People take aspirin for three main reasons: to relieve pain and fever, to prevent blood clots after a heart attack or stroke, and in some cases to lower the risk of certain cancers. It’s one of the oldest and most widely used medications in the world, but how and why people use it has shifted significantly in recent years as new evidence has changed medical guidelines.
How Aspirin Works in the Body
Aspirin permanently disables an enzyme called COX-1 in your cells. This enzyme is responsible for producing chemicals that trigger pain, inflammation, fever, and blood clotting. What makes aspirin unusual compared to other pain relievers is that its effect is irreversible. Once aspirin shuts down COX-1 in a platelet (the tiny blood cells that form clots), that platelet can never clot normally again for the rest of its 10-day lifespan. A single dose accomplishes this within 60 minutes.
This permanent effect is why aspirin is so useful for heart-related conditions but also why it carries bleeding risks that other pain relievers don’t. Your body has to manufacture entirely new platelets before normal clotting returns.
Pain, Fever, and Inflammation
At a standard dose of 300 mg, aspirin relieves mild to moderate pain, brings down a fever, and reduces swelling. It works well for headaches, muscle aches, toothaches, and menstrual cramps. For these short-term uses, aspirin functions similarly to ibuprofen or naproxen, all of which block the same inflammation pathway.
Most people reaching for aspirin in their medicine cabinet are using it this way, and occasional use for pain or fever is generally straightforward. The risks that make headlines are tied to taking aspirin daily over months or years, not to popping one for a headache.
Preventing a Second Heart Attack or Stroke
The most important medical use of aspirin today is “secondary prevention,” meaning it’s given to people who have already had a heart attack, stroke, or other cardiovascular event. Because aspirin stops platelets from clumping together and forming dangerous clots, a low daily dose (typically 75 to 100 mg) helps keep arteries open in people whose blood vessels are already damaged or narrowed.
This is where aspirin’s benefits are clearest and least controversial. If you’ve had a heart attack or a stent placed, daily aspirin significantly reduces the chance of another event. The bleeding risk exists but is much smaller relative to the benefit. In people with a history of cardiovascular disease, the increase in upper gastrointestinal bleeding risk from low-dose aspirin is about 40% compared to nonusers, but in absolute terms that translates to roughly 1 bleeding case per 391 patients treated for a year.
Primary Prevention Has Changed
For decades, many healthy adults took a daily “baby aspirin” hoping to prevent a first heart attack. That advice has been largely reversed. The U.S. Preventive Services Task Force now recommends against starting low-dose aspirin for heart disease prevention in adults 60 or older, concluding with moderate certainty that it offers no net benefit in this group. The bleeding risks in older adults essentially cancel out the clot-prevention benefits.
For adults aged 40 to 59 with elevated cardiovascular risk, the decision is more individualized and depends on factors like cholesterol levels, blood pressure, and diabetes status. The shift happened because newer studies showed that modern treatments for high blood pressure and cholesterol have gotten good enough that adding aspirin on top doesn’t move the needle much for people who haven’t already had a cardiac event.
Colorectal Cancer Risk Reduction
One of the more surprising findings about aspirin is its apparent ability to reduce the risk of colorectal cancer. An analysis of two large Harvard-led studies found that taking aspirin for six or more years was linked to a 19% decreased risk of colorectal cancer and a 15% decreased risk of any gastrointestinal cancer. The results were even more dramatic in people with Lynch syndrome, a genetic condition that sharply raises cancer risk: participants in one clinical trial who took aspirin had a 63% reduction in their relative risk of developing colorectal cancer compared to those given a placebo.
Multiple randomized trials have also shown that aspirin at any dose can reduce the development of colon polyps, which are precursors to colorectal cancer. The USPSTF previously noted that for people aged 50 to 59 who are already taking aspirin for cardiovascular reasons, cancer risk reduction is an added benefit, though it takes at least 10 years of use to see meaningful effects.
Preeclampsia Prevention in Pregnancy
Low-dose aspirin (81 mg daily) is recommended for pregnant people at high risk of preeclampsia, a dangerous condition involving high blood pressure that can threaten both mother and baby. High-risk factors include a history of preeclampsia, carrying multiples, chronic high blood pressure, diabetes, kidney disease, or autoimmune conditions. People with two or more moderate risk factors, such as being over 35, having a BMI above 30, or conceiving through IVF, may also benefit.
In these cases, aspirin is typically started after 12 weeks of pregnancy. This is one of the few situations where aspirin is specifically recommended during pregnancy rather than avoided.
Bleeding Risks and Who Should Avoid It
Aspirin’s most significant side effect is increased bleeding, particularly in the stomach and intestines. For people without cardiovascular disease taking it for primary prevention, the risk of upper gastrointestinal bleeding roughly doubles compared to nonusers. In absolute numbers, about 1 in 601 healthy people taking daily low-dose aspirin for a year will experience a bleeding event. That may sound small, but across tens of millions of users, it adds up to a lot of bleeding episodes that might not have happened otherwise.
The risk is higher in people who are older, who drink alcohol regularly, who have a history of ulcers, or who take other medications that affect clotting. This is precisely why the guidelines shifted: for someone who hasn’t had a heart attack, the small chance of preventing one may not outweigh the small but real chance of a serious bleed.
Why Children Should Not Take Aspirin
Aspirin is not safe for children or teenagers with viral illnesses. It’s been linked to Reye’s syndrome, a rare but serious condition that causes swelling in the liver and brain. Symptoms typically appear three to five days after a viral infection begins and can include persistent vomiting, unusual sleepiness, sudden behavior changes, and in severe cases, seizures and loss of consciousness. In younger children, it may show up as diarrhea and rapid breathing. Children with certain fatty acid metabolism disorders are especially vulnerable.
Because of this risk, aspirin should not be given to anyone under 18 who has the flu, chickenpox, or any upper respiratory infection. Acetaminophen and ibuprofen are safer alternatives for managing fever and pain in children.