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Drug Interactions


Drug/Drug ClassEffect on INRRecommendations/Comments
Acetaminophen/APAP↑ with higher dosesLimit APAP to 2000mg/day
Alcohol
↑ with binging
↓ with chronic use
Limit to 1-2 EtOH drinks/day
Amiodaroneslow ↑ over timeComplex interaction that takes place over 6-8 weeks. Ultimately expect 25-50% warfarin dose reduction. Check INR q1-2 weeks and make serial adjustments per INR.
Bosentan (Tracleer)
Capecitabine (Xeloda)
Carbamazepine
DicloxacillinMore significant if course >14 days.
DoxycyclineNot always clinically significant if pt not systemically ill.
Fibrates
Flu VaccineVaries from year to year, but may cause transient rise in INR.
FluconazoleHold warfarin x1 for single dose. Expect 25-50% warfarin dose reduction for extended course.
FluoroquinolonesNot always clinically significant if pt not systemically ill.
Mercaptopurine
Metronidazole (Flagyl)Expect 25-50% warfarin dose reduction.
Phenytoin↓ or ↑ Complex interaction: initially ↑ INR, but then ↓ after prolonged administration. Additionally, warfarin may alter serum phenytoin concentrations.
RifampinExpect 2- to 5-fold increase in warfarin dose requirements.
Sulfamethoxazole (Septra/Bactrim)Expect 25-50% warfarin dose reduction
Testosterone
Thyroid replacement

 

Increased Risk of Bleeding is Possible when Combining Warfarin With:

  • Antiplatelets
  • Fish Oil/Omega 3 Fatty Acids
  • NSAIDs
  • Other Anticoagulants
  • Steroids
  • Vitamin E