The present invention provides a method of ameliorating the drawbacks of anti-platelet drug named clopidogrel. The method of the resent invention comprises administration of an (S)-oxo-clopidogrel or its derivative of the Formula IIA in its free or pharmaceutically acceptable salt form for alleviating the symptoms of thrombosis or embolism by inhibiting blood platelet aggregation. Composition for use in such methods comprising an (S)-oxo-clopidogrel or its derivative of the Formula IIA or a pharmaceutically acceptable salt thereof are also provided.