RICHMOND, VA - Even though antibiotics may lead to an increase in international normalized ratio (INR) for older veterans on stable warfarin therapy, that may not result in clinically significant outcomes of bleeding or hospitalization, according to a new report.1

As a result, study authors suggest that antibiotics may be prescribed for patients more than 65 years old who take warfarin, as long as their INR is being routinely monitored.

Researchers from the Geriatric Pharmacotherapy Program at Virginia Commonwealth University in Richmond, VA, note, “Several classes of drugs, such as antibiotics, may interact with warfarin to cause an increase in warfarin’s anticoagulant activity, and the clinical relevance of warfarin-antibiotic interactions in older adults is not clear.”

Using medical record review from a VA outpatient anticoagulation clinic, they sought to determine the effect of oral antibiotics, such as amoxicillin, azithromycin, cephalexin, ciprofloxacin, levofloxacin, and moxifloxacin, on the INR of older patients taking warfarin. A secondary objective was to compare the effect of warfarin-antibiotic interactions on outcomes of over-anticoagulation.

Data was collected for patients 65 and older who were on stable warfarin therapy and had at least one prescription of a specific oral antibiotic from Jan. 1, 2003 to March 1, 2011.

With a total of 205 patients having 364 prescriptions for warfarin and antibiotics simultaneously, antibiotics related to significant increase in INR were amoxicillin (P = 0.0019), azithromycin (P < 0.0001), ciprofloxacin (P = 0.002), levofloxacin (P < 0.0001) and moxifloxacin (P < 0.0001). The researchers also found a significant association between type of antibiotic and secondary outcomes of over-anticoagulation, using cephalexin as the control.

1. Ghaswalla PK, Harpe SE, Tassone D, Slattum PW. Warfarin-Antibiotic Interactions in Older Adults of an Outpatient Anticoagulation Clinic. Am J Geriatr Pharmacother. 2012 Oct 19. doi:pii: S1543-5946(12)00126-2. 10.1016/j.amjopharm.2012.09.006. [Epub ahead of print] PubMed PMID: 23089199. J Crit Care. 2012 Oct 24. pii: S0883-9441(12)00305-X. doi: 10.1016/j.jcrc.2012.08.021. [Epub ahead of print]