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Vitamin K antagonist (anticoagulant) Pregnancy: Contraindicated in pregnancy - warfarin causes congenital malformations and foetal death when administered during pregnancy. Women of child-bearing age taking warfarin should use effective contraception during treatment. Can be used during breastfeeding (excreted in breast milk in small amounts; at therapeutic doses no effects on the breast-fed child are anticipated).

Warfarin sodium

Brand names: Coumadin, Marevan

Warfarin sodium is an oral vitamin K antagonist anticoagulant used for treatment and prevention of venous thromboembolism, stroke prevention in atrial fibrillation, and in patients with mechanical heart valves where a direct oral anticoagulant is not suitable.

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: typically 10 mg daily for 2 days, tailored to individual requirements. Maintenance: usually 3 to 9 mg daily
Route: Oral
Frequency: Once daily, at the same time each day
Perform a baseline coagulation screen and liver function tests before initiating. The exact maintenance dose depends on the prothrombin time (usually reported as INR) or other coagulation tests and must be adjusted to results; control tests at regular intervals. Once the maintenance dose is established it is rarely necessary to alter it. Elderly: as for adults but dosage may need to be lowered (generally more sensitive to warfarin, often require a smaller dose). In emergencies, initiate anticoagulation with heparin and warfarin together (discontinue heparin at least 6 hours before the first control test). Paediatric: dosage for children has not been established; the 1 mg/ml oral suspension is not recommended for use in children.

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.

Contraindications

  • Hypersensitivity to the active substance or excipients
  • Haemorrhagic stroke
  • Clinically significant bleeding
  • Use within 72 hours of surgery with risk of severe bleeding
  • Use within 48 hours postpartum
  • Pregnancy
  • Drugs where interactions lead to a significantly increased risk of bleeding
  • Any physical condition in which the risk of haemorrhage might be greater than the potential clinical benefits of anticoagulation

Side effects

  • Haemorrhage (most frequently reported adverse effect of oral anticoagulants; includes cerebral, gastrointestinal, epistaxis)
  • Skin necrosis (rare; mainly obese female patients, usually within 3-10 days of starting, associated with high induction doses)
  • 'Purple toes' syndrome
  • Rash, alopecia, purpura
  • Nausea, vomiting, diarrhoea
  • Jaundice and hepatic dysfunction

Interactions

  • Drugs that increase bleeding risk (e.g. NSAIDs)
  • Antibiotics
  • Antifungals
  • Botanical (herbal) products
  • Inhibitors and inducers of CYP2C9, 1A2 or 3A4 (perform more frequent INR monitoring when starting/stopping or changing doses of other drugs)

Clinical monograph

How it works

It inhibits vitamin K epoxide reductase, reducing regeneration of active vitamin K and impairing hepatic synthesis of clotting factors II, VII, IX and X and proteins C and S.

Prescribing in practice

  • It has a narrow therapeutic index requiring regular INR monitoring and dose titration; it is teratogenic and contraindicated in pregnancy except in specific high-risk valve situations under specialist care.
  • Numerous drug, food and alcohol interactions and intercurrent illness alter the INR, so review concurrent therapy and changes in diet or health.
  • There is an initial transient procoagulant effect from protein C depletion, so bridging anticoagulation is needed when starting in acute thrombosis.

Monitoring

Monitor the INR regularly against the target range for the indication, with more frequent checks after dose changes, new medicines or illness.

Counselling the patient

  • Take the dose at the same time each day, attend all INR appointments, and carry your anticoagulant alert card.
  • Report unusual bruising or bleeding, and tell any prescriber, dentist or pharmacist that you take warfarin.
  • Keep alcohol intake and vitamin K-rich diet consistent and avoid starting new medicines without advice.

Evidence & guidelines

Warfarin with INR-guided dosing remains the standard oral anticoagulant where vitamin K antagonism is indicated, including mechanical heart valves, per NICE guidance.

Reference: NICE NG185; BSH; 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).

Related

Curated clinical cross-links plus same-class fallbacks.

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