Vitamin B12 (Supplement)
Brand names: Neurobion, Methycobal, B12 Inject
Water-soluble vitamin
Vitamin B12 deficiency causes anaemia and irreversible nerve damage. It is common in vegans, elderly individuals, and patients on long-term metformin or PPIs. Supplementation is straightforward — but the route (oral vs injection) depends on the cause.
Drug Class
Water-soluble vitamin
Category
Vitamins & Supplements
Forms Available
Oral tablets (50–1000 mcg), Sublingual tablets
How It Works
Vitamin B12 (cobalamin) is essential for DNA synthesis, red blood cell maturation, and the maintenance of myelin sheaths around nerves. It acts as a cofactor for methionine synthase (important for folate metabolism and DNA synthesis) and methylmalonyl-CoA mutase. Deficiency causes megaloblastic anaemia and neurological damage.
What It's Used For
- Vitamin B12 deficiency — dietary or absorption-related
- Pernicious anaemia (autoimmune loss of intrinsic factor)
- Neuropathy associated with B12 deficiency
- Metformin-induced B12 deficiency
- Long-term PPI use
- Vegan and vegetarian supplementation
Available Forms
Side Effects
Common Side Effects
- Oral and injection forms are very well tolerated
- Injection site pain (IM)
- Mild diarrhoea (rarely)
Serious Side Effects
- Anaphylaxis (very rare, with injections)
- Hypokalaemia — during correction of severe deficiency as red cell production increases and potassium uptake rises
Drug Interactions
- Metformin — reduces B12 absorption in the terminal ileum
- PPIs and H2 blockers — reduce gastric acid needed to release B12 from food protein
- Nitrous oxide — inactivates B12; anaesthesia with N2O in B12-deficient patients can precipitate subacute combined degeneration
When to Avoid
- No absolute contraindications for cyanocobalamin at standard doses
- Hereditary optic neuropathy (Leber's disease) — use hydroxocobalamin, not cyanocobalamin
Patient Tips
- High-dose oral B12 (1000 mcg daily) is as effective as injections for most causes of deficiency — even without intrinsic factor — because ~1% is absorbed passively
- Injections remain preferred for pernicious anaemia (every 3 months after loading) and for patients with severe neurological symptoms needing rapid correction
- Plant foods contain virtually no B12 — vegans and vegetarians must supplement consistently
- Serum B12 can appear normal even with functional deficiency; methylmalonic acid (MMA) is a more sensitive marker
Neurological symptoms: the most serious consequence
B12 deficiency affecting the nervous system can cause subacute combined degeneration of the spinal cord — tingling and numbness in the feet, balance problems, and eventually weakness and cognitive impairment. These changes can become irreversible if the deficiency is prolonged.
Unlike anaemia (which resolves with treatment), neurological damage may only partially improve. Early detection and treatment are critical.
Oral vs injection: which is right for you?
High-dose oral B12 (1000–2000 mcg daily) achieves adequate blood levels via passive absorption even in pernicious anaemia. Randomised trials show equivalence with injections for most patients.
Injections are preferred when: rapid correction is needed for severe neurological symptoms, compliance is uncertain, or GI absorption is so severely impaired that even passive absorption is unreliable.
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Medical disclaimer
This page is for educational reference only. Always follow the dosage and instructions given by your prescriber or pharmacist. Do not start, stop, or change medicines without medical advice.
Reviewed 2026-06-01 · Looms Medical Team