DurgicalDurgical
EN
Open the app

Pharmacology · Antianaemia agents (iron, folic acid, vitamin B12)

Cyanocobalamin !

The common form of vitamin B12 used to treat and prevent B12 deficiency, including pernicious anaemia, by injection, nasal or oral route.

Updated 2026-11-01

Also called: vitamin B12

The figure shows human intrinsic factor holding cobalamin, the complex the ileum absorbs; in pernicious anaemia this factor is missing, so oral B12 is unreliable.

Target in 3D: Gastric intrinsic factor (absorption) and the B12-dependent enzymes. PDB structure 2PMV (RCSB PDB, public domain). View the structure at RCSB PDB

Mechanism of action

Cyanocobalamin is converted in the body to the two active coenzymes. Methylcobalamin lets methionine synthase regenerate tetrahydrofolate, so DNA synthesis resumes; adenosylcobalamin lets methylmalonyl-CoA mutase work, which matters for myelin. Dietary B12 needs gastric intrinsic factor to be absorbed in the ileum.

  1. Cobalamin is delivered to cells bound to transcobalamin
  2. Methylcobalamin lets methionine synthase recycle folate
  3. Adenosylcobalamin turns methylmalonyl-CoA into succinyl-CoA
  4. DNA synthesis and myelin recover

Pharmacokinetics

Absorption
Orally, needs intrinsic factor (ileum); injected intramuscularly or deep subcutaneously
Distribution
Stored in the liver
Elimination
Much of an injected dose appears in urine, more so if given intravenously

Indications

Adverse effects

Common

  • Diarrhoea
  • Itching, rash
  • Feeling of swelling of the whole body

Contraindications

  • Hypersensitivity to cobalamins
  • Early Leber's hereditary optic neuropathy (rapid optic atrophy reported)

Key interactions

Folic acid alone
Can correct the anaemia of B12 deficiency while nerve damage progresses; it is not a substitute
Marrow-suppressing drugs (e.g. chloramphenicol)
Blunt the response to vitamin B12

What is monitored

  • Reticulocyte count (rises within days if the diagnosis is right), haemoglobin and MCV
  • Potassium and platelets early in treatment of severe megaloblastic anaemia
  • Neurological symptoms over time

Administration and safety

  • Intramuscular or deep subcutaneous injection; the intravenous route is avoided because most is lost in urine
  • Nasal and oral forms are used for maintenance in selected patients

Nursing care

  • Check potassium in the first days of treating severe anaemia
  • Explain that pernicious anaemia needs treatment for life
  • Ask about numbness, unsteady gait and memory problems over time

Pregnancy and breastfeeding

B12 needs rise in pregnancy and breastfeeding; deficiency, common with strict vegan diets, also affects the breast-fed infant.

Test yourself

Think of the answer, then tap the question to see it.

Why is oral B12 unreliable in pernicious anaemia?

Intrinsic factor, needed for ileal absorption, is missing.

Which electrolyte can fall dangerously early in treatment?

Potassium.

Which two enzymes need vitamin B12?

Methionine synthase and methylmalonyl-CoA mutase.

Add to my cards (3)

← Back to the class: Antianaemia agents (iron, folic acid, vitamin B12)

Educational content on how drugs work; not a prescribing guide and no substitute for a health professional's advice. No doses are given.

Sources: DailyMed - CYANOCOBALAMIN injection, solution (DailyMed (U.S. National Library of Medicine), 2026); Cyanocobalamin Injection: MedlinePlus Drug Information (MedlinePlus (U.S. National Library of Medicine), 2026); Goodman & Gilman's The Pharmacological Basis of Therapeutics, 14th ed., ch. 45 (McGraw Hill, 2023); ATCDDD - ATC/DDD Index (WHO Collaborating Centre for Drug Statistics Methodology, 2026).

Spotted a mistake?We are always improving for the community. Tell us what is wrong and our team will review it.Report a mistake

Representative model, for educational purposes only.