Warfarin

How it works

It inhibits vitamin K epoxide reductase, depleting the reduced vitamin K needed to activate clotting factors II, VII, IX and X (and proteins C and S), thereby reducing the blood's capacity to clot.

Prescribing in practice

• Dosing must be titrated to the INR within a defined target range, as both over- and under-anticoagulation carry serious risk of major haemorrhage or thrombosis; check for the many drug, alcohol and dietary interactions before co-prescribing.
• Warfarin is teratogenic and is generally avoided in pregnancy, particularly the first trimester and near term, so confirm pregnancy status and provide contraceptive advice where relevant.
• Have a clear plan for reversal (vitamin K, prothrombin complex concentrate) and for peri-procedural bridging, and counsel on the higher bleeding risk with concurrent antiplatelets or NSAIDs.

Monitoring

Anticoagulation is monitored by regular INR testing, with the interval adjusted to stability and the dose modified to keep the INR within the indication-specific target range.

Counselling the patient

• Take it at the same time each day and attend all INR blood tests.
• Keep alcohol intake and vitamin K-rich foods consistent, and report any unusual bleeding, bruising or dark stools.
• Always tell pharmacists and prescribers you take warfarin before starting any new medicine, including over-the-counter or herbal products.

Evidence & guidelines

Warfarin's efficacy in stroke prevention and venous thromboembolism is supported by decades of trial evidence and remains endorsed by NICE, especially where direct oral anticoagulants are unsuitable such as mechanical valves.