Anti-clotting drug. Molecular model of the anti- clotting drug clopidogrel. The atoms are represented as rods and are colour-coded: carbon (red), hydrogen (white), oxygen (green), nitrogen (yellow), sulphur (blue) and chlorine (purple). Clopidogrel is used to prevent the formation of blood clots in patients with a history of heart attack, stroke, or peripheral vascular disease. It works by inhibiting adenosine diphosphate, a substance that induces blood platelet cells to aggregate and form blood clots. It can be used by patients unable to take aspirin.