Objective: To present the case of warfarin-cloxacillin interaction that resulted in an increased international normalized ratio (INR). Case summary: A 70-year-old man had been treated with warfarin for atrial fibrillation. He was hospitalized because of superficial thrombophlebitis of the left median cubital vein, which developed after venipuncture. An antibiotic therapy with cloxacillin was initiated immediately after the admission. Two days later, INR value increased from baseline 1.9 to 4.6. Anticoagulation therapy was discontinued and INR value was measured daily. His INR remained high for the entire duration of antibiotic therapy. Three days after the cloxacillin therapy was discontinued, the INR decreased to the baseline value. Discussion: In the presented case, the temporal relationship between the administration of cloxacillin and increased INR suggests that the cloxacillin was responsible for the enhanced warfarin activity. According to the Drug Interaction Probability Scale, a causal relationship between the warfarin-cloxacillin interaction and increased INR value was rated "probable". Conclusion: Interactions between warfarin and cloxacillin can result in serious adverse reactions. INR value should be closely monitored when patients are prescribed this combination of drugs.