Warfarin Institute of America

DEDICATED TO YOUR HEALTH SINCE 2000

THAT'S A GOOD QUESTION

My father is taking Coumadin.  His target INR is 2.0-3.0.  My question is why do they have him taking 2.5mg on 3 days of the week (Mon, Wed, Sat) and 5mg on the other days instead of just having him split his 2.5mg pills and take 1 ½ pills (3.75mg) every day?  It seems to me that taking the same dose each day would result in a much more steady concentration in the blood.

Coumadin® (warfarin) is a drug that has what we call a “long half-life.”  This means that it takes a long time for the drug to build up in your body and a long time to get cleared out of your body.  The half life of elimination for warfarin is 20 to 60 hours (1).  This means that, on average, it will take about two days for your body to get rid of 50% of any warfarin in your system.  Because of this, changes in the dose of warfarin from day to day will have only a very minimal effect on the concentration of warfarin in the body.  It is not at all uncommon for people to take a certain dose of warfarin some days of the week and take double that dose on the other days of the week.  In fact, we often think about warfarin dosing in terms of weekly doses instead of daily doses.  As long as some effort is made to spread the weekly dose throughout days of the week, the concentration of warfarin in your body will stay quite steady and the INR will be maintained in the therapeutic range. 

Now let’s look at your father’s dosing regimen.  Consider the following charts: 

Prescribed dosing regimen:

Proposed dosing regimen:

While there is nothing wrong in theory with taking the same dose of warfarin every day, doing what you propose would not be in concordance with the prescription written by your father’s physician. 

It is possible that a dosing regimen using the same dose of warfarin every day could be found that would keep your father’s INR within the therapeutic range.  However, warfarin dosing regimens are often changed due to the fact that there are many factors that affect INR.  If every warfarin dosing regimen was designed so that the same dose of warfarin was taken every day, we would end up dealing with quarter-tablets and eighth-tablets – which is hard to deal with and unnecessary for the reasons described above.

©2007 Sean Fitzpatrick  Used by permission.

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Last updated January 24, 2007