Major
Rifampicin × Warfarin
Antituberculous antibiotic×Vitamin K antagonist anticoagulants
Mechanism
Rifampicin strongly induces CYP2C9, the main S-warfarin clearance route. Warfarin plasma levels fall by 50–70% over 2–3 weeks; anticoagulant effect weakens.
Symptoms
INR drops below target. Clinically: increased thromboembolic risk – deep vein thrombosis, pulmonary embolism, ischaemic stroke.
Management
For short rifampicin courses (up to 2 weeks), check INR every 3–5 days and titrate warfarin to effect. For prolonged anti-TB therapy, warfarin dose typically rises 2- to 3-fold. After stopping rifampicin, induction persists for another 2 weeks – return warfarin to baseline dose gradually.