Drug Interactions with Warfarin
This list is not exhaustive, but provides a general guide
Many medications can interact with warfarin
It is important to increase the frequency of INR monitoring when any interacting medications are added OR stopped
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Drugs Expected to Increase the Anticoagulant Effect of Warfarin
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Allopurinol |
Gemfibrozil |
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Amiodarone |
Heparin |
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Aspirin |
Isoniazid (600 mg/d) |
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Bezafibrate |
Itraconazole |
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Cefotetan |
Ketoconazole |
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Cefoxitin |
Lanzoprazole |
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Ceftriaxone |
Metronidazole |
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Cephazolin |
Miconazole |
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Cimetidine |
Norfloxacin |
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Ciprofloxacin |
NSAIDs eg. diclofenac |
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Clarithromycin |
Ofloxacin |
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Cephamandole |
Omeprazole |
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Citalopram |
Paracetamol (> 2g/day >week) |
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Cotrimoxazole |
Paroxetine |
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Cyclophosphamide |
Phenytoin (transient increase) |
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Danazol |
Quinidine |
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Dextropropoxyphene |
Sertraline |
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Disulfiram |
Simvastatin |
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Doxycycline |
Sodium Valproate |
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Erythromycin |
Sulphamethoxazole |
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Fluconazole |
Tamoxifen |
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Fluoxetine |
Tetracycline |
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Flutamide |
Trimethoprim |
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Fluvastatin |
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Drugs which may decrease the anticoagulant effect of warfarin either by increasing its metabolism or indirectly by interfering with absorption:
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Drugs which may Decrease the Anticoagulant Effect
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Alcohol (>3 alcoholic drinks/day) |
Oral contraceptives & oestrogens |
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Antacids |
Penicillin (high dose intravenous) |
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Azathioprine |
Phenobarbitone |
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Carbamazepine |
Phenytoin |
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Carbimazole |
Primidone |
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Cholestyramine |
Propylthiouracil |
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Colestipol |
Rifampicin |
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Enteral feeds (some contain vitamin K) |
Sucralfate |
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Griseofulvin |
Vitamin K |
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Drugs Which Affect Platelet Function |
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Aspirin |
Dipyridamole |
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Clopidogrel |
Ticlopidine |
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Complementary Medicines Which may Increase the Anticoagulant Effect |
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Danshen |
Horse chestnut |
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Dong Quai |
Papain/Papaya extract |
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Ginger |
Vitamin E >400iu/day |
Complementary Medicines Which may Decrease the Anticoagulant Effect |
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Coenzyme Q10 |
Green Tea |
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Ginseng |
St John’s Wort |
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Complementary Medicines which Affect Platelet Function |
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Feverfew |
Ginkgo biloba |
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Garlic |
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Important: Some drugs are known to interact with warfarin. If a patient on warfarin requires any changes to their medication and in particular the introduction or withdrawal of drugs that could interact with warfarin, it is essential that the INR is monitored closely. |
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Note: This is not an exhaustive list. To check for an interaction or what action to take contact Medicines Information Service or your pharmacist. |