Coenzyme Q10 (Ubidecarenone) is marketed as a dietary supplement. Drug interaction between coenzyme Q10 and warfarin has previously been reported. In the present case, a 72-year-old female treated with warfarin showed less responsiveness to warfarin than previously. It appeared she had taken coenzyme Q10, and when this was stopped, her responsiveness to warfarin was the same as before. Coenzyme Q10 is chemically similar to K-vitamins, which may explain the interaction with warfarin. Patients in treatment with warfarin should be aware of the possible risk of treatment failure when taking coenzyme Q10. The need for questioning patients concerning not only medications but also use of dietary supplements and alternative medications is emphasised.