Aspirin is used to relieve pain, reduce fever, lower inflammation, and prevent blood clots. It’s one of the most versatile medications in existence, sold over the counter for headaches and prescribed by cardiologists to prevent heart attacks. A single 325 mg tablet can knock out a toothache, and a daily low-dose regimen can reduce the risk of a second stroke by nearly 20%.
How Aspirin Works in the Body
Aspirin belongs to a class of drugs called NSAIDs (nonsteroidal anti-inflammatory drugs), but it works differently from ibuprofen or naproxen. It permanently disables an enzyme called COX-1 by chemically attaching an acetyl group to it. This enzyme is responsible for producing prostaglandins, compounds that trigger pain, fever, inflammation, and the clumping of blood-clotting cells called platelets.
Because aspirin permanently modifies the enzyme rather than just temporarily blocking it, its effects on platelets last for the entire lifespan of those cells, roughly 7 to 10 days. That’s why a single dose can thin your blood for over a week, and why surgeons ask you to stop taking aspirin well before a procedure.
Pain and Fever Relief
The most common reason people reach for aspirin is everyday pain. It’s effective for headaches, muscle aches, toothaches, menstrual cramps, and the minor joint pain of arthritis. It also brings down fevers from colds and flu.
The standard adult dose is one or two 325 mg tablets every four hours, or three tablets every six hours, with a full glass of water. You should not exceed 12 tablets (3,900 mg) in 24 hours. At these doses, aspirin works comparably to ibuprofen for most types of mild to moderate pain.
Reducing Inflammation
Aspirin has a long history as an anti-inflammatory drug, particularly for conditions like rheumatoid arthritis. The catch is that meaningful anti-inflammatory effects require much higher doses, typically 3,600 to 5,000 mg per day, far above what you’d take for a headache. At those levels, aspirin relieves joint inflammation about as well as ibuprofen, but causes significantly more side effects, especially stomach problems. For that reason, most doctors now choose other NSAIDs or newer drugs for chronic inflammatory conditions.
One exception is Kawasaki disease, a childhood condition that inflames blood vessels. High-dose aspirin remains a standard part of treatment in that specific case, under close medical supervision.
Heart Attack Prevention and Emergency Use
Aspirin’s ability to stop platelets from clumping makes it a powerful tool against heart attacks. A clot forming inside a coronary artery is what causes most heart attacks, and aspirin directly interferes with that process.
If you or someone nearby is having a suspected heart attack, chewing a 325 mg aspirin tablet while waiting for emergency services can be lifesaving. Chewing gets the drug into the bloodstream faster than swallowing it whole. This is one of the few situations where taking a medication before reaching the hospital can make a meaningful difference in survival.
For long-term prevention, the picture is more nuanced. If you’ve already had a heart attack or stroke, daily low-dose aspirin (typically 81 mg) is a well-established strategy. A large meta-analysis found that antiplatelet therapy reduces the risk of serious vascular events, including repeat heart attacks, strokes, and vascular death, by about 25% in people with existing cardiovascular disease.
For people who have never had a heart attack or stroke, the guidelines have shifted. The U.S. Preventive Services Task Force recommends that adults aged 40 to 59 with a higher cardiovascular risk discuss with their doctor whether daily aspirin makes sense for them. Adults 60 and older should not start taking aspirin solely to prevent a first heart attack or stroke, because the bleeding risks begin to outweigh the benefits at that age.
Stroke Prevention
After an ischemic stroke (the type caused by a clot), aspirin is one of the first-line treatments to prevent another one. In trials comparing aspirin to placebo in people who had already suffered a stroke or a transient ischemic attack (mini-stroke), aspirin reduced the risk of a recurrent stroke by about 18%. For the combined outcome of stroke, heart attack, or vascular death, the reduction was 13%. These are modest numbers individually, but across millions of people taking aspirin after a stroke, they translate into tens of thousands of prevented events each year.
Blood Clot Prevention After Surgery
Joint replacement surgery and fracture repair carry a significant risk of blood clots forming in the deep veins of the legs, which can travel to the lungs and become life-threatening. Hospitals have traditionally used injectable blood thinners to prevent this, but recent evidence published in the New England Journal of Medicine found that low-dose aspirin (81 mg twice daily) works as a viable alternative after orthopedic surgery, with a more favorable safety profile. This is changing practice in many surgical centers, offering patients a pill instead of an injection during their hospital stay.
Risks and Side Effects
Aspirin’s biggest risk is bleeding, particularly in the gastrointestinal tract. Even at low doses taken daily, the incidence of GI bleeding ranges from about 0.5 to 3.6 cases per 1,000 people per year. Upper GI bleeding (stomach and upper intestine) is more common than lower GI bleeding. The risk climbs with age, higher doses, and concurrent use of alcohol or other blood-thinning medications.
Stomach irritation, heartburn, and nausea are the most frequent day-to-day complaints. Taking aspirin with food or using enteric-coated formulations can reduce stomach discomfort, though they don’t eliminate the underlying bleeding risk from long-term use.
Why Children Should Not Take Aspirin
Aspirin is not safe for children and teenagers with fevers. It’s linked to Reye’s syndrome, a rare but potentially fatal condition that causes swelling in the liver and brain. In the U.S., the FDA recommends against giving aspirin to anyone under 19 during episodes of fever. The UK sets the cutoff at 16. This applies to any product containing aspirin, including some combination cold medicines. Acetaminophen and ibuprofen are the standard alternatives for fever and pain in younger age groups.