| Drug | Usual effect on INR |
| Amiodarone | Increase |
| Aspirin | Increase or no change |
| Amoxicillin | Slight increase |
| Cefaclor | Increase |
| Carbamazepine | Decrease |
| Phenytoin | Decrease |
| Phenobarbital | Decrease |
| Cloxacillin | Increase |
| Prednisone | Increase |
| Co-trimoxazole | Increase |
| Ranitidine | Increase |
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