Drug Interactions with warfarin and Coumadin

Drug Interactions with warfarin and Coumadin

  • 09 Apr 0

Drug Interactions with Warfarin

Despite its effectiveness, treatment with warfarin can be complicated as many commonly used medications interact.  However, interaction doesn’t always mean incompatibility:  Often the interaction is predictable and the adjustment to the warfarin dose can be anticipated.  At other times, the influence on the INR is more variable and the INR will need to be monitored more frequently so that adjustments can be made to keep the INR in range.  

Always make sure each of your doctors knows about all of your medicines from other doctors and double check every medicine change with your anticoagulation provider.

The following is a list of more commonly seen interactions and the potential influence on the INR:

Drug/Drug Class Effect on INR Recommendations/Comments
Acetaminophen/APAP with higher doses Limit APAP to 2000mg/day
Alcohol with binging
with chronic use
Limit to 1-2 alcoholic drinks/day
Amiodarone slow over time Complex interaction that takes place over 6-8 weeks. Ultimately expect 25-50% warfarin dose reduction. Check INR every week and make adjustments per INR
Bosentan (Tracleer)  
Capecitabine (Xeloda)  >50% warfarin dose reduction possible
Dicloxacillin More significant if course >14 days
Doxycycline Not always clinically significant—check INR more frequently
Flu Vaccine Varies from year to year, but may cause transient rise in INR
Fluconazole Hold warfarin dose once for single dose. Expect 25-50% warfarin dose reduction for extended course
Fluoroquinolones Not always clinically significant—check INR more frequently
Metronidazole (Flagyl) Empiric 25-50% warfarin dose reduction
Phenytoin or Complex interaction: initially ↑ INR, but then ↓ after prolonged administration. Additionally, warfarin may alter serum phenytoin concentrations and both need to be monitored frequently
Rifampin Expect 2- to 5-fold increase in warfarin dose requirements if course is >2 weeks
Sulfamethoxazole (Septra/Bactrim) Empiric 25-50% warfarin dose reduction
Thyroid replacement  

These are all general guidelines and possible drug interaction situations with warfarin. All dosage adjustments should be made by your physician or pharmacist that is managing your warfarin therapy. The best practice while on warfarin therapy is weekly home INR testing in order to catch any INR rises or falls ahead of time as your medications, diet, or life changes.

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