Vitamin B12 — Megaloblastic Anaemia Pregnancy: Safe — essential nutrient; B12 deficiency in pregnancy associated with neural tube defects; IM supplementation as per standard dosing

Hydroxocobalamin

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

Adult dose

Dose: Vitamin B12 deficiency: 1 mg IM on alternate days for 2 weeks, then 1 mg every 3 months (maintenance). Pernicious anaemia: 1 mg IM every 3 months lifelong. Cyanide poisoning (Cyanokit): 5 g IV over 15 min

Route: IM (B12 deficiency) / IV (cyanide poisoning — Cyanokit formulation only)

Frequency: See dose notes

Max: 1 mg per injection (haematological); 15 g (3 doses of 5 g) for cyanide poisoning

Preferred over cyanocobalamin for maintenance therapy — longer plasma half-life. If neurological involvement (subacute combined degeneration): 1 mg IM on alternate days for 3 weeks, then every 2–3 months.

Paediatric dose

Dose: Seek specialist opinion mg/kg

Route: IM

Frequency: As per specialist guidance

Max: 1 mg per injection

BNFc: neonates and children — 250 micrograms to 1 mg IM initially, frequency determined by severity and response. Specialist paediatric haematology guidance required.

Dose adjustments

Renal

No dose adjustment required

Hepatic

No dose adjustment required

Paediatric weight-based calculator

Patient weight (kg)

BNFc: neonates and children — 250 micrograms to 1 mg IM initially, frequency determined by severity and response. Specialist paediatric haematology guidance required.

Clinical pearls

Subacute combined degeneration of the cord: B12 deficiency causing posterior and lateral column demyelination — commence B12 IM immediately; do not wait for serum levels
Pernicious anaemia: autoimmune gastric parietal cell destruction — lifelong IM therapy required; oral supplementation inadequate
Hypokalaemia risk on initiation of B12 therapy — rapidly expanding haematopoiesis demands potassium; monitor and supplement
Do not delay treatment for neurological B12 deficiency pending investigation results — irreversible cord damage can occur
Cyanokit (5 g IV) is first-line for confirmed/suspected cyanide poisoning from smoke inhalation — see emergency.json for full detail
Dietary B12 deficiency (vegans): oral cyanocobalamin 50–150 mcg daily or 1000 mcg weekly may suffice if malabsorption excluded

Contraindications

Cobalt hypersensitivity

Side effects

Injection site reactions
Hypokalaemia (initial treatment — increased haematopoiesis consumes potassium)
Acneiform rash (prolonged use)
Red/brown urine discolouration (harmless — expected with Cyanokit)
Hypertension (Cyanokit — transient)

Interactions

Chloramphenicol — impairs haematopoietic response to B12

Monitoring

FBC (at 8 weeks — reticulocyte response expected within days)
Serum B12
Potassium (first 48 hours)
Neurological examination

Reference: BNFc; BNF 90; BNFc; BCSH Guidelines B12 and Folate Deficiency (2014); BSH (2021). Verify against your local formulary and the latest BNF before prescribing.

Curated clinical cross-links plus same-class fallbacks.

Calculators

Pathways