Warfarin Institute of America
DEDICATED TO YOUR HEALTH SINCE 2000
AMIODARONE INTERACTIONS WITH WARFARIN (Coumadin, Jantoven)
Amiodarone and warfarin (Coumadin, Jantoven) are both prescribed for a condition called atrial fibrillation. When this irregular rhythm of the heart occurs, the risk of stroke is greatly increased. warfarin (Coumadin, Jantoven) cuts this risk down. Physicians sometimes choose amiodarone to bring the heart back into normal rhythm. Therefore, this interaction is common. It is also one of the most significant of all warfarin (Coumadin, Jantoven) interactions. DO NOT UNDERESTIMATE THE SERIOUSNESS OF THIS INTERACTION.
Amiodarone has what is called a long-half life. This means that it is slow to start acting and it is extremely slow to be cleared from the body. This is what makes management of this interaction very difficult.
How soon the interaction starts to show itself in the INR test seems to depend upon whether or not a loading dose was given. A loading dose could be given IV or orally. Any IV dose is a loading dose. An oral loading dose is usually 200 mg of amiodarone twice a day. The amiodarone dose is then usually decreased to 200 mg every day. If a loading dose was given, the INR will usually start to climb after about one week. If no loading dose was given, it may take two weeks for the INR to start to rise. Several downward adjustments of the warfarin (Coumadin, Jantoven) dose may have to be made before this interaction is stabilized. If you are taking warfarin (Coumadin, Jantoven) and started on amiodarone, it is very important that your INR be frequently monitored. The interaction may stabilize in about one month. The dose of warfarin (Coumadin, Jantoven) needed the may be 30% to 50% less than what was needed before the amiodarone was started.
The really tricky part of managing the warfarin (Coumadin, Jantoven) - amiodarone interaction is when amiodarone is discontinued but warfarin (Coumadin, Jantoven) is continued. Because of its long half-life, amiodarone can take up to six months after the last dose is taken to be cleared from the body. This is so long that it is difficult to remember the amiodarone and realize what is happening. Typically a warfarin (Coumadin, Jantoven) dose increase needs to be made about three months after the amiodarone is stopped. Further warfarin (Coumadin, Jantoven) dose increases may need to be made up to about six months after the amiodarone is stopped.
Amiodarone may also affect the thyroid since it contains a lot of iodine. The thyroid also plays a part in warfarin (Coumadin, Jantoven) regulation, so this may also affect the amount of warfarin (Coumadin, Jantoven) which is needed. For a discussion of the effects of amiodarone on the thyroid click here.
SEE ALSO THE THYROID THEORY OF WHY warfarin (Coumadin, Jantoven) IS HARD TO MANAGE
References:
Mancano M. Drug Interactions with Warfarin: Part 3. Pharmacy Times July 2000 pages 22 - 24.
Gage BF et al. Use of pharmacogenetics and clinical factors to predict the maintenance dose of warfarin. Thromb Hemost. 2004;9:87-94.
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Last modified March 25, 2006
