A daily low-dose aspirin is one of the most effective drugs in medicine for one group of people and a net harm, or at best a coin-toss, for almost everyone else. Whether it helps you turns on a single question: have you already had a cardiovascular event, or been diagnosed with the disease that causes them? This chapter sets out why, with the trial numbers, where the UK and US now stand, the few other situations in which aspirin is recommended, and the one thing not to do: change it on your own.
1. How it works, and why the dose is low
Aspirin permanently switches off an enzyme (COX-1) inside each platelet, the cell fragments that start a clot. Platelets have no nucleus, so they cannot make more of the enzyme; each one stays switched off for the rest of its 7–10 day life, and only about a tenth of the platelets are replaced each day. That is why a small daily dose — 75 mg in the UK, 81 mg in the US, 100 mg in Australia — keeps most of them disabled. The same enzyme protects the lining of the stomach, which is why aspirin causes ulcers and bleeding, and why higher doses add stomach harm without adding heart protection. The exception is the 300 mg dose chewed in a suspected heart attack: it works faster, not better, reaching every circulating platelet within minutes.
2. After a heart attack or stroke: among the strongest tools in medicine
For people who already have cardiovascular disease — secondary prevention — the evidence is decades old and has never wavered.
- In the heart attack itself: the ISIS-2 trial of 1988 gave 17,187 patients aspirin (160 mg daily) or placebo; deaths from vascular causes over five weeks fell from 11.8% to 9.4%, about 24 lives saved per 1,000 treated, without a significant rise in brain bleeds.
- In the years after: the 2009 Antithrombotic Trialists' analysis of the long-term trials found serious vascular events fell from about 8.2% to 6.7% a year: roughly one fewer heart attack, stroke or vascular death per 100 people each year, far more than the extra bleeds it causes.
Who this applies to: anyone after a heart attack, an ischaemic stroke or TIA, a stent, bypass surgery, or with angina or peripheral artery disease. After a stroke the UK usually switches to clopidogrel after two weeks (preventing the next stroke); after a heart attack aspirin is lifelong, alongside a second antiplatelet for the first year (after a heart attack). If you are in this group, the question is not whether aspirin works but how to take it safely (section 6).
3. If you have never had an event: why the advice changed
For people without cardiovascular disease — primary prevention — aspirin was widely recommended for decades. Three large trials published in 2018 changed that:
| Trial | Who | What it found |
|---|---|---|
| ASPREE | About 19,000 healthy people, mostly over 70, on 100 mg daily for about 4.7 years | No change in disability-free survival. Major bleeding rose from 2.8% to 3.8%. Slightly more deaths in the aspirin group, mainly from cancer: an unexplained finding that may be chance, but not one that supports aspirin |
| ASCEND | About 15,500 people with diabetes but no cardiovascular disease | Serious vascular events fell from 9.6% to 8.5%; major bleeding rose from 3.2% to 4.1%. The benefit and the harm roughly cancelled out, although most of the extra bleeds were non-fatal stomach bleeds while the prevented events included strokes and heart attacks |
| ARRIVE | About 12,500 people at moderate cardiovascular risk | No significant benefit; more stomach bleeding, mostly mild |
Why the old benefit shrank. The early trials were done when fewer people took statins, blood pressure was less well treated, and more people smoked. As those improved, people's baseline risk of a heart attack fell, and with it the number of events aspirin could prevent. Its bleeding risk did not fall. At today's risk levels, for most people the two sides of the ledger now come out about even, and bleeds are harder to reverse than the events they are traded against.
Where the guidelines now stand. NICE does not recommend aspirin for primary prevention. The US Preventive Services Task Force (2022) advises against starting it at 60 or over, and calls it an individual decision for people aged 40–59 with a 10-year risk of 10% or more and no raised bleeding risk, where the net benefit is small. Canadian and Australian guidance is broadly the same.
4. The coronary calcium exception, and how firm it is
The CAC chapter and the calculator say a low-dose aspirin "discussion" is reasonable when the calcium score is 100 or more. That rests on modelling from observational data (the MESA cohort), which estimated that at a score of 100 or more, with a low bleeding risk, the heart attacks prevented would outnumber the serious bleeds caused, and that below it they would not. It has not been tested in a randomised trial. The American 2019 primary-prevention guideline allows aspirin to be "considered" in selected adults of 40–70 at higher risk and without raised bleeding risk, which is the space the calcium score is used to fill; NICE has no equivalent. So: a reasonable conversation to have with a clinician if your score is 100 or more and your bleeding risk is low, not a prescription, and not something to start on your own.
