Abstract
Warfarin is an oral anticoagulant that is indicated for the prevention and treatment of various cardiovascular disorders. Potential drug interactions of warfarin are observed to be one of the most frequently appearing challenge that may alter the pharmacokinetics and pharmacodynamics of the drug thus alter the overall therapeutic response. The aim of this study was to determine the effect of antibiotics on INR values over time in patients > 50 years receiving stable warfarin therapy. Data for this retrospective cohort study were collected through a medical record review of patients in an ambulatory anticoagulation clinic. A total of 121 patients from 185 prescriptions qualified the inclusion criteria and were included in the study. The results revealed that there was a significant increase in INR values because of antibiotic and warfarin interactions. The results indicated that the pattern of INR change for the 5 antibiotics was significantly different across the time. The mean INR increase was significant for amoxicillin (0.191, p < 0.05), ciprofloxacin (0.13, p < 0.05), azithromycin (0.32, p < 0.05), levofloxacin (0.32, p < 0.05), and cefuroxime (0.24, p < 0.05). It has been concluded that the use of antibiotics in older patients receiving a stable warfarin therapy, may lead to an increase in the INR. However, this increase in INR may not be too significant as long as the INR is routinely monitored.