Drug Interactions
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 |