Patients having a predisposition for thrombus formation are effectively treated with compositions of acetylsalicylic acid, citric acid, thiamine and/or a zinc salt. The combination of aspirin and citric acid has been found to be significantly more effective in inhibiting platelet aggregability than aspirin alone, and thiamine contributes to the reduction in thrombotic potential by reducing plasma fibrinogen levels. Accordingly, new methods and compositions are disclosed for reducing the thrombotic potential of human or animal subjects, such as in the prophylaxis or treatment of atherosclerosis, vascular surgery patients or other cardiovascular diseases.