Fish Oil-Induced Elevated International Normalized Ratio in Patients Taking Warfarin: A Case Report and Literature Review

Warfarin is an oral anticoagulant commonly used to prevent thromboembolism. Its pharmacodynamics can be affected by interactions with other drugs and dietary supplements, although there is only limited information available on these interactions. One dietary supplement that has been associated with increased international normalized ratio (INR) levels, potentially leading to supratherapeutic levels, is fish oil. However, the data available on this interaction are not extensive. This case study focused on a sixty-nine years old Thai female who was prescribed warfarin at a weekly dose of 30 milligrams (maintaining an INR range of 2.5-3.5) to prevent thromboembolism resulting from atrial fibrillation (AF) following a mechanical valve replacement at the mitral position. The patient began taking fish oil supplements for a period of 2 months. After initiating the fish oil supplement in addition to the warfarin, the patient’s INR had increased from 2.94 to 4.13, although no bleeding symptoms were observed. When combining fish oil with warfarin, it is important for medical professionals to exercise caution and to monitor the INR levels in all cases to ensure safety and optimize efficacy. KEYWORDS warfarin, fish oil, dietary supplement