Warfarin is a vitamin K antagonist (VKA) anticoagulant that reduces the body’s ability to form blood clots. It works by inhibiting vitamin K epoxide reductase (VKORC1), an enzyme required for recycling vitamin K in the liver. Without active vitamin K, the liver cannot effectively produce clotting factors II, VII, IX, and X, or the anticoagulant proteins C and S. As a result, warfarin thins the blood and lowers the risk of harmful clots forming in veins or arteries. Unlike newer direct oral anticoagulants (DOACs), warfarin has a variable dose–response relationship influenced by genetics, diet, and interactions. This makes frequent blood testing (INR monitoring) essential to maintain therapeutic safety.
Warfarin has been used for decades worldwide and is highly effective for preventing strokes in atrial fibrillation, treating venous thromboembolism, and protecting patients with mechanical heart valves. It remains the anticoagulant of choice in certain populations (e.g., patients with severe kidney disease or mechanical prosthetic valves) despite the increasing use of DOACs like apixaban or rivaroxaban. The key to safe warfarin therapy is careful balance: too little drug and patients risk clots; too much and patients risk dangerous bleeding. Patients must be counselled to attend INR checks, maintain a consistent diet, and be aware of drug and food interactions.
Therapeutic range is usually INR 2.0–3.0 for most indications, though higher ranges (2.5–3.5) may be required in mechanical valves. Warfarin is taken orally, once daily, and is available in multiple tablet strengths to allow fine adjustment. Dose adjustments are always guided by INR blood tests and clinical judgment. Education, adherence, and partnership with healthcare providers are critical for safe outcomes.
Warfarin is prescribed for prevention and treatment of blood clots in many clinical settings:
Dosing is highly individualised. Patients usually start on a small daily dose (e.g., 2–5 mg), then adjust according to INR results. Frequent testing is needed at initiation (every few days), then intervals can extend to every 4–12 weeks once stable. Dose changes must never be made outside medical supervision. Patients are often provided with “warfarin booklets” or apps to record INR values, dose changes, and counselling notes. Consistent diet (especially vitamin K intake from leafy greens), alcohol moderation, and adherence are vital to keeping INR within range. Patients must be instructed to never double-dose if they miss a tablet.
The major risk of warfarin is bleeding, but many patients tolerate therapy well when INR is controlled. Side effects vary from mild to life-threatening.
Any signs of uncontrolled bleeding—vomiting blood, black/tarry stools, sudden severe headache, dizziness, vision changes, coughing blood—require emergency evaluation. Warfarin patients should carry alert cards or bracelets and inform healthcare providers about their therapy before any procedures or new prescriptions.
Patients should be educated to recognise symptoms of both bleeding and clotting (e.g., new swelling/pain in a leg, sudden chest pain, or neurological symptoms suggesting stroke). Warfarin therapy requires lifelong commitment to monitoring and safe use practices.
Warfarin is subject to numerous clinically significant interactions:
Patients should always tell healthcare providers and pharmacists they are on warfarin, check before starting any new medicine, and avoid abrupt dietary changes (especially leafy green vegetables or vitamin K-fortified foods).
Overdose: Presents with excessive bleeding, high INR, bruising, or haemorrhage. Management depends on severity: stop warfarin, administer vitamin K (oral or IV), use prothrombin complex concentrate or fresh frozen plasma in emergencies, and admit for monitoring.
Missed dose: If remembered the same day, take it promptly; if not until the next day, skip and resume the usual schedule. Never double-dose. Document the missed dose and inform the clinician at next INR review.
Accidental double dose: Do not panic; contact the anticoagulation clinic. INR testing may be needed to check effect. Patients should monitor for unusual bruising, nosebleeds, or gum bleeding.
Education: Emphasise consistent daily intake, the importance of INR testing, and clear instructions on what to do if bleeding or clotting symptoms arise. Provide written plans.
Warfarin tablets are colour-coded by strength in many countries (e.g., UK, US) to reduce errors; patients should confirm their dose each time to avoid mistakes.
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