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A tenet of modern medicine tells us that Aspirin prevents heart attack and stroke. An Aspirin a day keeps the doctor away, the saying goes – a belief so deeply ingrained that about half of older adults take low-dose acetylsalicylic acid (ASA) daily.

But a flurry of new research is throwing cold water on Aspirin’s reputation as a panacea. Like all challenging of established dogma, however, the shift in scientific thinking requires some explanation, provisos and cautions.

People who have had a heart attack (a clot that stops blood flow to the heart) or an ischemic stroke (a clot that stops blood flow to the brain) – as well as those who have had a stent installed – are at significantly greater risk of having another.

Those who don’t take a daily Aspirin have two to three times the risk of another cardiac event. So it’s a well-established treatment.

This is known as secondary prevention, and the recommendations haven’t changed. But most people who take low doses (or ‘baby’ Aspirin) daily have never had a heart attack or stroke.

They do so based on the intuitive notion that, if blood clots can cause heart attacks and strokes, and Aspirin lessens clotting, then the drug will provide protection.

This theory was first floated in the 1960s, and by the early 1980s, large studies showed that that those who took a daily dose of ASA did indeed have fewer fatal heart problems.

That research has become less valid because, over the years, smoking rates have declined and we’ve become better at treating high blood pressure, so the number of fatal heart attacks and strokes have fallen sharply. In other words, Aspirin works as it always has, but its relative importance for prevention is not as high.

There is also greater recognition of the fact that every drug that has benefits also has risks.

In the case of Aspirin, the danger is gastrointestinal bleeding. To reduce that possibility, people take lower doses (which is good) and enteric-coated Aspirin (though there is little evidence that makes a difference.)

In a nutshell, new research published in the New England Journal of Medicine says that for people over 70 who have never had a heart attack or stroke, the risks of taking daily Aspirin outweigh the benefits. (Older people are at higher risk of internal bleeding. They are also far more likely to be taking other medications for conditions such as arthritis, and anticoagulants for illnesses such as chronic obstructive pulmonary disease (COPD), which also add to the risks of bleeding.)

Another study, published a month ago, reached a similar conclusion about prescribing Aspirin to patients with diabetes, saying the “absolute benefits were largely counterbalanced by the bleeding hazards.”

Does that mean people should stop taking Aspirin? Not quite. That’s because research has shown that those who stop this therapy abruptly could see their risk of heart attack and stroke jump – a rebound effect.

And what about people under 70? Current U.S. guidelines say that people 50 to 69 should only take daily Aspirin if they are at high risk of heart attack or stroke, but the definition of “high risk” varies.

The Canadian Cardiovascular Society is far less equivocal. Its guidelines state that daily Aspirin therapy is “not recommended without evidence of manifest vascular disease.”

The fact remains though that many doctors don’t follow the guidelines, and that many people self-medicate. And it’s no wonder they do, because Aspirin has a reputation as a pennies-a-day cure-all, or more precisely, prevent-all.

In addition to working its magic on the heart, past research has shown that daily consumers of Aspirin have lower rates of cancer (particularly colon cancer) and dementia. But the new research also revealed no reduction in cancer risk among daily Aspirin users – although that aspect of the study received far less media attention.

The notion that Aspirin therapy will reduce the risk of dementia is speculative, again based on the idea that if circulation of blood to the brain is better, dementia risk will fall. Proving that will be difficult at best.

The lack of clarity on the benefits and risks of Aspirin is frustrating for the public, particularly those who want to take control of their health. It is also a sobering reminder that there are no silver bullets.

The underlying causes of cardiovascular disease, cancer, diabetes and dementia are complex, so we shouldn’t expect prevention to be any less complicated.

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