Herbal medicine is usually safe and popular in Iranian communities. Peppermint herbal products may interact with warfarin, which is an essential lifelong drug used in patients with prosthetic heart valves. Herein, we report a case of possible interaction between warfarin and peppermint herbal tea.We described a 68-year-old woman who was on warfarin treatment due to bileaflet mechanical aortic valve, and mitral valve replacement presented with severe dyspnea in the setting of large obstructive prosthetic mitral valve thrombosis diagnosed by echocardiography. Therefore, cardiac surgery was conducted in the emergent setting. Warfarin was started the day after the procedure. She developed melena with a volatile and elevated international normalized ratio (INR) of 4.8 and prothrombin time > 30 s on the 7th post-operation day. She reported no changes in drug or dietary habits except for drinking high amounts of peppermint tea the day before. After the discontinuation of warfarin and medical consultation for gastrointestinal bleeding, the INR level decreased to 1.8 within two days.Consumption of peppermint herbal products are usually safe; however, caution should be taken with patients under warfarin treatment. Full disclosure for patients on anticoagulant treatment is necessary before discharge and during clinical follow-up