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Drug Interactions

These drugs affect your INR (International Normalized Ratio) test, which measures how quickly your blood clots. We use the results of this test to determine how much Coumadin (warfarin) is in your system.

  • The lower your INR value, the higher your risk of having a blood clot or stroke.
  • The higher your INR value, the higher your risk of bleeding and increased bruising.

Many drugs can affect how your body absorbs Coumadin, so it is very important to inform your anticoagulation provider of any changes in your medication routine.

Prescription drugs

Increased INR

  • Amiodarone
  • Anabolic Steroids
  • Antifungals
    • Fluconazole (diflucan)
    • Ketoconazole (nizoral)
    • Itraconazole (sporano)
  • Aspirin and Salicylates
  • Cephalosporins
  • Chloral Hydrate
  • Cimetidine (tagamet)
  • Clofibrate
  • Cranberry Juice (CYP2C9 inhibitor)
  • Danazol (danocrine)
  • Diflunisal (dolobid)
  • Disulfiram (Antabuse)
  • Fluvoxamine (Luvo)
  • Ginkgo Biloba
  • Heparin
  • Isoniazid (INH)
  • Erythromycin
  • Clarithromycin
  • Metronidazole (flagyl)
  • NSAIDS
  • Omeprazole (prilosec)
  • Paroxetine (paxil)
  • Pepto-Bismol
  • Quinilones
  • Tamoxifen
  • Tetracycline
  • Synthroid (thyroxine)
  • Ticlopidine (ticlid)
  • Trimethoprim-Sulfate (bactrim, Septra)
  • Vitamin E
  • Levaquin

Decreased INR

  • Barbiturates
  • Carbamaziepine (tegretol)
  • Oral contraceptives
  • Rifampin
  • Vitamin K
  • American Ginseng

May increase or
decrease INR

  • Allopurinol
  • Corticosteroids (Prednisone)
  • Penicillin
  • Phenytoin (dilantin)

Over the counter drugs

Increased INR

  • Aspirin / Bayer / Excedrin
  • Tylenol (more than twice daily)
  • Advil / Motrin / Ibuprofen
  • Pepto-Bismol
  • Pepcid A.C.
  • Zantac
  • Alka-Seltzer
  • Nyquil
  • Thera-Flu
  • Benadryl
  • Tylenol PM
  • Chondroitin; Glucosamine

Decreased INR

  • Green Tea

No effect

  • Mylanta
  • Tums
  • Imodium A.D.
  • Robitussin
  • Psuedophedrine / Sudafed
  • Tylenol (less than twice daily)
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