A 67-year-old white woman had been taking warfarin for 1.5 years due to recurrent transient ischaemic attacks. She had her international normalised ratio (INR) checked and was followed monthly. Prior to the interaction, her INR was therapeutic for 5 months while she was taking warfarin 1.5 mg/day. The patient admitted to doubling her fish oil dose from 1000 to 2000 mg/day. Without dietary, lifestyle or medication changes, her INR increased from 2.8 to 4.3 within 1 month. The INR decreased to 1.6, 1 week after subsequent fish oil reduction, necessitating a return to the original warfarin dosing regimen. The authors believe that the INR change was probably caused by her fish oil intake.