Warfarin is the most widely used oral anticoagulant in Australia. Although it can prevent thrombosis, it can cause life-threatening haemorrhages. Patients taking warfarin should have their INR measured regularly. More frequent tests are needed when patients start, stop or alter the dose of their other medications. Educating the patients about warfarin helps them to take their treatment correctly. They should report any abnormal bleeding and have their INR measured. A very high INR is an indication for admission to hospital to have the effects of warfarin controlled.