A Case of PT-INR Fluctuation Owing to Drug-drug Interaction Caused By the Combination of Warfarin, Rifampicin, and Sulfamethoxazole-trimethoprim

Warfarin potassium (WF) is widely used to treat and prevent thromboembolism. Rifampicin (RFP) induces various drug-metabolizing enzymes, whereas sulfamethoxazole/trimethoprim (ST) inhibits these enzymes. There is not enough information on drug interaction when WF, RFP, and ST are used together. In this case, when a combination of RFP and ST was used to treat pyogenic spondylitis in a patient taking WF, the prothrombin time-international normalized ratio (PT-INR) fluctuated, and the WF dosage was required to make frequent adjustments.