For decades, a daily low-dose aspirin was one of the most familiar habits of middle age. It was cheap, it was available without a prescription, and it seemed to make obvious sense: if aspirin prevents blood clots, and clots cause heart attacks and strokes, why not take it before the first event? Then, within a few weeks in 2018, three large trials changed the picture. Four years later the US Preventive Services Task Force formally reversed its advice. The story of aspirin is a story about bleeding and benefit — and about how medicine corrects itself when better evidence arrives.
Two very different situations
Everything in this article depends on one distinction. Secondary prevention means preventing a further event in someone who has already had a heart attack, an ischaemic stroke, a stent or bypass surgery, or who has known arterial disease. Primary prevention means preventing a first event in someone who has none of these.
The Antithrombotic Trialists' Collaboration (2009) pooled individual data from both settings. In secondary prevention, aspirin reduced serious vascular events from 8.2% to 6.7% per year — a large absolute benefit. In primary prevention, the reduction was from 0.57% to 0.51% per year, a 12% relative reduction but only 0.06 percentage points in absolute terms, while major gastrointestinal and other bleeds outside the brain rose from 0.07% to 0.10% per year. Already then, the balance in healthy people looked fine-grained.
2018: three trials, one message
ARRIVE
12,546 people at moderate cardiovascular risk, average age 64, aspirin 100 mg or placebo for a median of five years. Cardiovascular events: 4.3% vs 4.5% — no significant difference. Gastrointestinal bleeding: 0.97% vs 0.46%, roughly doubled.
ASCEND
15,480 people with diabetes but no cardiovascular disease, followed for 7.4 years. Serious vascular events fell from 9.6% to 8.5% (12% relative reduction), but major bleeding rose from 3.2% to 4.1%. Per 1,000 people: about 11 events prevented, about 9 major bleeds caused. Gastrointestinal cancer was unchanged.
ASPREE
19,114 healthy people aged 70 or older (65 for Black and Hispanic participants), median age 74, followed for 4.7 years. Aspirin did not prolong disability-free survival and did not significantly reduce cardiovascular events (10.7 vs 11.3 per 1,000 person-years). Major haemorrhage rose from 6.2 to 8.6 per 1,000 person-years. Unexpectedly, all-cause mortality was higher with aspirin (5.9% vs 5.2%), driven by cancer deaths.
Why did the older trials look better? Part of the likely answer is that the world changed: fewer people smoke, and many more take statins and blood pressure drugs, so the baseline risk that aspirin can reduce is lower — while the bleeding risk it adds is not.
2022: the guidelines reverse
In 2016, the USPSTF had recommended that adults aged 50–59 with a 10-year cardiovascular risk of 10% or more start low-dose aspirin, partly because it was also expected to prevent colorectal cancer. In April 2022, after reviewing the new evidence, it changed course. For adults aged 40–59 with a 10-year risk of 10% or more, starting aspirin is now an individual decision with a small net benefit (Grade C). For adults aged 60 and older, the Task Force recommends against starting aspirin for primary prevention (Grade D). Its pooled estimates: about 12% fewer non-fatal heart attacks and ischaemic strokes, but about 58% more major gastrointestinal bleeding and about 31% more bleeding inside the skull. People already taking aspirin may consider stopping at around age 75.
Europe had moved earlier. The 2021 European Society of Cardiology (ESC) prevention guideline states that antiplatelet therapy is not recommended for people at low or moderate risk because of the increased risk of major bleeding (Class III). For people with diabetes at high or very high cardiovascular risk, low-dose aspirin may be considered in the absence of clear contraindications (Class IIb).
What about colorectal cancer?
The hope that aspirin prevents bowel cancer was one of the strongest arguments for taking it. The USPSTF now considers the evidence inadequate: in the Women's Health Study, an apparent effect did not persist with longer follow-up, and ASPREE showed more colorectal cancer deaths with aspirin. A 2026 follow-up of ASPREE over a median of 8.6 years found no effect on overall cancer incidence (hazard ratio 0.98) or colorectal cancer (1.01), but higher cancer mortality (1.15). In healthy older adults, aspirin should not be taken for cancer prevention.
Medicine did not get aspirin wrong out of carelessness. It acted on the best evidence available — and then changed its advice when larger, better trials in today's patients showed a different balance. That is how evidence-based medicine is supposed to work.
How medicine corrects itself
The aspirin story illustrates several lessons. Plausible mechanisms are not proof of benefit. Older trials may not apply to populations whose risk has since changed. Relative reductions can conceal tiny absolute gains. And harms that are rare per person — a bleed here, a haemorrhagic stroke there — add up when millions of healthy people take a drug. The reversal was not a failure of science; it was science working as intended.
Do not stop on your own
If you take aspirin because you have had a heart attack, stroke, stent, bypass surgery or have known arterial disease, the evidence for benefit remains strong. Do not stop without talking to your doctor. Even if you take aspirin for primary prevention, discuss stopping with your doctor rather than deciding alone.
What this means for you
Ask yourself — and your doctor — why you take aspirin. If you have never had a cardiovascular event, ask: What is my 10-year risk? How high is my personal bleeding risk? Would a statin or better blood pressure control achieve more with less risk? If you are over 60 and have no cardiovascular disease, starting aspirin is generally not recommended.
Bottom line
Low-dose aspirin for people who already have cardiovascular disease: recommended (Evidence: ★★★ High). For healthy people, especially from age 60: not recommended for routine primary prevention, because bleeding largely cancels out the small benefit (Evidence: — Not recommended). Possible exceptions in younger people with high risk and low bleeding risk are an individual decision.
References
- Antithrombotic Trialists' (ATT) Collaboration. Aspirin in the primary and secondary prevention of vascular disease: collaborative meta-analysis of individual participant data from randomised trials. Lancet. 2009;373(9678):1849-1860. doi:10.1016/S0140-6736(09)60503-1
- Gaziano JM, Brotons C, Coppolecchia R, et al. Use of aspirin to reduce risk of initial vascular events in patients at moderate risk of cardiovascular disease (ARRIVE): a randomised, double-blind, placebo-controlled trial. Lancet. 2018;392(10152):1036-1046. doi:10.1016/S0140-6736(18)31924-X
- ASCEND Study Collaborative Group; Bowman L, Mafham M, et al. Effects of aspirin for primary prevention in persons with diabetes mellitus. N Engl J Med. 2018;379(16):1529-1539. doi:10.1056/NEJMoa1804988
- McNeil JJ, Wolfe R, Woods RL, et al. Effect of aspirin on cardiovascular events and bleeding in the healthy elderly. N Engl J Med. 2018;379(16):1509-1518. doi:10.1056/NEJMoa1805819
- McNeil JJ, Nelson MR, Woods RL, et al. Effect of aspirin on all-cause mortality in the healthy elderly. N Engl J Med. 2018;379(16):1519-1528.
- US Preventive Services Task Force. Aspirin use to prevent cardiovascular disease: US Preventive Services Task Force recommendation statement. JAMA. 2022;327(16):1577. doi:10.1001/jama.2022.4983
- Visseren FLJ, Mach F, Smulders YM, et al. 2021 ESC Guidelines on cardiovascular disease prevention in clinical practice. Eur Heart J. 2021;42(34):3227-3337.
- Orchard SG, Polekhina G, Zalcberg J, et al. Cancer incidence and mortality with aspirin in older adults: follow-up of the ASPREE trial. JAMA Oncol. 2026;12(3):285-294. doi:10.1001/jamaoncol.2025.6196
This article is general information and not individual medical advice. Decisions about tests and treatment belong in a conversation with your own physician.