Warfarin

How it works

It inhibits vitamin K epoxide reductase, reducing synthesis of vitamin K-dependent clotting factors II, VII, IX and X and the anticoagulant proteins C and S.

Prescribing in practice

• Older people have a higher bleeding risk and greater sensitivity, so initiate at lower doses with careful INR monitoring; assess bleeding versus thrombotic risk and consider falls and cognitive ability to manage therapy.
• It has a narrow therapeutic range and numerous drug, food and alcohol interactions (including many antibiotics and changes in dietary vitamin K), all of which can destabilise the INR.
• It is contraindicated in pregnancy and requires prompt action with bridging or reversal around bleeding, surgery or supratherapeutic INRs.

Monitoring

Monitor the INR regularly against the target range, more frequently after starting, dose changes or any interacting medicine, especially in older patients.

Counselling the patient

• Attend all INR appointments and keep dietary vitamin K intake consistent.
• Report unusual bleeding or bruising, and tell any clinician you take an anticoagulant.
• Carry an anticoagulant alert card and check before starting new medicines.

Evidence & guidelines

Trials such as BAFTA showed warfarin reduces stroke in older patients with atrial fibrillation, and NICE supports anticoagulation with careful monitoring in this group.