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For decades, aging men and women have been told that a baby aspirin a day will keep heart attacks and strokes at bay, and millions have dutifully popped a low-dose acetylsalicylic acid (ASA) tablet daily.

But the longstanding conventional belief has slowly been overtaken by evidence, to the point where people over the age of 60 are now being advised that the daily routine probably does more harm than good.

Of course, the advice is more nuanced than a simple “it’s good or bad,” so it’s worth looking at the new recommendations of the influential U.S. Preventive Services Task Force (USPSTF) in some detail.

In a statement published in the Journal of the American Medical Association, the USPSTF says that people over 60 should not be prescribed (or self-prescribe) low-dose ASA because it has “no net benefit.”

More specifically, the slight decrease in the risk of heart attack and stroke that comes from taking ASA is offset by the risk of bleeding.

Acetylsalicylic acid hinders blood clotting; that’s good for preventing the formation of clots that cause myocardial infarctions and ischemic strokes, but it can also cause life-threatening bleeds in the stomach, intestines and brain.

The USPSTF says that for adults aged 40 to 69, there is a “small net benefit” from daily aspirin therapy, but the approach should be discussed with a patient’s physician.

The blue-ribbon committee also states that no one over the age of 75 should be taking daily ASA, as the risk of bleeds increases with age.

Finally, what is probably the most significant shift in the recommendations has received little media attention. The USPSTF, in a complete reversal of its 2016 advice, now says baby aspirin should not be taken to prevent colorectal cancer, because there is growing evidence doing so actually increases risk.

(A side note: In Canada, Aspirin is a brand name, and other ASA brands like Entrophen and Novasen are sold; in the U.S., ASA is known generically as aspirin.)

There are some important caveats in the recommendations, the most important of which is that people should not stop doing what they’re doing now, especially when it comes to the prevention of cardiovascular events.

If you’re taking baby aspirin now, continue. And the next time you have a doctor’s appointment, discuss whether this approach is still right for you.

And don’t assume your doctor knows about the new recommendations. It is hard enough for health professionals to keep up with the dizzying array of constantly updated guidelines and even harder for them to inform patients.

But baby aspirin therapy is something that deserves special attention because it is so common. According to Heart & Stroke (formerly the Heart and Stroke Foundation), about 5.3 million Canadians take low-dose ASA daily, including 2.4 million who do so without being directed by a physician.

The conditions we’re trying to prevent are also common – about 60,000 Canadians have heart attacks annually, and about the same number have strokes.

It’s also worth underscoring, in bold letters, that these recommendations are about primary prevention – meaning they pertain to how ASA should be used for people who have never had a heart attack or stroke, but may be at higher risk.

Taking baby aspirin remains a standard and important tool for secondary prevention – for people who have already had a heart attack or stroke or who have an implanted device like a stent.

The benefits of daily ASA therapy can also outweigh the risks for some people at high risk of cardiovascular events. (High-risk is defined as having a 10 per cent or greater risk of a cardio event in the next 10 years. There are many good risk calculators that can be found online, such as the Framingham CVD risk/benefit calculator.)

It can be frustrating for the public to see accepted wisdom crumble. How is it that baby aspirin was once a highly touted prevention tool, and now it’s viewed with skepticism?

Well, circumstances change. The initial studies that showed the value of low-dose ASA were done in the 1970s. They were done only on men, at a time when smoking was common and when we did a much worse job of controlling cardiovascular risk factors like high blood pressure and high cholesterol. Research at the time also tended to gloss over unintended consequences, like bleeding.

People also live longer. Taking baby aspirin daily at age 40 is very different than at age 70; and taking a drug for years or decades can also have consequences.

In short, the environment changed, our knowledge evolved, and our behaviour needs to follow suit.

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