Warfarin Institute of America
Dedicated To Your Health
TRIMETHOPRIN/SULFAMETHOXAZOLE INTERACTION WITH WARFARIN
This is a very common interaction. Many physicians are very careless about this interaction. This can cause an increase in the INR. It does not seem to cause much harm if the trimethoprim/sulfamethoxazole is to be taken for 10 days or less. If the INR is high after 3 to 5 days and the trimethoprim/sulfamethoxazole is to be taken for 10 days, usually holding a dose every other day until the trimethoprim/sulfamethoxazole is completed is sufficient to prevent a problem. However if the trimethoprim/sulfamethoxazole is to be taken for longer than this, it will probably be necessary to adjust the warfarin dose down.
If the warfarin dose is adjusted down, then it is important to remember that it will have to be adjusted up when the trimethoprim/sulfamethoxazole is stopped.
It is very important to remember that stopping an interacting medication can have adverse effects. Many people overlook this fact. Think of warfarin as sitting on one side of a teeter-totter. Whatever you do to one side will affect the other side.
I have heard of pharmacists refusing to fill prescriptions for this because the patient was already on warfarin. I do not think that this is a wise course of action. The patient should not be made to suffer for an interaction that will usually take several days to occur. A much wiser course of action would be to inform the patient that they need to contact the person managing their warfarin therapy as soon as possible to have their INR checked. This is a much better discharge of the pharmacists professional duty than merely refusing to fill the prescription.
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Last updated November 21, 2004