5. Other situations where aspirin is recommended
| Situation | The advice |
|---|---|
| Pregnancy at higher risk of pre-eclampsia | NICE advises 75–150 mg daily from 12 weeks until the birth for women at high risk (including previous pre-eclampsia, chronic kidney disease, lupus or antiphospholipid syndrome, diabetes or chronic high blood pressure), or with two or more moderate risk factors. In the ASPRE trial it cut early (preterm) pre-eclampsia by more than half. Prescribed by the midwife or obstetrician; see also heart disease in women |
| Lynch syndrome | An inherited condition with a high risk of bowel cancer. NICE advises considering daily aspirin for at least two years, on the basis of the CAPP2 trial, as a decision with the genetics or bowel team. This is the one cancer-prevention use in UK guidance |
| After a blood clot, if anticoagulation stops | Aspirin 75–150 mg roughly halves recurrence for a couple of years, well short of an anticoagulant; an option for some people at the three-month review (after a clot) |
| Bowel cancer prevention in everyone else | No longer recommended. Older data suggested a benefit and it was once part of the American advice; it was dropped in 2022 after ASPREE failed to confirm it |
| Atrial fibrillation | Not recommended for stroke prevention: it barely reduces stroke in AF and bleeds as much as an anticoagulant (the rhythm chapter) |
| Long flights and car journeys | Not recommended for preventing clots in the legs (sitting still) |
6. Who should not take it, and taking it safely
- Children and teenagers under 16 should not be given aspirin unless a specialist prescribes it (for example in Kawasaki disease), because of the risk of Reye's syndrome, a rare but serious illness of the liver and brain.
- Aspirin allergy, or asthma made worse by aspirin or ibuprofen: avoid it and tell every doctor.
- A current stomach ulcer or recent bleeding: only with specialist advice.
- With an anticoagulant (warfarin, apixaban, rivaroxaban and others): the combination roughly doubles bleeding and is used only where a cardiologist has decided the benefit justifies it, usually for a limited time after a stent.
- Stomach protection: people over about 65, or with previous ulcers or reflux, or on a second antiplatelet, are usually given a proton-pump inhibitor (such as omeprazole or lansoprazole) with it. Take aspirin with food. Avoid ibuprofen and similar painkillers alongside it; paracetamol is the choice.
- Before surgery or dental work: most dental work is done on aspirin. For surgery, the decision to stop is the prescriber's and the surgeon's together, not the patient's.
7. Do not stop it, or start it, on your own
The guidelines above are about starting aspirin. Stopping is a different question. In a Swedish national study, people who stopped long-term low-dose aspirin had about a third more heart attacks and strokes over the following years than those who carried on, and the risk appeared within months. For anyone on it after an event, stopping can be dangerous. For anyone who started it years ago for primary prevention, stopping may well be the right answer now, but it is a conversation to have with your GP rather than a decision to make from a video or this page.
8. Aspirin against lifestyle: comparing like with like
It is often said that lifestyle "beats" aspirin for prevention. The direction is right; the arithmetic is usually not. The benefits of lifestyle are typically quoted as relative reductions and aspirin's as absolute ones, which flatters lifestyle. The Mediterranean diet trial PREDIMED reported about a 30% relative reduction in cardiovascular events, which was an absolute difference of roughly 1 to 1.5 percentage points over about five years — the same order as ASCEND. (PREDIMED's original paper was also retracted and republished in 2018 because of randomisation problems; the result held, but the evidence is a little weaker than it is often presented.) The Diabetes Prevention Programme's 58% reduction was in developing diabetes, not in heart attacks. Blood pressure lowering is powerful — about a fifth fewer major cardiovascular events for every 10 mmHg — but that evidence comes mostly from drug trials.
The fair summary: lifestyle changes carry no bleeding risk and help far more than the heart, which is their real advantage; they are not an alternative to aspirin for someone who needs it after an event, and the two are not competitors. The eating plan and the heart diet chapter are where to start.
9. Four things people believe that are not so
- "A lower dose tailored to my platelet test protects me as well." Adjusting antiplatelet treatment to platelet-function tests has been tried in trials (GRAVITAS, ARCTIC) without clear benefit, and doses far below 75 mg have not been shown to protect.
- "Willow bark has been used for thousands of years, so daily aspirin is safe." Long use of a plant remedy says nothing about the bleeding risk of a daily dose, which is well measured.
- "More is better: 300 mg a day, or twice a day." For heart protection it is not; above 75–100 mg only the stomach risk rises. Anyone taking higher doses should review it with their doctor.
- "Aspirin causes anaemia by interfering with iron." Aspirin can cause anaemia, but through slow, hidden bleeding from the gut; if you are on it and become tired or pale, that needs checking.
10. Where UK and US guidance differ
- Dose: 75 mg in the UK, 81 mg in the US (a quarter of the old 325 mg tablet), 100 mg in much of Europe and Australia. The trials support all three.
- Primary prevention: NICE does not recommend it. The USPSTF allows an individual decision at 40–59 with a 10-year risk of 10% or more and advises against starting at 60 or over; the American College of Cardiology and American Heart Association allow it to be considered in selected higher-risk adults of 40–70 without raised bleeding risk.
- Secondary prevention, the acute 300 mg dose, pre-eclampsia and Lynch syndrome are the same in both.