<Note>Effects of Tramadol Coadministration on Prothrombin Time-International Normalized Ratio in Patients Receiving Warfarin

Several case studies have reported a possible drug interaction between warfarin and tramadol where tramadol coadministration enhanced the antithrombotic effects of warfarin. To assess this drug interaction, changes in prothrombin time-international normalized ratio (PT-INR) before and after tramadol coadministration were investigated in patients receiving warfarin. For this study, we examined 54 patients (male/female: 22/32, 68.4±12.7 years) who were being treated with warfarin for deep vein thrombosis, atrial fibrillation, arteriosclerosis obliterans, congestive heart failure, and other vascular diseases. Significant increases in PT-INR were observed 9.5 (1-118) d after coadministration of tramadol (1.81±0.56 vs. 2.47±1.10, p