2 Warfarin Therapy

How warfarin works

Warfarin is an oral vitamin K antagonist used to prevent and treat thromboembolism. It inhibits vitamin K epoxide reductase, reducing activation of vitamin K-dependent clotting factors II, VII, IX, and X. It also reduces the anticoagulant proteins C and S.

The existing circulating clotting factors are not removed immediately. Consequently, warfarin’s full antithrombotic effect develops over several days.

INR goals and monitoring

Warfarin dose adjustment is guided by the prothrombin time expressed as the international normalized ratio (INR). An INR range of 2–3 is common for venous thromboembolism and atrial fibrillation, but this is not a universal target; some mechanical heart valves require a different target.

INR monitoring is more frequent when treatment starts and when relevant medicines, diet, or health status change.

Individualized dosing

Warfarin dosing is individualized according to INR response and the condition being treated. Age, body weight, other medicines, comorbidities, and genetic variation in CYP2C9 or VKORC1 can affect dose needs. Older adults may be more sensitive to warfarin and often need lower doses.

Patients should not adjust their own dose. If a dose is missed, they should not make it up by doubling the next day.

Interactions and diet

Medicines, diet, illness, and other factors can change warfarin’s effect. Antibiotics and other medicines may alter the INR or bleeding risk, and vitamin K-rich foods can affect response.

When a patient on stable warfarin starts an antibiotic or makes a sharp increase in leafy-green intake, they should promptly contact their anticoagulation clinician. The clinician may arrange closer INR monitoring and determine whether a dose change is needed. The general dietary goal is consistent vitamin K intake—not avoiding nutritious foods or making sudden, large dietary changes.

Pregnancy-related risks

Warfarin can harm the fetus, including through congenital malformations and fetal bleeding, and is generally contraindicated during pregnancy. For a pregnant patient with a mechanical heart valve, stopping warfarin may also pose serious risks. Treatment decisions in this situation require individualized discussion with specialists.

Excess anticoagulation and bleeding

Management of excessive anticoagulation depends on the INR, whether clinically relevant bleeding is present, and the patient’s circumstances. For an INR above 4.5 but below 10 without clinically relevant bleeding, guidance suggests temporarily withholding warfarin rather than routinely adding vitamin K. A clinician should assess bleeding and clotting risks and arrange follow-up INR testing; management may differ in other clinical circumstances.

For major, life-threatening warfarin-associated bleeding, the general approach is to stop warfarin and urgently manage the bleeding. Guidance supports intravenous vitamin K plus four-factor prothrombin complex concentrate (4F-PCC); plasma may be used if 4F-PCC is unavailable. Reversal is emergency treatment directed by clinicians.