EFFECT OF CIPROFLOXACIN ON THE PHARMACOKINETICS AND PHARMACODYNAMICS OF WARFARIN

To determine if ciprofloxacin therapy alters the response to warfarin treatment, 36 adult patients attending three university-affiliated out patient anticoagulation clinics randomly received a 12-day course of c iprofloxacin (750 mg twice daily) and a 12-day course of placebo; each course was separated by a 2-week washout period. Prothrombin times (P Ts), concentrations of S-warfarin and R-warfarin (the isomers of warfa rin), and concentrations of clotting factors II and VII were determine d three times weekly for 9 weeks, By day 12 of ciprofloxacin therapy, concentrations of S-warfarin remained unchanged compared with those af ter placebo therapy, but R-warfarin concentrations increased significa ntly (1.15 times those after placebo therapy; P = .001); concentration s of clotting factors II and VII decreased (0.903 and 0.872 times thos e after placebo therapy, respectively, P less than or equal to .020). The mean PT ratio after 12 days of ciprofloxacin therapy increased sli ghtly (1.032 times that after placebo therapy; P = .057), but no patie nt had bleeding or a change in PT that required alteration in warfarin or ciprofloxacin therapy. We conclude that warfarin therapy is not a contraindication to the use of ciprofloxacin.