One study looked at more than 12,000 patients at moderate risk of heart problems because of other health issues, such as high blood pressure, high cholesterol or smoking. The results showed no benefit.
Doctor J. Michael Gaziano of Brigham and Women's Hospital suggests that is because those people already were taking other medications that lowered their risk.
"Risk that a person has changes over time, and some of that change is due to some of the things that we do, like managing their risk factors and taking care of them when they develop symptoms," Gaziano said.
Dr. Jane Armitage of the University of Oxford led another study of more than 15,000 adults with diabetes that found the benefits of aspirin were outweighed by a greater risk of serious bleeding.
"We also saw almost a 30 percent increased risk in major bleeding," Armitage said. "So that was bleeding bad enough to get you into hospital. Mainly from the gut, or bleeding into the eye or the brain and if it was into the eye, it was bad enough to threaten your sight."
Based on results of the studies, disclosed over the weekend at the European Society of Cardiology, doctors say aspirin best benefits patients who already have heart disease.