Anticoagulants prevent the formation of harmful thrombi by decreasing the ability of the blood to clot. They are prescribed for a number of reasons, such as after replacement of a heart valve to prevent thrombosis or after myocardial infarction to prevent another infarct or stroke. However, there is an increased risk of severe bleeding among patients who are taking such medications. In order to optimize the therapeutic effect and avoid dangerous side effects like bleeding, careful monitoring is required during treatment, as directed by the 2006 American Heart Association (AHA)/American College of Cardiology (ACC) guidelines and the 2002-2003 Japanese guideline on anticoagulant therapy. Heparin is used in hospitals immediately before and after cardiac surgery, while oral medications like warfarin or aspirin are being adjusted. As oral anticoagulants are long-acting drugs, so these medications need to be stopped before elective surgery and the patient is switched to intravenous or subcutaneous shorter-acting heparin in hospital before the operation. However, heparin-induced thrombocytopenia (HIT) is a potentially life-threatening problem that is encountered in patients having cardiac surgery, and it is increasingly being recognized with the widespread use of heparin. The evidence suggests that heparin should not be used and that alternative anticoagulant therapy with one of the newer agents such as argatroban would be of benefit for avoiding harmful events.