- In most studies, aspirin has been shown to be effective for a period of 1 year in reducing the frequency of saphenous vein bypass graft occlusion when begun 1 day before surgery, on the day of surgery, or the day after surgery. Effective doses of aspirin range from 100 to 975 mg/d. Aspirin in combination with dipyridamole is effective in the prevention of saphenous vein bypass graft occlusion if aspirin and dipyridamole therapy is started 1 or 2 days before surgery or aspirin therapy is started on the day of surgery but dipyridamole therapy is started before surgery or if treatment with both aspirin and dipyridamole is started 1 day after surgery. Aspirin in combination with dipyridamole is not more effective than aspirin alone in the prevention of saphenous vein graft occlusion. Bleeding is higher among patients treated with aspirin alone than among controls if aspirin therapy is started 1 day before surgery. Bleeding is not greater in comparison to controls if aspirin therapy is started the day of surgery or 1 day after surgery. When aspirin and dipyridamole are used in combination, bleeding is higher than in controls, and bleeding is higher than with aspirin alone. The continued use of aspirin for 2 additional years after an initial year of aspirin therapy for the prevention of saphenous vein bypass graft occlusion showed no additional long-term benefit on graft patency at the end of the third year.