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Vitamin K Antagonist (Anticoagulant)

Warfarin

Brand names: Coumadin, Marevan

Warfarin is a vitamin-K-antagonist oral anticoagulant for atrial fibrillation, venous thromboembolism, mechanical heart valves and other indications, dose-adjusted to the INR.

Dosing — being independently re-sourced

ClinCalc Pro is rebuilding its dose data from primary open sources — the manufacturer SmPC (eMC), the WHO Model Formulary and other official references — under clinician review. This drug's structured dose is not yet published here. Confirm all doses against the product SmPC and your local formulary before prescribing.

US labelling (FDA)

Reference — US labelling, may differ from UK

Individualize dosing regimen for each patient, and adjust based on INR response. ( 2.1 , 2.2 ) Knowledge of genotype can inform initial dose selection. ( 2.3 ) Monitoring: Obtain daily INR determinations upon initiation until stable in the therapeutic range. Obtain subsequent INR determinations every 1 to 4 weeks. ( 2.4 ) Review conversion instructions from other anticoagulants. ( 2.8 ) 2.1 Individualized Dosing The dosage and administration of warfarin sodium tablets must be individualized for each patient according to the patient’s International Normalized Ratio (INR) response to the drug. Adjust the dose based on the patient’s INR and the condition being treated. Consult the latest …

Source: US FDA prescribing information (openFDA / DailyMed), label dated 2025-06-17. Accessed 2026-06-12. US dosing and indications can differ from UK practice — use UK sources for prescribing decisions.

Clinical monograph

How it works

It inhibits vitamin K epoxide reductase, reducing synthesis of the vitamin-K-dependent clotting factors (II, VII, IX, X) and proteins C and S.

Prescribing in practice

  • It has a narrow therapeutic index requiring regular INR monitoring and dose adjustment, with a slow onset/offset that sometimes needs heparin 'bridging'.
  • It has many food and drug interactions (antibiotics, amiodarone, miconazole and others) that shift the INR — review every new medicine.
  • Bleeding is the main risk; it is reversed with vitamin K (and prothrombin complex concentrate for major bleeding). It is teratogenic — avoid in pregnancy other than specific specialist mechanical-valve situations.

Monitoring

Monitor the INR against the target range for the indication, watch for bleeding, and review interacting drugs and diet.

Counselling the patient

  • Attend your INR checks and take the dose exactly as directed.
  • Keep your vitamin-K intake (green vegetables) steady rather than avoiding it; tell any clinician or pharmacist you take warfarin before starting a new medicine.
  • Report unusual bruising or bleeding, black stools or red/brown urine; carry your anticoagulant alert card.

Evidence & guidelines

Established for stroke prevention in AF and for VTE/valves (NICE NG196 and others), individualised to the INR.

Reference: NICE NG196 (Atrial Fibrillation); MHRA Warfarin Safety Update; Drug verified in RxNorm (NLM); confirm dosing against the manufacturer SPC (eMC). Verify against your local formulary and current prescribing references before prescribing. Monograph status: clinician-reviewed (2026-07-04).

Related

Curated clinical cross-links plus same-class fallbacks.

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