For decades the belief was that taking a baby aspirin daily would be effective in prevention of a heart attack, and it has been. Originally it was just a benefit for men until someone realized that the original study included men only. A subsequent study was conducted on both men and women and concluded that an aspirin daily was just as effective for women. Such a surprise! So for decades physicians have recommended that their patients, both men and women, take an 81 mg aspirin daily for heart attack prevention. The results showed a mild decrease in heart attacks.
In a more recent study conducted in cooperation between the United States and Australia it was found that even though the aspirin did indeed prevent heart attacks it also was the culprit in deaths due to internal bleeding. In fact the study concluded that more people died due to the bleeding side effect than the number who avoided heart attacks.
So the guideline has once again changed. If you have suffered a heart attack in the past, then, yes, you absolutely need to take the baby aspirin daily. However if you have never experienced heart disease, then you no longer are recommended to take that tiny aspirin.
This change in philosophy clearly represents how medical research evolves. A good idea is adopted following rigorous testing. The results are then reviewed over time. If unexpected side effects develop, such as the internal bleeding complication with aspirin, then that original idea is modified. This is precisely what has happened with the aspirin recommendation.