Hydroxocobalamin - Cyanocobalamin (B12) 💊
💉 Hydroxocobalamin is the usual UK first-line vitamin B12 injection 🇬🇧 because it is retained longer than cyanocobalamin,
so maintenance injections are typically every 2–3 months. 🧠 If there are neurological features, treat urgently and more intensively.
📚 Always check the BNF for current dosing and product guidance.
📘 About
- 📚 Always check the BNF for up-to-date prescribing guidance.
- 🇬🇧 Hydroxocobalamin is preferred in UK practice because it binds and is stored more effectively, allowing less frequent maintenance dosing.
- 🩸 Treats megaloblastic anaemia due to B12 deficiency and prevents irreversible neurological injury when given promptly.
- 🧪 Also used as an antidote to cyanide poisoning (high-dose IV), which is a completely different dosing context.
⚙️ Mode of Action (why B12 matters)
- 🧬 Vitamin B12 is essential for:
- 🩸 DNA synthesis → normal red cell maturation (prevents macrocytosis/megaloblastosis).
- 🧠 Myelin maintenance → normal peripheral nerve and spinal cord function.
- 🔁 Hydroxocobalamin replenishes B12 stores and acts as a cofactor for:
- 🧠 Methionine synthase: homocysteine → methionine (supports DNA methylation and cell division).
- ⚡ Methylmalonyl-CoA mutase: methylmalonyl-CoA → succinyl-CoA (fatty acid/energy pathways).
- 🧪 Biochemical clues:
- 📈 ↑ homocysteine and ↑ methylmalonic acid (MMA) support B12 deficiency (MMA is more specific).
- ⚠️ Folate can “fix the anaemia” while neuro damage progresses → always check B12 before/with folate replacement.
🧩 Causes of B12 deficiency (useful clinical framing)
- 🧬 Pernicious anaemia (autoimmune intrinsic factor deficiency) — common and usually lifelong IM therapy.
- 🍽️ Dietary deficiency (e.g. strict vegan diet) — can often be treated with oral therapy if absorption intact.
- 🦠 Malabsorption: coeliac disease, Crohn’s, ileal resection, pancreatic disease.
- 💊 Drug-related: metformin (long-term), PPIs (possible contribution), nitrous oxide exposure (functional B12 inactivation).
📋 Indications & Example Doses (Hydroxocobalamin)
- 🩸 Prophylaxis / maintenance in established deficiency: 1 mg IM every 2–3 months.
- 🧬 Pernicious anaemia (no neurological involvement):
1 mg IM three times weekly for 2 weeks, then 1 mg IM every 2–3 months.
- 🧠 Pernicious anaemia (with neurological involvement):
1 mg IM on alternate days until no further improvement, then 1 mg IM every 2 months.
🚨 Treat early — neuro recovery can be slow and may be incomplete if delayed.
- 👁️ Tobacco amblyopia / Leber’s optic atrophy:
1 mg IM daily for 2 weeks → 1 mg IM twice daily until no further improvement → then 1 mg IM every 1–3 months.
- ☠️ Cyanide poisoning (antidote):
5 g IV over 15 minutes; may repeat once (over 15 min–2 h) depending on severity, diluted in 0.9% sodium chloride.
(This is a specialist/emergency indication.)
🩺 Practical monitoring & response
- 📈 Hb/MCV usually improve over weeks; reticulocytosis can occur within ~1 week once marrow responds.
- 🧠 Neurological symptoms may take months; document baseline neuro exam (gait, vibration sense, proprioception, reflexes).
- 🧂 Consider checking/ replacing folate and iron if mixed deficiencies (common in older adults and malabsorption states).
- 🔎 In pernicious anaemia, consider associated autoimmunity (thyroid disease) and follow local guidance on gastric cancer risk counselling.
📌 Cyanocobalamin (UK context)
- 💊 Can be used orally for dietary deficiency when absorption is intact, but UK practice often favours hydroxocobalamin for parenteral replacement.
- 🧾 If using cyanocobalamin, follow the BNF monograph for exact regimens.
⚠️ Adverse effects / safety
- 🟠 Usually very well tolerated; injection-site discomfort is commonest.
- ⚡ Rare hypersensitivity reactions can occur (treat as per anaphylaxis protocol if severe).
- 🧪 Rapid correction of severe anaemia can unmask hypokalaemia (rare; consider in very severe cases).
📖 References
