Bruises on warfarin patient's shoulder. Extensive bruising on the upper arm, shoulder and chest of a 92-year-old woman who is taking warfarin. Warfarin is a blood-thinning drug, and one of its side-effects is severe bruising and bleeding if the patient injures themselves. This injury was sustained following a fall. This patient is taking warfarin to help prevent side-effects of atrial fibrillation.