When the Prothrombin Ratio (PR) is used for monitoring warfarin therapy, it is referred to as the INR.
The INR (International Normalised Ratio) allows for a valid comparison between different thromboplastin reagent's sensitivity (ISI), to warfarin-induced changes in clotting factors.
INR = (PR)ISI
(ISI International Sensitivity Index assigned to a thromboplastin reagent by calibrating against an International Reference Preparation (IRP))
The following drugs tend to potentiate warfarin (increase the INR):
Allopurinol
Amiodarone
Anabolic Steroids
Aspirin
Bezafibrate
Cefotetan
Cefoxitin
Ceftriaxone
Cephamandole
Cephazolin
Cimetidine
Ciprofloxacin
Citalopram
Clarithromycin
Clopidogrel
Co-trimoxazole
COX-2 Inhibitors
Cyclophosphamide
Danazol
Dextropropoxyphene
Diflunisal
Dipyridamole
Disulfiram
Doxycycline
Erythromycin
Fluconazole
Fluoxetine
Flutamide
Fluvastatin
Gemfibrozil
Heparin
Isonizid
Itraconazole
Ketoconazole
Lanzoprazole
Metronidazole
Miconazole
Norfloxacin
NSAIDs
Ofloxacin
Omeprazole
Paracetamol
Paroxetine
Phenytoin
Quinidine
Sertraline
Simvastatin
Sodium Valproate
Sulphamethazole
Tamoxifen
Tetracyclines
Thyroxine
Ticlopidine
Thrimethoprim
Vitamin E (high dose)
The following drugs tend to decrease the INR in warfarinised patients:
Alcohol (>3 drinks/day)
Antacids
Azathioprine
Carbamazepine
Carbimazole
Cholestyramine
Colestipol
Ginseng
Griseofulvin
Oral contraceptives
Penicillin (high dose)
Phenobarbitone
Phenytoin
Primidone
Propylthiouracil
Rifampicin
St Johns Wart
Sucralfate
Drugs which affect platelet function:
Aspirin
Clopidogrel
Dipyridamole
Feverfew
Garlic
Gingko biloba