Hematinics · Hematology
Cyanocobalamin (vitamin B12)
vitamin B12 · hydroxocobalamin
Replace it before folate, or the blood improves while the spinal cord does not.
Also: injection, oral liquid
Mechanism
A cofactor for methionine synthase and methylmalonyl-CoA mutase: without it, DNA synthesis fails in the marrow and myelin maintenance fails in the nervous system.
Indications
- B12 deficiency from any cause
- Pernicious anemia
- Post-gastrectomy or ileal resection
- Long-term metformin or proton pump inhibitor use
Formulations
Dosing concepts
Oral versus injection
High-dose oral replacement works even in pernicious anemia, because roughly one percent is absorbed by passive diffusion independent of intrinsic factor. Injections remain standard where absorption or adherence is in doubt.
Neurological disease
Neurological involvement is treated more intensively and for longer, and recovery is incomplete if treatment is delayed.
Concepts, not prescribing instructions. Practice numbers live in current references and local protocols.
Pharmacokinetics
- Bioavailability
- Intrinsic-factor-dependent at physiological doses; about 1% passive at high doses
- Half-life
- Body stores last years
- Elimination
- renal: Excess is excreted in urine; toxicity is essentially unknown
Adverse effects
- Injection-site reactioncommon
- Hypokalemia during rapid correctionseriousA brisk marrow response consumes potassium; watch it in severe anemia.
- Acne-like eruptioncommon
Contraindications & precautions
Contraindications
- Leber hereditary optic neuropathy (cyanocobalamin form)
Precautions
- Severe deficiency, where potassium falls during correction
- Coexisting folate deficiency
Interactions
No curated interaction entries yet for this agent.
Monitoring
Reticulocyte response at about a week
Confirms the diagnosis as much as the treatment
Potassium during correction of severe anemia
It falls as the marrow consumes it
Neurological symptoms
Recovery is slower and less complete than the blood count's
The cause
Pernicious anemia needs lifelong treatment; dietary deficiency does not
Counselling points
- If your deficiency is from pernicious anemia or surgery, this is lifelong.
- Tell us about numbness, tingling or unsteadiness; nerve symptoms need more intensive treatment.
- Take oral doses on an empty stomach.
Clinical pearls
- Giving folate to a B12-deficient patient corrects the anemia while subacute combined degeneration of the cord progresses. Check B12 first, always.
- Metformin and long-term proton pump inhibitors both reduce B12 absorption, which makes them a common and checkable cause in an older patient with neuropathy.
References & review
- Cyanocobalamin product monograph (consult the current version for your market) (monograph)
- Goodman & Gilman's The Pharmacological Basis of Therapeutics, 14th ed. (reference-work)
Last reviewed 2026-08-16 · jurisdiction: global · drug information changes; verify against current references before practice use.