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Drug Interactions


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 EtOH drinks/day
Amiodarone slow ↑ over time Complex 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)
Dicloxacillin More significant if course >14 days.
Doxycycline Not always clinically significant if pt not systemically ill.
Flu Vaccine Varies from year to year, but may cause transient rise in INR.
Fluconazole Hold warfarin x1 for single dose. Expect 25-50% warfarin dose reduction for extended course.
Fluoroquinolones Not always clinically significant if pt not systemically ill.
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.
Rifampin Expect 2- to 5-fold increase in warfarin dose requirements.
Sulfamethoxazole (Septra/Bactrim) Expect 25-50% warfarin dose reduction
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