Aspirin for Primary Prevention: Should Healthy People Take It Every Day?

For most of the second half of the 20th century and the first decades of the 21st, daily low-dose aspirin (typically 75–100mg) was one of the most widely recommended, simple "heart health" interventions — not just for patients who had already had a heart attack or stroke, but for healthy middle-aged and older adults who wanted to lower their risk before anything happened. The drug was cheap, available without a prescription, and had genuine, well-documented successes in a different patient group behind it, which easily translated in public understanding into a blanket "it's worth taking preventively."

The problem is that those documented successes came almost entirely from research on secondary prevention — that is, in people who had already had a heart attack, a stroke, or another cardiovascular event. In those patients, aspirin genuinely reduces the risk of a repeat event, and that recommendation still stands. Far less well studied for years was the other situation: primary prevention, meaning aspirin use in people who had never had a heart attack or stroke, taken purely "just in case" or because of age.

Three large randomized trials published in recent years — two analyses from the ASPREE trial and the ARRIVE trial — finally provided good-quality data specifically for primary prevention, and the results turned out to be consistent enough, and different enough from earlier practice, that they drove a genuine change in cardiology guidelines in many countries.

ASPREE (ASPirin in Reducing Events in the Elderly) is, to date, the largest randomized trial evaluating daily aspirin in pure primary prevention among older adults — the population where "just in case" intuitively seems to make the most sense given age alone.

In other words: in this large, well-designed group of healthy older adults, aspirin did not significantly reduce cardiovascular events — the difference between groups fell within the range of chance — while consistently and significantly increasing the risk of major bleeding. That directly contradicts the assumption that even if aspirin "doesn't help much," it "surely can't hurt" — ASPREE showed the harm was real and measurable, while the benefit was not.

The same ASPREE research program published a second analysis in parallel, in the same issue of NEJM, focused not on cardiovascular events but on all-cause mortality in that same group of participants. The result was unexpected enough that it deserves its own honest discussion — not as a sensational finding, but as a serious signal that requires careful interpretation.

It's essential to note that this finding concerns healthy people with no prior cardiovascular disease, taking aspirin purely preventively. It does not apply to patients already taking aspirin because of a prior heart attack or stroke — in that group, the documented benefits of preventing a repeat cardiovascular event most likely outweigh a signal of this kind.

One could argue that the ASPREE finding is specific to a very elderly population (≥70) and doesn't necessarily generalize to younger, moderate-risk healthy adults. The ARRIVE trial, published the same year in The Lancet, addresses exactly that question — in a different, younger group with a different baseline risk profile.

Aspirin works as an antiplatelet agent by irreversibly inhibiting the cyclooxygenase-1 (COX-1) enzyme in platelets, blocking production of thromboxane A2 — a substance essential for platelet aggregation and clot formation. That is the exact same mechanism that prevents a dangerous clot in a coronary or cerebral artery, while simultaneously impairing the body's ability to stop bleeding anywhere else — most commonly in the gastrointestinal tract, where the mucosal lining is especially vulnerable, but potentially intracranially as well.

In someone who already has an unstable atherosclerotic plaque and an elevated risk of a clot in a coronary vessel (typically, a patient with a prior heart attack or stroke), that platelet inhibition provides a clear, measurable benefit that statistically outweighs the bleeding risk. In a healthy person without such a plaque and without an elevated near-term risk of a clotting event, that same mechanism has, in practice, much less to prevent — while the bleeding risk remains identical to that of a high-risk patient, since the platelet-inhibiting mechanism operates the same way regardless of whether the person is actually at risk of a clot right now.

In other words: aspirin's benefit-to-risk balance depends almost entirely on a person's baseline risk of a clotting event. The lower that baseline risk — and in a healthy person with no prior cardiovascular event it is, by definition, lower than in a post-heart-attack patient — the smaller the potential benefit, while the bleeding risk stays roughly constant. That's why the same drug, at the same dose, can be clearly beneficial in one group of patients and neutral or harmful in another.

This distinction isn't an academic nuance — it has direct clinical relevance for millions of people who, over the years, took aspirin preventively based on general, unspecific advice like "take an aspirin for your heart," without ever clearly identifying which of these two groups they actually belonged to. It's worth noting that the change in guidance reflects the average risk-benefit balance across a population — an individual patient's specific risk factors may differ, which is why this decision should always be made together with a doctor rather than from an independent reading of the trial results.

The data from ASPREE and ARRIVE reflect the average risk-benefit balance in the studied populations — they don't mean preventive aspirin is pointless for absolutely every healthy person under every circumstance. Some cardiology guidelines still allow for considering low-dose aspirin in narrowly defined cases of very high documented cardiovascular risk in people without a prior event — but that decision requires an individual physician assessment accounting for the patient's specific risk profile, including bleeding risk, rather than a blanket, universal recommendation.

It's rare for a shift in decades-old, widely accepted clinical practice to rest on such consistent evidence: two independent trials (ASPREE and ARRIVE), in two different age groups, run by different research teams, reached the same conclusion — daily aspirin in healthy people with no prior cardiovascular event provides no significant cardiac benefit, while consistently increasing bleeding risk. The additional, serious signal on all-cause mortality from ASPREE — though it needs further research before it's fully explained — only strengthens the case for an individual, physician-guided decision rather than routine, self-directed use "just in case."