Dear At
Most multivitamins contain vitamin K. The vitamin K will counteract the effect of the warfarin, because vitamin K was originally called the "coagulation vitamin" (in other words, it makes your blood clot, while warfarin has been prescribed to ensure that you blood remains thin). This is why the doctor usually recommends that patients taking warfarin should not take any vitamin supplements or eat large quantities of green leafy vegetables (esp broccoli, cabbage, and dark lettuce leaves which are also a rich source of vitamin K), during their treatment. So avoid these foods and stop taking your multivitamins until the warfarin treatment has been completed.
Best regards
DietDoc