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Vitamin B12 Supplement / Cyanide Antidote Pregnancy: Safe and essential in pregnancy if B12 deficient — B12 deficiency in pregnancy associated with NTDs and fetal neurological impairment.

Hydroxocobalamin (Vitamin B12)

Brand names: Cobalin-H (IM), Neo-Cytamen (IM), Cyanokit (IV — cyanide antidote)

Adult dose

Dose: B12 deficiency without neurological involvement: 1mg IM 3 times per week for 2 weeks, then 1mg IM every 3 months (maintenance for life in pernicious anaemia / malabsorption). B12 deficiency WITH neurological involvement: 1mg IM every other day until neurological improvement (no time limit), then 1mg every 2 months. Cyanide poisoning (Cyanokit): 5g IV over 15 minutes; second dose of 5g IV if needed.
Route: IM (B12 replacement) / IV (cyanide antidote)
Frequency: See dosing above
Max: 1mg per IM injection; 10g total (cyanide antidote)
Hydroxocobalamin preferred over cyanocobalamin in UK — retained in body longer (injection every 3 months vs. monthly). Pink discolouration of skin, mucous membranes, and urine for 48h after injection (harmless — warn patient). Oral B12 (cyanocobalamin 50 micrograms OD) is an alternative for dietary deficiency but NOT for pernicious anaemia (lack intrinsic factor — oral absorption requires IF).

Paediatric dose

Route: IM
Frequency: Varies by indication
Max: 1mg per IM injection
BNFc: Neonatal B12 deficiency (maternal): seek specialist neonatologist opinion. Children: 1mg IM every other day for 2 weeks (initial), then every 3 months. Cyanide poisoning (Cyanokit): 70mg/kg IV. Seek specialist paediatric haematology opinion.

Dose adjustments

Renal

No dose adjustment required.

Hepatic

No dose adjustment required.

Clinical pearls

  • Pernicious anaemia: lifelong IM injections required — PA is caused by autoimmune destruction of gastric parietal cells → no intrinsic factor → no oral B12 absorption. Oral B12 supplements are ineffective in PA (unless very high dose — passive absorption)
  • Neurological involvement: if subacute combined degeneration of the spinal cord (loss of posterior column — proprioception/vibration; and lateral column — UMN signs) is present, treat aggressively with alternate-day injections until improvement — neurological recovery is incomplete if delayed
  • Cyanide antidote: Cyanokit 5g IV is first-line for cyanide poisoning (fire victims with impaired consciousness, lactic acidosis) — mechanism: hydroxocobalamin binds cyanide to form cyanocobalamin (non-toxic, renally excreted)
  • Hypokalemia monitoring: when severe megaloblastic anaemia is treated, potassium is rapidly taken up by newly proliferating red blood cell precursors — serum K⁺ can fall dangerously within 48h; check K⁺ after starting treatment

Contraindications

  • Hypersensitivity to hydroxocobalamin or cobalt (relative — anaphylaxis rare)

Side effects

  • Injection site pain (common)
  • Pink/red discolouration of skin and urine (48h — harmless — warn patient)
  • Hypokalaemia (as B12 treatment drives cellular potassium uptake during Hb recovery — monitor in first 48h)
  • Hypertension (large IV doses for cyanide — transient)
  • Allergic reactions (rare)

Interactions

  • Chloramphenicol — may diminish haematopoietic response to B12
  • No significant pharmacokinetic interactions

Monitoring

  • FBC (Hb rise confirms response; reticulocyte peak at 5–7 days)
  • Serum B12 (before and at 3 months)
  • Serum potassium (48h after starting treatment — hypokalemia risk)
  • Neurological examination (improvement in B12 neuropathy)

Reference: BNFc; BNF 90; BSH Guidelines on Cobalamin and Folate Deficiency 2014; NICE CG126. Verify against your local formulary and the latest BNF before prescribing.

Related

Curated clinical cross-links plus same-class fallbacks.

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