In addition to maintaining patients within the narrow therapeutic INR range, perhaps the other major challenge with warfarin is the long list of multiple drugs and foods that will interact with warfarin.  Some of the more common drugs include antibiotics, antiarrhythmics such as amiodarone, aspirin and aspirin-containing products, and the nonsteroidal antiinflammatory drugs (NSAIDs).  In addition, many patients take “health” supplements that can influence warfarin without their physicians knowing they are taking them.  This list includes the co-enzyme Q10 drugs, fish oils, St. John’s Wort, and obviously any supplements that include vitamin K.  The Figure also lists some of the food and beverages that can affect warfarin, including alcohol, the foods that are high in vitamin K, such as soy beans, and canola oils (which I think a lot of people underestimate), and black licorice and cranberries.

Rothman SA. Am J Med 2013; 126: 00-00.