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

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 Paracetamol (> 2g/day >week) 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