Common side effects may include: Nausea, vomiting, Mild stomach pain, Bloating, gas, Altered sense of taste Serious side effects may include: Pain, swelling, hot or cold feeling, skin changes, or discoloration anywhere on your body, Sudden and severe leg or foot pain, foot ulcer, purple toes or fingers, Sudden headache, dizziness, or weakness, Unusual bleeding (nose, mouth, vagina, or rectum), bleeding from wounds or needle injections, any bleeding that will not stop, Easy bruising, purple or red pinpoint spots under your skin, Blood in your urine, black or bloody stools, coughing up blood or vomit that looks like coffee grounds, Pale skin, feeling light-headed or short of breath, rapid heart rate, trouble concentrating, Dark urine, jaundice (yellowing of the skin or eyes), Pain in your stomach, back, or sides, Urinating less than usual or not at all, Numbness or muscle weakness, Any illness with diarrhea, fever, chills, body aches, or flu symptoms Seek emergency medical help if you have any signs of an allergic reaction including hives, difficulty breathing and/or swelling of your face, lips, tongue, or throat. If you have not been eating well, if you have an illness or infection that causes fever, vomiting, or diarrhea for more than 2 days, or if you start using any antibiotic medications, contact your doctor or pharmacist immediately because these conditions can affect how Coumadin works.