Warfarin Drug Interactions

Aka: Warfarin Drug Interactions, Drug Interactions with Coumadin, Coumadin Drug Interactions

1. See Also
   1. Coumadin (Warfarin)
   2. Coumadin Protocol
   3. Coumadin Protocol for the Perioperative Period
2. Risk Factors: Drug Interaction with Warfarin
   1. Older age
   2. Acutely ill
   3. Dietary change
   4. Concurrent infection
3. Precautions
   1. Avoid using agents with major Drug Interactions with Warfarin
   2. When using medications known to affect INR, monitor INR closely
      1. Check INR 5 days after starting medication
      2. Check INR after stopping the medication
4. Drug Interactions: Agents that increase INR or bleeding risk
   1. Acetaminophen (Tylenol)
   2. Alcohol
   3. Amiodarone (Cordarone)
      1. Adjustment per Amiodarone maintenance dose
         1. Amiodarone 400 mg/day: Reduce Warfarin dose 40%
         2. Amiodarone 300 mg/day: Reduce Warfarin dose 35%
         3. Amiodarone 200 mg/day: Reduce Warfarin dose 30%
         4. Amiodarone 100 mg/day: Reduce Warfarin dose 25%
      2. Reference
         1. Sanoski (2002) Chest 121:19-23 [PubMed]
   4. Anabolic Steroids
   5. Antifungal Medications
      1. Fluconazole (Diflucan)
      2. Ketoconazole (Nizoral)
      3. Itraconazole (Sporanox)
      4. Miconazole (Monistat)
   6. Aspirin and Salicylates
   7. Cephalosporins
      1. Cefoperazone (Cefobid)
      2. Cefamandole (Mandol)
      3. Cefotetan (Cefotan)
      4. Cefmetazole (Zefazone)
   8. Chloral Hydrate
   9. Cimetidine (Tagamet)
   10. Clofibrate
   11. Cranberry Juice (CYP2C9 inhibitor)
       1. Appears safe in at least one clinical trial
       2. Greenblatt (2006) Clin Pharmacol Ther 79:125-33 [PubMed]
   12. Danazol (Danocrine)
   13. Diflunisal (Dolobid)
   14. Disulfiram (Antabuse)
   15. Ginkgo Biloba (independent effect due to antiplatelet activity)
   16. Heparin
   17. HMG CoA Reductase inhibitors
   18. Isoniazid (INH)
   19. Macrolides
       1. Erythromycin
       2. Clarithromycin
   20. Metronidazole (Flagyl)
       1. High risk Drug Interaction (can significantly increase INR with bleeding risk 2-4 fold over baseline)
       2. Avoid if possible and if not, consider decreasing Septra dose by 25-40%
   21. Nalidixic Acid
   22. NSAIDs
   23. Omeprazole (Prilosec)
   24. Paroxetine (Paxil)
   25. Penicillin
   26. Propafenone (Rythmol)
   27. Quinidine
   28. Quinolones
   29. Selective Serotonin Reuptake Inhibitors (SSRI) and Serotonin Norepinephrine Reuptake Inhibitors (SNRI)
       1. Fluvoxamine (Luvox)
       2. Fluoxetine (Prozac)
       3. One additional patient on Warfarin for Atrial Fibrillation with SNRI or SSRI will have have a major bleeding event
          1. (2014) Presc Lett 21(11): 65
          2. Sansone (2009) Psychiatry 6(7): 24–29 [PubMed]
   30. Sulfinpyrazone (Anturane)
   31. Tamoxifen
   32. Tetracycline
   33. Thyroid Hormone (Thyroxine or Synthroid)
   34. Ticlopidine (Ticlid)
   35. Trimethoprim-Sulfamethoxazole (Bactrim, Septra)
       1. High risk Drug Interaction (can significantly increase INR with bleeding risk 2-4 fold over baseline)
       2. Avoid if possible and if not, consider decreasing Septra dose by 25-40%
   36. Vitamin E
       1. Theoretical bleeding risk, but appears to be safe in clinical trials
       2. Dereska (2006) J Surg Res 132:121-9 [PubMed]
5. Drug Interactions: Drugs that decrease INR or increase clotting risk
   1. American Ginseng (no effect on Warfarin with Asian Ginseng)
   2. Barbiturates
   3. Binding Resins
   4. Carbamazepine (Tegretol)
   5. Oral Contraceptives
   6. Penicillin
   7. Rifampin
      1. Avoid if possible and if not, consider increasing Warfarin dose by 25-40%
   8. St. John's Wort
   9. Vitamin K
6. Drug Interactions: Drugs that have a variable effect on INR or bleeding risk
   1. Allopurinol
   2. Corticosteroids
   3. Phenytoin (Dilantin)
7. Drug Interactions: Lowest risk antibiotics for interaction
   1. Amoxicillin
   2. Azithromycin
   3. Cepahlosporins
8. References
   1. Horton (1999) Am Fam Physician [PubMed]
   2. Gage (2000) Am J Med 109:484 [PubMed]