Combining low-dose aspirin with another antithrombotic agent, such as clopidogrel, dipyridamole or a vitamin K antagonist, greatly increases the risk of serious upper gastrointestinal (GI) bleeding, say Danish researchers. They conducted a case-control study, involving 1443 cases of serious upper GI bleeding and 57 720 age- and sex-matched controls, and measured exposure to low-dose aspirin, antithrombotic agents and some other drugs. They found that combination therapy was associated with higher rates of bleeding. The greatest risk came from combining aspirin with clopidogrel (odds ratio [OR], 7.4), followed by aspirin with a vitamin K antagonist (OR, 5.3). The researchers said that this effect reflected a true synergism as the effect of combined treatment was more than a simple addition of the effects of the individual drugs.
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