Vitamin B12 (cobalamin) is essential for red blood cell formation, neurological function, and DNA synthesis. The correct dosage of B12 varies considerably depending on whether you are maintaining adequate levels, preventing deficiency, or treating diagnosed B12 deficiency. For healthy UK adults, the Reference Nutrient Intake is 1.5 micrograms daily, easily obtained through diet. However, therapeutic doses for treating deficiency range from high-dose oral supplements (1,000–2,000 µg daily) to intramuscular injections of 1 mg, particularly for pernicious anaemia or malabsorption conditions. Understanding the appropriate B12 dosage for your individual circumstances ensures optimal health outcomes and prevents neurological complications.

What Is the Correct Dosage of B12 for Adults?

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 correct dosage varies depending on whether you are maintaining adequate levels or treating a deficiency.

For healthy adults, the Reference Nutrient Intake (RNI) established by the UK Department of Health is 1.5 micrograms (µg) daily. This amount can typically be obtained through a balanced diet containing animal products such as meat, fish, dairy, and eggs. Most multivitamin supplements contain between 2.5 and 25 µg of B12, which is well above the RNI but considered safe due to the vitamin's low toxicity profile.

For individuals following plant-based diets, fortified foods (such as breakfast cereals, plant-based milk alternatives, and yeast extracts) or supplements become essential sources. The British Dietetic Association recommends that vegans take a daily supplement of at least 10 µg or a weekly supplement of 2,000 µg to ensure adequate intake.

Absorption considerations are important when determining appropriate dosage. The body absorbs B12 through a complex mechanism involving intrinsic factor, a protein produced in the stomach. Only about 1% of oral B12 is absorbed passively when intrinsic factor is absent or impaired. This means that higher oral doses may be required in certain circumstances to achieve therapeutic effects. Excess B12 from high oral doses is primarily excreted through urine, which contributes to its low toxicity profile.

B12 Dosage for Deficiency and Pernicious Anaemia

When B12 deficiency is diagnosed, treatment dosages are substantially higher than maintenance levels. The approach differs significantly between deficiency with and without neurological involvement.

According to the NICE Clinical Knowledge Summary (CKS) and the British National Formulary (BNF), the standard treatment for B12 deficiency without neurological symptoms involves:

• Hydroxocobalamin 1 mg intramuscularly three times weekly for two weeks

• Followed by maintenance doses of 1 mg every three months

For patients with neurological symptoms (such as paraesthesia, numbness, cognitive changes, or gait disturbances), more intensive treatment is required:

• Hydroxocobalamin 1 mg intramuscularly on alternate days until no further improvement occurs

• Followed by 1 mg every two months for maintenance

Pernicious anaemia, an autoimmune condition where the body cannot produce intrinsic factor, requires lifelong treatment. The standard regimen is hydroxocobalamin 1 mg intramuscularly every three months indefinitely. Some patients report symptom recurrence before their next scheduled injection and may benefit from more frequent dosing, though this should be discussed with their GP.

Oral high-dose B12 (1,000-2,000 µg daily) may be considered as an alternative for patients with dietary deficiency without malabsorption who prefer this route or cannot tolerate injections. However, injectable therapy remains the first-line treatment in UK practice for pernicious anaemia and severe deficiency, as it bypasses absorption issues entirely. The MHRA-approved hydroxocobalamin formulation is the preferred injectable form in the UK, as it has a longer duration of action compared to cyanocobalamin.

Importantly, if both B12 and folate deficiencies are present, B12 should be corrected first to avoid potentially worsening neurological symptoms.

Oral vs Injectable B12: Dosage Differences

The route of B12 administration significantly affects the required dosage due to differences in bioavailability and absorption mechanisms.

Injectable B12 (hydroxocobalamin) is administered intramuscularly at doses of 1 mg (1,000 µg). This route provides reliable systemic levels as it bypasses the gastrointestinal tract entirely. The injection delivers B12 directly into the bloodstream, making it the most reliable method for patients with absorption problems, pernicious anaemia, or gastrointestinal conditions affecting B12 uptake (such as Crohn's disease, coeliac disease, or following gastric surgery).

Oral B12 supplementation requires much higher doses to achieve similar therapeutic effects. While the RNI for maintenance is only 1.5 µg daily, therapeutic oral doses typically range from:

• 50-150 µg daily for mild deficiency prevention in at-risk groups

• 1,000-2,000 µg daily for treating confirmed deficiency without malabsorption

• Some formulations contain up to 5,000 µg, though there is no proven additional benefit over 1,000-2,000 µg for most patients

The reason for this dosage discrepancy lies in absorption efficiency. When intrinsic factor-mediated absorption is functioning normally, only about 1.5-2 µg can be absorbed per meal through this active mechanism. However, approximately 1% of any oral dose is absorbed passively through diffusion across the intestinal mucosa, regardless of intrinsic factor. Therefore, a 1,000 µg oral dose would yield approximately 10 µg of absorbed B12.

Sublingual formulations (tablets or sprays placed under the tongue) are marketed as having superior absorption, but current evidence does not conclusively demonstrate that they offer significant advantages over standard oral tablets. Both ultimately rely primarily on passive diffusion for absorption.

For patients with confirmed pernicious anaemia or malabsorption disorders, injectable therapy remains the recommended first-line treatment according to NICE CKS guidance, as oral supplementation—regardless of dose—cannot reliably correct deficiency in these circumstances.

Signs You May Need a Higher B12 Dosage

Recognising when current B12 intake or supplementation may be inadequate is important for preventing progression to more serious deficiency states. Several clinical indicators suggest the need for dosage reassessment.

Persistent or recurring symptoms despite treatment may indicate inadequate dosing. These include:

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

• Haematological signs: persistent fatigue, weakness, pale or jaundiced skin, or shortness of breath

• Glossitis: a sore, red, smooth tongue

• Cognitive symptoms: difficulty concentrating, confusion, or "brain fog"

Seek urgent medical attention if you experience rapidly worsening neurological symptoms, significant difficulty walking, severe paraesthesia, or marked cognitive decline, as these may require immediate assessment.

If you are receiving maintenance B12 injections but symptoms return before your next scheduled dose, this should be discussed with your healthcare provider. Some patients may require more frequent administration, but this should only be adjusted under clinical supervision after excluding other causes of persistent symptoms.

Certain medical conditions and medications increase B12 requirements:

• Long-term use of metformin (diabetes medication) or proton pump inhibitors (PPIs for acid reflux) can impair B12 absorption

• Gastrointestinal conditions such as Crohn's disease, coeliac disease, or previous gastric surgery

• Autoimmune conditions beyond pernicious anaemia may affect B12 metabolism

When to contact your GP: If you are taking B12 supplements or receiving injections but continue to experience symptoms, request a review. Blood tests measuring serum B12 levels can help determine if your current dosage is adequate. More specialised tests such as active B12 (holotranscobalamin) or methylmalonic acid may be available through specialist referral, though availability varies across the UK. Never increase prescription B12 injection frequency without medical supervision, as the underlying cause of persistent symptoms should be investigated. However, increasing oral supplement doses (within safe limits) while awaiting medical review is generally considered safe, as B12 has very low toxicity and excess is excreted through the kidneys.

Frequently Asked Questions