Warfarin sodium

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.