Aspirin to prevent a heart attack: what the evidence says about taking it daily

For decades, taking a baby aspirin every morning was sold as something close to life insurance for your heart. Today the picture is more nuanced: taking it because you have already had a heart attack or a stroke is not the same as taking it to prevent a first event when you are healthy. That distinction, the one separating secondary from primary prevention, is what has changed the recommendations in recent years. Here we explain what the major trials and guidelines say, with their figures, so you understand why the answer is no longer a yes for everybody.

What aspirin does in the body

Low-dose acetylsalicylic acid is an antiplatelet drug: it reduces the ability of platelets to clump together and form clots. It does so by blocking the synthesis of substances called prostaglandins, derived from arachidonic acid, both in the endothelium lining the blood vessels and in the platelets themselves. Put simply, it makes the blood clot less easily. That same mechanism explains both the good and the bad: if it prevents clots from blocking an artery, it protects against heart attack and stroke, but if it makes clotting harder in general, it also makes bleeding more likely.

Secondary prevention: when an event has already happened

Here the evidence is solid and in favour. In people who have already had a heart attack or a stroke, or who have established arterial disease, the large ATT Collaboration meta-analysis showed that aspirin reduced serious vascular events from 8.2% to 6.7% per year (p<0.0001), with reductions of around one fifth in total stroke (2.08% versus 2.54% per year, p=0.002). Coronary events fell by a similar proportion, also around one fifth (4.3% versus 5.3% per year, p<0.0001).

That is why the European guidelines continue to support its use in this setting. The 2021 ESC guideline recommends, for secondary prevention, treatment with aspirin at 75-100 mg a day or, in cases of intolerance, clopidogrel at 75 mg a day. The key point is that in these patients the risk of a new event is high, and that benefit outweighs the bleeding risk.

Primary prevention: here the balance shifts

The debate concerns healthy people who have never had an event. In primary prevention, the ATT Collaboration meta-analysis of 95,000 patients across six trials found a 12% proportional reduction in serious vascular events (0.51% with aspirin versus 0.57% in the control group per year, p=0.0001). It sounds good, but the caveat matters: bleeding risk was around 50% higher, and that offset much of the benefit. When your baseline risk of a heart attack is low, the absolute protection you gain is small and the danger of a haemorrhage stops being acceptable collateral damage.

The ASPREE trial made this fairly clear in healthy older adults. After a median of 4.7 years, cardiovascular disease was only slightly lower with aspirin (10.7 versus 11.3 events per 1,000 person-years; HR 0.95; 95% CI 0.83-1.08), a non-significant difference. Major bleeding, on the other hand, was higher: 3.8% versus 2.8% (HR 1.38; 95% CI 1.18-1.62; p<0.001). Nor did aspirin prolong disability-free survival over 5 years.

There was also a signal that should be interpreted cautiously: in ASPREE, all-cause mortality was 12.7 per 1,000 person-years with aspirin versus 11.1 with placebo (HR 1.14; 95% CI 1.01-1.29), and cancer was the main contributor, with 1.6 excess deaths per 1,000 person-years. The authors themselves called for caution: this is a signal, not an established causal relationship, and it should not be read as aspirin causing cancer.

What the guidelines say today

The recommendations have followed the evidence. The US Preventive Services Task Force (USPSTF) recommends that the decision to start low-dose aspirin for primary prevention in adults aged 40 to 59 with a 10-year cardiovascular risk of 10% or higher be taken on an individual basis (grade C), and recommends against starting it in adults aged 60 or over (grade D). In Europe, the 2021 ESC guideline advises against antiplatelet drugs in people at low or moderate cardiovascular risk, precisely because of the bleeding risk.

Even so, inertia carries weight. It has been estimated that between 23% and 46% of older adults in the United States are still taking low-dose aspirin for primary prevention despite the change in recommendations. That is a US population figure and cannot be extrapolated to Spain, but it illustrates how hard it is to unlearn a habit that for years was considered prudent.

Available products and authorised uses in Spain

In Spain, low-dose aspirin is marketed under brands including Adiro 100 mg, as gastro-resistant tablets authorised by the AEMPS (the Spanish Agency for Medicines and Medical Devices; the 300 mg presentation is authorised but not currently marketed), and as A.A.S. 100 mg. The authorised indications for A.A.S. 100 mg include prophylaxis of myocardial infarction and reinfarction in unstable angina and a history of infarction, prevention of aortocoronary bypass occlusion, and treatment of transient ischaemic attacks in men to reduce the risk of stroke.

There is one important practical detail: because of its delayed-release formulation, Adiro is not indicated in acute myocardial infarction; in an emergency it is recommended that the tablet be crushed or chewed to speed up absorption. And here is a myth worth dismantling with data: higher doses of aspirin have not shown greater efficacy and do increase the number of complications. More is not better.

When it is not appropriate, and precautions

A.A.S. 100 mg is contraindicated in patients with active or recurrent peptic ulcer, a history of gastric bleeding or perforation after taking aspirin or other NSAIDs, a bleeding disorder or bleeding risk, and in those with asthma or nasal polyps induced or worsened by acetylsalicylic acid. In addition, aspirin should not be given to under-16s with feverish illnesses, influenza (flu) or chickenpox, because of its association with Reye's syndrome, a rare but serious condition.

In summary, and what you can do today

What to take away: aspirin remains valuable when there has already been a heart attack, a stroke or established vascular disease, but in healthy people its role has narrowed because the bleeding outweighs the benefit. The decision depends on your real cardiovascular risk, and that calls for individual assessment, not an inherited habit. If you are also going through the menopause and want a doctor to look at your situation as a whole, what does exist in Spain today is menopause treatment with a prior assessment, prescription when appropriate by a registered doctor, and ongoing follow-up. The first step is to check whether you are eligible.