This report explains the potential interaction between warfarin and astaxanthin in a 69-year-old Thai woman with history of ischemic stroke. Before taking astaxanthin, the patient used constant doses of warfarin, atenolol, digoxin, aspirin, omeprazole, and simvastatin concomitantly for 17 days without any signs and symptoms of adverse events. One day after astaxanthin was supplemented to her treatment regimen, ecchymosis was found on the right side of her groin and thigh. On the next day, area of ecchymosis was larger. International normalized ratio (INR) values increased from 1.4 to 10.38. Warfarin and astaxanthin were withheld and vitamin K was given. Two days later, INR reduced to 1.43 and symptoms of ecchymosis were better. Causality assessment of adverse drug reaction indicated a probable relationship between bleeding symptoms and astaxanthin supplementation. Counseling the patients and caregivers as well as monitoring for potential interactions with dietary supplements should be considered as an active process in order to prevent negative outcomes to patients undergoing warfarin therapy. .