International normalized ratio elevated with warfarin

Parker DL, Hoffmann TK, Tucker MA, Gerschutz GP, Malone PM. Elevated International Normalized Ratio associated with concurrent use of ophthalmic erythromycin and warfarin. Am J Health Syst Pharm 2010 67(1) 38-41. 

Either UFH or LMWHs should be administered to patients with non-ST-segment-elevation ACS. Therapy should be continued for up to 48 hours or until the end of the angiography or PCI procedure. In patients initiating warfarin therapy, UFH or LMWHs should be continued until the international normalization ratio (INR) with warfarin is in the therapeutic range. Data supporting the addition of UFH to aspirin stems from a meta-analysis of six randomized trials demonstrating a 33% reduction in the risk of death or MI at 6 weeks with UFH... 

Rofecoxib is metabolized in the liver, primarily by cytosolic enzymes, with little renal excretion of unchanged drug. Rofecoxib at 75 mg daily modestly increased methotrexate concentrations, and at 50 mg daily modestly elevated the International Normalized Ratios (INRs) of warfarin patients. Additionally, rifampin can decrease rofecoxib concentrations. Clinically significant interactions were not observed when rofecoxib was administered with crmetidine, digoxin, oral contraceptives, or ketoconazole. Rofecoxib inhibits CYP450 1A2 and may increase serum theophylline area under the curve. 

There may be a marked elevation of the International Normalized Ratio (INR) and prolongation of prothrombin time when fluoxetine is combined with warfarin. ... 

Stoner SC, Lea JW, Dubisar BM, Farrar C. Possible international normalized ratio elevation associated with celecoxib and warfarin in an elderly psychiatric patient. J Am Geriatr Soc... 

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