BACKGROUND: Excessive anticoagulation due to warfarin use is associated with hemorrhage. Subcutaneously administered vitamin K has not been evaluated for the treatment of warfarin-associated coagulopathy, yet it is widely used. OBJECTIVE: To show that oral vitamin K is more effective than subcutaneous vitamin K in the treatment of warfarin-associated coagulopathy. DESIGN: Randomized, controlled trial. SETTING: Two teaching hospitals. PATIENTS: Patients with an international normalized ratio (INR) between 4.5 and 10.0. INTERVENTION: Warfarin therapy was withheld, and 1 mg of vitamin K was given orally or subcutaneously. MEASUREMENTS: The primary outcome measure was the INR on the day after administration of vitamin K. Secondary outcome measures were hemorrhage and thrombosis during a 1-month follow-up period. RESULTS: 15 of 26 patients receiving oral vitamin K and 6 of 25 patients receiving subcutaneous vitamin K had therapeutic INRs on the day after study drug administration (P = 0.015; odds ratio, 4.32 [95% CI, 1.13 to 17.44]). CONCLUSION: Oral vitamin K lowers INR more rapidly than subcutaneous vitamin K in asymptomatic patients who have supratherapeutic INR values while receiving warfarin.