Low dose aspirin is typically 75 to 100 mg per day, depending on where you live. In the United States, the standard low dose tablet is 81 mg. In the UK and much of Europe, it’s 75 mg. Both are far below the 300 to 325 mg tablets used for pain relief, and they serve a completely different purpose: preventing blood clots rather than treating headaches or inflammation.
Why the Dose Varies by Country
The reason you’ll see different numbers is simply the tablet sizes manufactured in different regions. The US settled on 81 mg (historically one-quarter of a standard 325 mg tablet), while British and European pharmacies stock 75 mg tablets. The World Health Organization references 75 mg in its guidelines, and the American College of Obstetricians and Gynecologists uses 81 mg. Both fall squarely in the low dose range and work the same way in the body.
You may also encounter 100 mg tablets in some countries. Anything at or below 100 mg taken daily is generally considered low dose aspirin. Once you move into the 300 to 325 mg range, you’re at a standard pain-relief dose, which carries a different risk profile.
How Low Dose Aspirin Works
At these small doses, aspirin isn’t acting as a painkiller. Instead, it permanently disables an enzyme inside platelets, the tiny blood cells responsible for clotting. When that enzyme is knocked out, platelets can no longer produce a substance called thromboxane A2, which normally tells platelets to clump together. Because platelets have no nucleus and can’t repair themselves, each platelet affected by aspirin stays disabled for its entire lifespan of about 7 to 10 days. Your body replaces roughly 10% of its platelets each day, so after stopping aspirin, it takes about a week for clotting to fully return to normal.
This is what makes aspirin unique compared to other anti-inflammatory drugs like ibuprofen or naproxen. Those drugs also affect the same enzyme, but they bind to it temporarily. Their antiplatelet effect wears off within hours as the drug clears your system. Aspirin’s effect is permanent and irreversible on each platelet it touches, which is why even a very small daily dose is enough to meaningfully reduce clotting.
Who Takes It and Why
Low dose aspirin is used in two very different situations, and the recommendations for each have diverged significantly in recent years.
After a Heart Attack or Stroke
For people who have already had a heart attack, stroke, or heart bypass surgery, daily low dose aspirin is a cornerstone of ongoing care. This is called secondary prevention. A large trial called ADAPTABLE compared 81 mg to 325 mg daily in patients with stable heart disease and found no significant difference in effectiveness or safety between the two doses. Because of this, most people in this group take the lower 81 mg dose, since it works just as well with no added benefit from the higher amount. In the period immediately following a heart attack, stroke, or bypass surgery, doses up to 300 mg daily may be used temporarily before stepping down.
For People Without Heart Disease
This is where guidelines have shifted dramatically. The U.S. Preventive Services Task Force now recommends against starting low dose aspirin for heart disease prevention if you’re 60 or older and have never had a cardiovascular event. For adults aged 40 to 59 with an estimated 10-year cardiovascular risk of 10% or greater, the decision is individualized. The task force describes the net benefit in this group as small, and it only applies to people who aren’t already at increased risk for bleeding.
The shift happened because newer, larger trials showed that the bleeding risks of daily aspirin often offset the heart benefits in otherwise healthy people, particularly as they age.
Low Dose Aspirin in Pregnancy
Pregnant women at high risk for preeclampsia, a dangerous blood pressure condition, are often advised to take low dose aspirin daily. The American College of Obstetricians and Gynecologists recommends 81 mg per day, ideally started between 12 and 16 weeks of pregnancy and continued until delivery. High-risk factors include a history of preeclampsia, chronic high blood pressure, diabetes, kidney disease, autoimmune conditions, or carrying multiples. The WHO and UK guidelines recommend 75 mg daily for the same purpose, reflecting regional tablet availability.
Bleeding Risks Are Real, Even at Low Doses
The word “low dose” can make aspirin sound harmless, but it carries measurable risks. In primary prevention studies, taking 100 mg or less of aspirin daily increased the risk of major gastrointestinal bleeding by about 58% and hemorrhagic stroke (bleeding in the brain) by about 27% compared to not taking aspirin. In practical terms, that translates to roughly 2 extra major bleeding events per 1,000 people per year among younger individuals with no other bleeding risk factors.
These numbers climb for people who are older, have a history of stomach ulcers, take blood thinners, or use other anti-inflammatory drugs regularly. This is precisely why the guidelines now weigh bleeding risk so heavily against cardiovascular benefit, especially for people over 60 who haven’t had a heart attack or stroke.
Enteric-Coated vs. Plain Aspirin
Many low dose aspirin tablets come with an enteric coating, a shell designed to keep the tablet intact through the stomach so it dissolves in the small intestine instead. The idea is to protect the stomach lining. In practice, the evidence is mixed. Enteric coating does reduce direct erosion of the stomach lining, but meta-analyses have not convincingly shown that it reduces actual gastrointestinal bleeding rates compared to uncoated aspirin.
There’s also a trade-off with absorption. Studies have found that uncoated aspirin achieves greater than 95% inhibition of the clot-promoting enzyme in all patients, while enteric-coated aspirin reaches that threshold in only about 87%. A secondary analysis of the ADAPTABLE trial found no significant clinical difference in heart attacks, strokes, deaths, or bleeding between enteric-coated and uncoated formulations. So while enteric coating isn’t harmful, it may slightly reduce aspirin’s antiplatelet potency without clearly improving safety.
The 81 mg vs. 325 mg Question
If you’ve been told to take daily aspirin, you might wonder whether the higher 325 mg dose offers better protection. The evidence suggests it doesn’t. The ADAPTABLE trial, which enrolled patients with established heart disease, found no meaningful difference in cardiovascular outcomes or bleeding between 81 mg and 325 mg. Most current guidelines point to 81 mg as the reasonable choice for long-term use, reserving higher doses for short-term situations like the acute phase after a cardiac event.