amiodarone and warfarin

Amiodarone inhibits the CYP2C9 and CYP3A4 isoenzymes, which are responsible for the metabolism of warfarin. This inhibition leads to increased plasma concentrations of warfarin and a consequent enhancement of its anticoagulant effect. Because amiodarone has a very long half-life, the interaction may persist for weeks or months after amiodarone is discontinued.

The anticoagulant effect of warfarin is significantly potentiated, with an increased risk of bleeding, including serious or life-threatening haemorrhage. The INternational Normalised Ratio (INR) can rise markedly and unpredictably. The onset of the interaction may be delayed, occurring days to weeks after starting amiodarone, and it can persist long after stopping amiodarone.

Avoid the combination if possible. If concurrent use is necessary, reduce the warfarin dose, typically by 30-50%, and monitor INR frequently (at least weekly) during the first few weeks of therapy and after any dose change. Continue to monitor INR for several weeks after amiodarone is stopped, as its effects are prolonged. Consider alternative antiarrhythmic agents where appropriate.

This interaction is well established through clinical observation and pharmacokinetic studies. It is widely documented in prescribing information and major drug interaction databases. The mechanism is supported by in vitro studies and clinical case reports.