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A case of presumed interaction between warfarin and nalidixic acid is reported in an 84 year-old female in whom INR rose from 1.9 to 9.6 after start of treatment with nalidixic acid. The interaction may be caused by several factors: 1) Nalidixic acid may displace warfarin from plasma proteins and thereby increase the anticoagulant effect. 2) The fluoroquinolone drug enoxacin reduces the hepatic clearance of the R-stereomere of warfarin but without prolongation of the prothrombin time ratio. 3) Fluoroquinolones may affect the vitamin-K producing bacteria in the gut and thereby influence the vitamin-K: warfarin ratio in plasma. Only some patients develop hypoprothrombinaemia during concomitant therapy with warfarin and quinolone antimicrobials. It is hypothesized that the interaction may partly be related to age, sex or concomitant disease. Frequent control of INR in patients treated with both warfarin and quinolone antimicrobials is recommended.
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