I recently developed hives on warfarin, but I am sure it is an allergy. When I started taking benedryl they went away for awhile. That indicates allergy. I read on-line that they only have one case of someone being allergic to warfarin itself and many cases of allergies to the color and fillers. It is something to discuss with your doctor if you are having allergy indicators such as hives, swelling, joint swelling.