Stop using warfarin and call your doctor at once if you have a serious side effect such as: 1  pain, swelling, hot or cold feeling, skin changes, or discoloration anywhere on your body; 2  sudden and severe leg or foot pain, foot ulcer, purple toes or fingers; 3  sudden headache, dizziness, or weakness;IDE EFFECTS: Nausea, loss of appetite, or stomach/abdominal pain may occur. If any of these effects persist or worsen, tell your doctor or pharmacist promptly. Remember that your doctor has prescribed this medication because he or she has judged that the benefit to you is greater than the risk of side effects.