthesia), difficulty walking, balance problems, memory difficulties, or mood changes

• Haematological signs: ongoing fatigue, weakness, pale skin, or shortness of breath suggesting anaemia hasn't resolved

• Glossitis: a sore, red tongue that hasn't improved

• Cognitive issues: continued difficulty concentrating or memory problems

These symptoms, particularly progressive neurological deficits or severe anaemia with cardiorespiratory compromise, may require urgent medical attention. Contact your GP for reassessment if symptoms persist after starting treatment.

Blood test results provide objective evidence for dose adjustment. Your healthcare provider will monitor your response to treatment through full blood count, mean corpuscular volume (MCV), and reticulocyte response rather than serum B12 levels after injection therapy. For oral therapy, serum B12 levels may be checked against your laboratory's specific reference range (units may be reported as pmol/L or ng/L depending on the laboratory). In cases of diagnostic uncertainty, additional tests such as holotranscobalamin or methylmalonic acid may be considered.

Changes in health status may necessitate dose modification. New diagnoses of gastrointestinal conditions, starting medications that affect B12 absorption, or undergoing gastric surgery all require reassessment. Similarly, pregnancy or the decision to adopt a vegan diet means reviewing your B12 intake.

Unusual symptoms such as acne-like skin reactions or hypokalaemia (low potassium) during initial high-dose treatment are rare but should prompt medical consultation. Never adjust prescribed B12 injection schedules without discussing with your GP, as this could lead to symptom recurrence. If you experience any concerning symptoms or your current regimen isn't providing expected benefits, seek professional guidance rather than self-adjusting doses.

Frequently Asked Questions