Description

Tamoxifen dramatically increases warfarin anticoagulation, causing large INR elevations and a substantially elevated risk of serious bleeding.

Mechanism

Tamoxifen and its metabolites inhibit CYP2C9, markedly reducing S-warfarin clearance; the effect on CYP3A4 further reduces R-warfarin clearance.

Clinical Significance

S-warfarin AUC may increase by >100%; major hemorrhage including intracranial bleeding has been documented. This is one of the most potent warfarin interactions.

Management

Reduce warfarin dose by 30–50% when initiating tamoxifen; monitor INR very frequently (every 3–7 days) until stable. Consider alternative anticoagulants.

Medical Disclaimer

This content is for educational and informational purposes only. It is not a substitute for professional medical advice, diagnosis, or treatment. Always consult a qualified healthcare provider before making medication decisions.

Data sources: ChEMBL, PubChem, DailyMed.