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.
International Normalized Ratio, Warfarin, Antibiotics, Drug interaction.