Drug Interactions

One of the reasons warfarin patients require close monitoring is that a number of drugs and foods impact on its effectiveness. Some drugs taken with warfarin will increase the effects of the warfarin, while others will work against it.

Drugs Expected to Increase the Anticoagulant Effect of Warfarin

Acetaminophen (> 2g/day >week) Allopurinol Amiodarone Aspirin
Bezafibrate Cefotetan Cefoxitin Ceftriaxone
Cephazolin Cimetidine Ciprofloxacin Clarithromycin
Cephamandole Citalopram Cotrimoxazole Cyclophosphamide
Danazol Dextropropoxyphene Disulfiram Doxycycline
Erythromycin Fluconazole Fluoxetine Flutamide
Fluvastatin Gemfibrozil Heparin Isoniazid (600 mg/d)
Itraconazole Ketoconazole Lanzoprazole Metronidazole
Miconazole Norfloxacin NSAIDs eg. diclofenac Ofloxacin
Omeprazole Paroxetine Sertraline Sodium Valproate
Quinidine Phenytoin (transient increase) Simvastatin Tetracycline
Tamoxifen Sulphamethoxazole Trimethoprim


Drugs which may decrease the anticoagulant effect of warfarin either by increasing its metabolism or indirectly by interfering with absorption:

Alcohol (>3 alcoholic drinks/day) Antacids Azathioprine Carbamazepine
Enteral feeds (some contain vitamin K) Carbimazole Cholestyramine Colestipol
Oral contraceptives & oestrogens Griseofulvin Phenobarbitone Phenytoin
Penicillin (high dose intravenous) Primidone Propylthiouracil Rifampicin
Sucralfate Vitamin K

This is not an exhaustive list – if in doubt – ask a pharmacist or doctor.

Drugs Which Affect Platelet Function

Aspirin Clopidogrel
Dipyridamole Ticlopidine