Interaction between diflunisal and warfarin

The effect of diflunisal on steady‐state warfarin dynamics and kinetics was studied in five healthy men taking daily subtherapeutic doses of warfarin. Diflunisal 500 mg twice daily for 2 wk increased the percentage unbound warfarin from 1.02% to 1.34% and lowered total warfarin from 741 to 533 ng/ml, but did not alter the anticoagulant response (prothrombin complex activity, PCA). When diflunisal was discontinued but warfarin continued, there was a loss of anticoagulant effect and a difference in the rates at which percentage unbound warfarin and total warfarin concentration returned to prediflunisal levels. There was a correlation between plasma diflunisal and percentage unbound warfarin. Diflunisal reduced both the maximum plasma protein‐binding capacity for warfarin and the apparent association constant. Clinical Pharmacology and Therapeutics (1980) 28, 493–498; doi: 10.1038/clpt.1980.193