A study review suggests that taking people who do not have heart disease to aspirin daily to prevent heart attacks and strokes may increase the risk of severe brain bleeding beyond any potential benefit.
American doctors have long advised adults who have not had a heart attack or a stroke but are very vulnerable to these crises to take aspirin pills daily as a form of primary prevention.
Although there is clear evidence that aspirin is useful, many doctors and patients are hesitant to follow recommendations because of the risk of rare but fatal internal bleeding.
During the current study, the researchers reviewed data from 13 clinical trials on aspirin effects involving more than 134,000 adults.
The risk of brain hemorrhage was rare and the study found that taking aspirin was associated with two additional cases of this type of internal bleeding per 1,000 people.
But the risk of bleeding was 37 percent higher in people taking aspirin than those who did not.
"Intrapartic hemorrhage is a particular concern because it is strongly associated with increased risk of death and poor health over the years of life," said Dr Ming Li of the Changjung University School of Medicine in Taiwan.
"These results suggest that caution should be exercised with regard to low-dose aspirin use in individuals who do not have symptoms of cardiovascular disease," he said in an e-mail.
For people who have already had a heart attack or a stroke, there is evidence to take advantage of low-dose aspirin to prevent other major complications in the heart. But the researchers wrote that the value of aspirin is less clear in healthy people who may have a greater risk of bleeding than aspirin.
The guidelines for taking aspirin for primary prevention of heart disease in the United States, Europe and Australia already indicate the need to balance potential benefits with the risk of bleeding. For older people who are at risk of bleeding more than younger adults, the risk may be greater than any benefit from aspirin.