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Vitamin B12 — Megaloblastic Anaemia Pregnancy: §4.6: hydroxocobalamin injection should NOT be used for the treatment of megaloblastic anaemia of pregnancy. Breast-feeding — hydroxocobalamin is secreted into breast milk but is unlikely to harm the infant. Fertility — no data available.

Hydroxocobalamin

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

Used in: Anaemia

Hydroxocobalamin is a form of vitamin B12 given by intramuscular injection to treat and prevent vitamin B12 deficiency, including pernicious anaemia and deficiency due to malabsorption.

Adult dose

Dose: Vitamin B12 deficiency / megaloblastic anaemia — this page's indication, and a licensed indication of this product (§4.1: 'Treatment of Addisonian pernicious anaemia. Prophylaxis and treatment of other macrocytic anaemias due to vitamin B12 deficiency.'). WITHOUT neurological involvement — initially 250 micrograms to 1000 micrograms intramuscularly on alternate days for one or two weeks, then 250 micrograms weekly until the blood count is normal; maintenance 1000 micrograms every two or three months. WITH neurological involvement — initially 1000 micrograms on alternate days as long as improvement continues; maintenance 1000 micrograms every two months. PROPHYLAXIS of macrocytic anaemia associated with B12 deficiency resulting from gastrectomy, ileal resection, certain malabsorption states and vegetarianism — 1000 micrograms every two or three months.
Route: Intramuscular injection (§4.2, 'Method of Administration: Intramuscular injection').
Frequency: Alternate days initially, then weekly, then every two or three months for maintenance — see the dose above for which schedule belongs to which regimen.
§4.2 states no maximum dose for this indication, so none is given here; the largest single amount it prints is 1000 micrograms per injection, which it does not describe as a ceiling. SCOPE: this record covers the B12-deficiency / megaloblastic anaemia indication only. The Cyanokit cyanide-antidote product also named on this page is a different product and is NOT covered by this source — no cyanide-poisoning dose is given here. §4.1 also licenses this product for tobacco amblyopia and Leber's optic atrophy; those are different indications and their regimens are deliberately not carried onto this haematology page. §4.4: confirm the diagnosis of vitamin B12 deficiency before giving hydroxocobalamin; if megaloblastic anaemia fails to respond, investigate folate metabolism. Doses in excess of 10 micrograms daily may produce a haematological response in patients with folate deficiency, and indiscriminate administration may mask the true diagnosis.

Paediatric dose

Dose: No separate paediatric figure is printed. §4.2 opens 'The following dosages are suitable for children and adults' — the regimens above apply to children unchanged. Nothing has been scaled or derived.
Route: Intramuscular injection
Frequency: As for the adult regimens above
Verify against a children's formulary and specialist paediatric haematology guidance before use.

Dose adjustments

Renal

No renal dose adjustment is stated in this SPC.

Hepatic

No hepatic dose adjustment is stated in this SPC.

Dose independently re-sourced from UK Summary of Product Characteristics (SPC) via the eMC — clinician-reviewed. Always confirm against the product SmPC and your local formulary before prescribing.

Contraindications

  • Hypersensitivity to the active substance or to any of the excipients listed in section 6.1

Side effects

  • Reactive thrombocytosis can occur during the first weeks of use in megaloblastic anaemia
  • Initial hypokalaemia — and cardiac arrhythmias secondary to hypokalaemia
  • Hypersensitivity reactions including rash, itching and exanthema; anaphylaxis
  • Antibodies to hydroxocobalamin-transcobalamin II complex have developed during therapy
  • Headache, paraesthesia, tremor; dizziness
  • Nausea, vomiting, diarrhoea
  • Injection site reactions — pain, erythema, pruritus, induration and swelling
  • Fever, chills, hot flushes, malaise, pain
  • Acneiform and bullous eruptions
  • Chromaturia (discoloured urine)
  • All frequencies in §4.8 are classed 'Not known' — the SPC states there are no modern clinical studies available to determine the frequency of undesirable effects

Interactions

  • Parenteral chloramphenicol may attenuate the effect of hydroxocobalamin in anaemia
  • Oral contraceptives — the serum concentration of hydroxocobalamin may be lowered
  • §4.5 notes the above interactions are unlikely to be of clinical significance but should be taken into account when performing assays for blood concentrations
  • Vitamin B12 assays by microbiological techniques are invalidated by antimetabolites and most antibiotics

Monitoring

  • Confirm the diagnosis of vitamin B12 deficiency before giving hydroxocobalamin — indiscriminate administration may mask the true diagnosis
  • Regular monitoring of the blood; in the non-neurological regimen, 250 micrograms weekly is continued until the blood count is normal
  • Plasma potassium during initial therapy — cardiac arrhythmias secondary to hypokalaemia have been reported during this period
  • If megaloblastic anaemia fails to respond, investigate folate metabolism

Clinical monograph

How it works

It is converted to the active coenzymes methylcobalamin and adenosylcobalamin, which are required for methionine synthesis and methylmalonyl-CoA metabolism, supporting DNA synthesis, normal erythropoiesis and myelin integrity.

Prescribing in practice

  • In severe deficiency, starting treatment can precipitate hypokalaemia as erythropoiesis resumes, so potassium should be considered during the initial response.
  • Hydroxocobalamin is preferred over cyanocobalamin in the UK because its longer retention allows less frequent maintenance injections.
  • Neurological involvement requires prompt and adequate replacement, and the cause of deficiency should be established as parenteral therapy is usually lifelong in malabsorption.

Monitoring

Monitor the haematological response and, in severe cases, serum potassium during early treatment, with periodic review of ongoing maintenance need.

Counselling the patient

  • Vitamin B12 is given as a course of injections and then usually maintained for life when due to absorption problems.
  • Tell your clinician about any numbness, tingling or balance problems.
  • Keep maintenance injection appointments to prevent the deficiency returning.

Evidence & guidelines

Hydroxocobalamin is the standard UK parenteral vitamin B12 preparation for pernicious anaemia and B12-deficiency states, reflecting its favourable retention compared with cyanocobalamin.

Reference: BCSH Guidelines B12 and Folate Deficiency (2014); BSH (2021); Drug verified in RxNorm (NLM); confirm dosing against the manufacturer SPC (eMC). Verify against your local formulary and current prescribing references before prescribing. The structured dose values shown have been reviewed by a clinician. Monograph status: clinician-reviewed (2026-07-04).

Related

Curated clinical cross-links plus same-class fallbacks.

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