The short answer
- Low dose: 75 mg (UK), 100 mg (Australia) or 81 mg (US) once daily to prevent clots after a heart attack, stroke, stent or in some other conditions. It does not relieve pain.
- Pain-relief dose: 300–900 mg every 4–6 hours, maximum 4 g a day for adults. Not for anyone under 16.
- Don’t start or stop low-dose aspirin on your own. For healthy people, bleeding risks usually outweigh the benefit.
One molecule, two very different jobs
Aspirin (acetylsalicylic acid) permanently switches off the COX-1 enzyme in platelets, the blood cells that start clots. Platelets can’t make new enzyme, so a tiny daily dose keeps them less “sticky” for their whole 7–10 day life. That is the antiplatelet effect, and it needs only 75–100 mg.
Relieving pain, fever and inflammation requires blocking COX enzymes in tissues all over the body, which recover quickly. That takes much larger and repeated doses, typically 300–900 mg every 4–6 hours. At these doses aspirin behaves like other anti-inflammatories (NSAIDs) such as ibuprofen, with the same stomach and kidney cautions.
|
Low-dose aspirin |
Regular / high-dose aspirin |
| Dose |
75–100 mg once a day (81 mg in the US) |
300–900 mg every 4–6 h; max 4,000 mg/day |
| Purpose |
Prevent clots (heart attack, stroke) |
Pain, fever, migraine, inflammation |
| Typical products |
Cartia, Astrix, Aspro Clear low dose, “baby aspirin”, Bayer 81 mg |
Disprin, Aspro Clear, Bayer 325 mg |
| Usually |
Long term, on medical advice |
Short term, a few days |
| Main risks |
Bleeding (stomach, brain), bruising |
Stomach irritation and bleeding, asthma flare, ringing in the ears at high doses |
| Children |
Only on specialist advice (e.g. Kawasaki disease) |
Never under 16 (Reye’s syndrome) |
Who should take low-dose aspirin?
For people who have already had a heart attack, a stroke caused by a clot, a coronary stent or bypass, or who have peripheral arterial disease, daily low-dose aspirin (or another antiplatelet) clearly reduces the risk of another event. This is “secondary prevention”.
For healthy people without heart disease (“primary prevention”), recent large trials found that aspirin prevents few heart attacks while causing a similar number of serious bleeds. The US Preventive Services Task Force (2022) recommends against starting aspirin for primary prevention at age 60 or older, and only an individual decision with a doctor for people aged 40–59 at high cardiovascular risk. UK and Australian guidelines are similar. Aspirin may also be prescribed in pregnancy to people at high risk of pre-eclampsia, at specific doses and times decided by their obstetric team.
Don’t stop low-dose aspirin suddenly if a doctor prescribed it after a heart attack, stroke or stent: the risk of a clot can rise. Before surgery or dental work, ask the prescriber or surgeon what to do.
Aspirin for pain and migraine
Adults can take 300–900 mg every 4–6 hours as needed, up to 4 g a day, with food or water (soluble forms dissolve in water). A single 900 mg dose taken early is an evidence-based treatment for migraine. Paracetamol or ibuprofen is usually preferred for everyday pain because aspirin is harder on the stomach. See the pain relief guide to compare them.
Combining aspirin with other painkillers
- Paracetamol + aspirin: fine for adults.
- Pain-dose aspirin + ibuprofen, naproxen or diclofenac: avoid; two NSAIDs (see why NSAIDs shouldn’t be combined).
- Low-dose aspirin + occasional ibuprofen: ibuprofen can block aspirin from reaching the platelet enzyme. The US FDA advises taking ibuprofen at least 30 minutes after immediate-release low-dose aspirin, or more than 8 hours before it. Regular ibuprofen on top of low-dose aspirin also adds bleeding risk, so paracetamol is the better painkiller for people on aspirin.
Frequently asked questions
Is 300 mg aspirin the same as four 75 mg tablets?
The amount of aspirin is the same (300 mg), but low-dose tablets are often enteric-coated, so they act more slowly and less predictably for pain. Don’t use low-dose tablets as a painkiller, and don’t change your prescribed low dose without advice.
Does Nurofen contain aspirin?
No. Nurofen contains ibuprofen. Both are NSAIDs, so people with aspirin-sensitive asthma or aspirin allergy should avoid ibuprofen too.
Can I take Nurofen after taking aspirin?
If the aspirin was a pain-relief dose, wait until it would be due again (4–6 hours) and use paracetamol in between instead of mixing NSAIDs. If you take low-dose aspirin daily, follow the timing above and ask your pharmacist whether ibuprofen is suitable for you.
Why is aspirin sometimes prescribed twice a day after surgery?
After some orthopaedic operations, such as hip, knee or ankle surgery, surgeons may prescribe aspirin (for example 81 mg or 100 mg twice daily, or other regimens) for several weeks to prevent blood clots in the legs. Follow your surgeon’s specific instructions and don’t add other NSAIDs without asking.
Sources and further reading
CanforaMedicine Guide
