What is known and objective: Vitamin K deficiency is known to cause impaired coagulation. We report a case of marked prolongation of the prothrombin time‐international normalized ratio (PT‐INR) associated with warfarin and vitamin K deficiency caused by endoscopic nasobiliary drainage (ENBD). Case presentation: Oral administration of warfarin was initiated in a 67‐year‐old man after left hemihepatectomy. He developed a biliary fistula after surgery that was treated by ENBD, which resulted in significant prolongation of the PT‐INR. What is new and conclusion: The effect of warfarin was enhanced in this patient due to reduced absorption of vitamin K as a result of external biliary drainage.