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.
- 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.
- 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.
- 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.
- 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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