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Warfarin
(Coumadin®)
Drug Interactions -
Numerous medicines can interact with warfarin. This includes
prescription and over-the-counter medicines, and herbals. Taking
warfarin with these medicines can increase your chances of clotting
or bleeding. Many interactions can be very serious, even fatal.
For this reason, it is important that you check with your doctor
or pharmacist before starting any new medicine while on warfarin.
It is also important that you report any changes in your medicines
to the clinician monitoring your warfarin.
Over-the-counter medicines that can increase the effects of warfarin
are listed in Table 1. Tylenol® has
been reported to increase the effects of warfarin when large
doses are taken for a duration
of greater than one week. Many cough and cold products contain
some of the pain medicines listed, so it is important to check
with a healthcare provider before taking these medicines.

Several herbals and supplements can also interact with warfarin.
Herbals are of particular concern to healthcare professionals.
Many people do not consider herbals to be “drugs.” It
is important to note that herbals are not regulated by the
Food and Drug Administration.
Their potency can vary from product
to product, which can affect the severity of the interaction.
Herbals that can increase or decrease the effect of warfarin
are listed in Table 2. Supplements that contain vitamin K can
decrease warfarin’s effect and can lead to clotting.

There are also many prescription medicines that can interact
with warfarin. Some of the more common prescription drug interactions
with warfarin are listed in Table 3.

This information does not include every medicine that interacts
with warfarin. It is very important to check with a doctor or
pharmacist before changing any medicine, including over-the-counter,
prescription, and herbals.
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