Although commonly used to treat blood clots, warfarin (Coumadin, Jantoven) can have dangerous side effects or interactions that can place you at risk of bleeding. Here are precautions to take to avoid warfarin side effects.If you've been prescribed warfarin (Coumadin, Jantoven) to prevent blood clots, you probably already know that this powerful drug can save your life if you are at risk of or have had blood clots.But you may not realize how serious warfarin side effects can be.Warfarin, especially if taken incorrectly, increases your risk of dangerous bleeding. Warfarin side effects can also include interactions with some foods, prescription medicines and over-the-counter supplements.ut you may not realize how serious warfarin side effects can be. Warfarin, especially if taken incorrectly, increases your risk of dangerous bleeding. Warfarin side effects can also include interactions with some foods, prescription medicines and over-the-counter supplements.