What do the data say about aspirin for primary prevention?

Early trials that treated patients with aspirin during a heart attack or stroke found they were more likely to survive. But these trials also found that leaving patients on aspirin for months and years afterwards reduced future heart attacks and strokes.

For every 100 patients who’ve had a heart attack or stroke and stay on a daily aspirin, five recurrent heart attacks or strokes are prevented in the next year. While there is a small risk of serious bleeding with aspirin (mostly stomach bleeding, but also bleeding in the brain), fewer than one in 100 patients experience this. Therefore, everyone agrees that the benefits of aspirin outweigh the risks for people who have had a heart attack or stroke.

But for patients with no history of a heart attack or stroke, the data say that the risk of serious bleeds and the benefit of reducing heart attacks and strokes are about equal for a population of middle-aged and older adults.

According to the new calculations in the task force guidelines, for every 100 men 55-60 years old with an average risk of a first heart attack or stroke of 1% per year, starting a daily aspirin would have a lifetime effect of avoiding about two heart attacks and one stroke among those 100 men, but causing about three serious stomach bleeds and about one hemorrhagic stroke.