cyanocobalamin
vitamin B12· Brands: Cytamen, Cyanoject, CyominIMPOSC
Mechanism of action
Cyanocobalamin is a form of vitamin B12 that is converted in the body to the active coenzymes methylcobalamin and adenosylcobalamin. These coenzymes are essential for DNA synthesis, red blood cell production, and neurological function. Methylcobalamin is required for the conversion of homocysteine to methionine, while adenosylcobalamin is needed for the conversion of methylmalonyl-CoA to succinyl-CoA. Deficiency leads to megaloblastic anaemia and neurological damage.
Pharmacokinetics
Cyanocobalamin is absorbed from the gastrointestinal tract after oral administration, but absorption is dependent on intrinsic factor. After parenteral administration, it binds to transcobalamin II and is transported to tissues. It is stored in the liver, and excess is excreted in bile and urine. The half-life is approximately six days.
Indications & dosing
Vitamin B12 deficiency (initial treatment)
For 1 week, then weekly for 4 weeks, then monthly.
- adult · IM: 1000 micrograms daily · For 1 week, then weekly for 4 weeks, then monthly.
Vitamin B12 deficiency (maintenance)
After initial treatment.
- adult · IM: 1000 micrograms monthly · After initial treatment.
Vitamin B12 deficiency (oral replacement)
For mild deficiency or maintenance.
- adult · PO: 1000–2000 micrograms daily · For mild deficiency or maintenance.
Prophylaxis of vitamin B12 deficiency in malabsorption
E.g., after gastrectomy, in pernicious anaemia.
- adult · IM: 1000 micrograms every 3 months · E.g., after gastrectomy, in pernicious anaemia.
Contraindications & precautions
- Hypersensitivity to cyanocobalamin or any component of the formulation, including cobalt · May cause anaphylaxis.
- Leber's hereditary optic neuropathy · May cause severe and irreversible optic nerve atrophy.
- Exclude folic acid deficiency in patients with megaloblastic anaemia, as cyanocobalamin may correct the blood picture while allowing neurological damage to progress.
- Monitor blood counts and reticulocyte count during therapy, especially in severe deficiency.
- Use caution in patients with cobalt hypersensitivity or a history of allergy to cobalamins.
- Avoid rapid intravenous administration; use intramuscular or subcutaneous routes as recommended.
Adverse effects
- Injection site pain, redness, and swelling
- Diarrhoea
- Nausea
- Headache
- Anaphylaxisserious
- Rash and pruritus
Monitoring, pregnancy & toxicity
- Serum vitamin B12 · Baseline and periodically during therapy
- Haemoglobin and red cell indices · Baseline and after 4-8 weeks
- Reticulocyte count · Initial response
- Serum potassium · When treating megaloblastic anaemia
- Serum ferritin · Baseline
- Pregnancy: Cyanocobalamin is safe in pregnancy at recommended doses. Vitamin B12 is essential for fetal development, and deficiency should be corrected. No teratogenic effects have been reported.
- Toxicity: Hypersensitivity reactions, including anaphylaxis, have been reported with cyanocobalamin injections. · Discontinue injection; provide supportive care and treat anaphylaxis per protocol.
Clinical notes
Cyanocobalamin is used to treat vitamin B12 deficiency, including pernicious anaemia. Oral administration is effective for mild deficiency in patients with intact intrinsic factor, while parenteral administration is preferred for severe deficiency, pernicious anaemia, or malabsorption. It is commonly given intramuscularly or subcutaneously. Note that hydroxocobalamin is more widely used in the UK for long-term therapy because it has a longer duration of action. The dose and route depend on the severity and cause of deficiency. Monitor serum vitamin B12 levels and clinical response.
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BNF; local formulary