Amiodarone and Warfarin Interaction

JT is a 61 year old male.  He has recently been diagnosed with atrial fibrillation and placed on amiodarone.  He is also on warfarin with a history of DVT/PE.  The amiodarone and warfarin interaction can be a little more challenging than other drug interactions.  What really makes this unique is the long duration of action and half life of amiodarone.

Here’s some stats on amiodarone from Lexicomp:

  • Onset: 2 days to 3 weeks
  • Peak effect: 1 week to 5 months
  • Half-life: average 58 days

So here’s some things to consider when identifying a patient on amiodarone and warfarin.

  1. How long have they been on these agents?  This can help you determine if an interaction remains relevant or is already accounted for in INR monitoring.
  2. How often is reasonable to check INR when first initiating the amiodarone.  Patient factors are going to play a role in this question.  Due to the unique pharmacokinetics of amiodarone, this interaction can take up to weeks to ramp up.  Checking a couple of times per week or once a week would potentially be reasonable until we feel the patient has stabilized.
  3. Is the patient a candidate for a newer oral anticoagulant that might not have this concern?  One way to manage an interaction is to avoid a medication.  This isn’t always possible, but should always be thought about.
  4. How long are we going to more closely monitor the amiodarone and warfarin interaction?  If we’ve done weekly checks for 2, 3, 4 weeks with no change in an at goal INR, most clinicians will go back to routine monthly monitoring.

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Written By Eric Christianson

August 10, 2016

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