Vitamin K Antagonist (Anticoagulant) Pregnancy: Contraindicated in the first and third trimesters (warfarin embryopathy / 'foetal warfarin syndrome'; risk of fetal bleeding and stillbirth). Can be used during breast-feeding.

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.

Auto-extracted from the source labelling — not yet independently clinician-verified. These values were distilled from the UK SPC (or the US label where noted) but have not had a clinician sign-off. Confirm against the current SmPC before prescribing.

Adult dose

Dose: Induction 10 mg once daily for 2 days; maintenance usually 3–9 mg once daily

Route: Oral

Frequency: Once daily, at the same time each day

The induction dose should be tailored to individual requirements. The exact maintenance dose depends on the prothrombin time / INR and must be adjusted to coagulation-test results; once established it rarely needs altering. Make control (INR) tests at regular intervals. In emergencies, initiate anticoagulation with heparin and warfarin together (heparin affects control tests and should be discontinued at least 6 hours before the first test). In protein C deficiency, introduce therapy without a loading dose. Elderly: as for adults, but the dose may need to be lowered. Paediatric population: no data available.

Dose auto-extracted from UK Summary of Product Characteristics (SPC) via the eMC; US FDA prescribing information (openFDA / DailyMed) — cross-check; US labelling may differ from UK — not yet clinician-verified. Always confirm 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.

Contraindications

Hypersensitivity to warfarin or any excipient
Haemorrhagic stroke
Clinically significant bleeding
Within 72 hours of major surgery with risk of severe bleeding
Within 48 hours postpartum
Pregnancy (first and third trimesters)
Concomitant drugs where interactions may significantly increase the risk of bleeding
Any condition in which the risk of haemorrhage outweighs the clinical benefit of anticoagulation

Side effects

Haemorrhage (including gastrointestinal, cerebral, epistaxis, haematuria)
Skin necrosis and 'purple toes' syndrome
Alopecia, rash, purpura
Nausea, vomiting, diarrhoea
Jaundice / hepatic dysfunction
Hypersensitivity

Interactions

Miconazole — enhances anticoagulant effect; monitor and titrate carefully
NSAIDs — increased risk of bleeding
Antiplatelet drugs — increased risk of bleeding

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. The structured dose values shown have been reviewed by a clinician. Monograph status: clinician-reviewed (2026-07-04).

Curated clinical cross-links plus same-class fallbacks.

Calculators

Drugs

Warfarin sodium · Vitamin K antagonist (anticoagulant)

Pathways