Daily aspirin is taken to prevent blood clots that cause heart attacks and strokes. It works by permanently disabling an enzyme in platelets that promotes clotting, making it harder for dangerous blockages to form in your arteries. But the advice on who should take it has changed significantly in recent years, and for many people, the risks now outweigh the benefits.
How Aspirin Prevents Blood Clots
Aspirin blocks an enzyme called COX-1 inside platelets, the tiny blood cells responsible for clotting. What makes aspirin unique among pain relievers is that it does this permanently. Other anti-inflammatory drugs like ibuprofen or naproxen temporarily slow COX-1, but once those drugs wear off, platelet function returns to normal. Aspirin knocks COX-1 out for the entire lifespan of the platelet, which is about 7 to 10 days. Since your body produces new platelets continuously, taking aspirin daily keeps a large portion of your platelets unable to clump together and form clots.
This is why aspirin is so effective after a heart attack or stroke. The clots that cause these events form when platelets pile up at the site of a ruptured plaque in an artery. By keeping platelets less “sticky,” daily aspirin reduces the chance of that happening again.
Who Benefits Most: Secondary Prevention
The strongest case for daily aspirin is in people who have already had a heart attack, stroke, or been diagnosed with cardiovascular disease. This is called secondary prevention, meaning you’re trying to stop a second event. In large pooled analyses covering tens of thousands of patients, daily aspirin reduced the risk of dying from a cardiovascular cause by about 15%. Among 20,000 patients with a history of prior heart attack, aspirin produced a 4% absolute reduction in serious vascular events over two years.
Those numbers may sound modest in percentage terms, but for someone who has already survived a heart attack, a 4% absolute reduction translates to meaningful protection. For this group, the benefits clearly outweigh the bleeding risks, and daily aspirin remains a cornerstone of long-term treatment.
Why Guidelines Changed for Healthy Adults
For decades, doctors routinely recommended daily aspirin to healthy middle-aged adults as a preventive measure. That changed in 2022 when the U.S. Preventive Services Task Force updated its guidance based on accumulating evidence that bleeding risks were eating into the benefits for people who had never had a cardiovascular event.
The current recommendation is straightforward: adults 60 and older should not start taking daily aspirin for primary prevention. The task force concluded with moderate certainty that there is no net benefit for this age group. For adults aged 40 to 59 who have a 10% or greater chance of a cardiovascular event in the next 10 years, the decision is individual, meaning the benefit is small enough that it depends on your personal risk factors and preferences.
The shift happened because newer, larger trials consistently showed that when people are already managing their blood pressure, cholesterol, and blood sugar with modern treatments, the additional clot prevention from aspirin adds less benefit than it did in earlier decades. Meanwhile, the bleeding risk stays the same regardless of what other treatments you’re on.
The Bleeding Trade-Off
Aspirin’s clot-preventing power is also its main liability. By making platelets less able to clump, it makes all bleeding harder to stop, not just the dangerous kind. Low-dose aspirin (100 mg or less daily) increases the risk of major gastrointestinal bleeding by about 58% compared to not taking it. That’s a relative increase, and the absolute numbers are small for most people: roughly 1 to 6 extra major GI bleeding events per 10,000 people per year, depending on baseline risk.
Brain bleeding is the more serious concern. In a study of older adults, bleeding events inside the skull were 38% higher among those taking daily aspirin compared to placebo. These events were uncommon overall (187 total across the study), but hemorrhagic strokes and brain bleeds carry severe consequences when they do occur. The risk climbs with age, which is a key reason why starting aspirin after 60 is no longer recommended for people without existing heart disease.
81 mg vs. 325 mg
The ADAPTABLE trial, the largest head-to-head comparison of the two most common aspirin doses, found no meaningful difference in cardiovascular outcomes or major bleeding between 81 mg and 325 mg daily. People on the higher dose were more likely to stop taking it or switch to the lower dose on their own. Given comparable effectiveness, 81 mg is the preferred dose for most people on long-term aspirin therapy. It provides the same platelet-blocking benefit with better tolerability.
Why You Shouldn’t Stop Without a Plan
If you’ve been taking daily aspirin for a long time, stopping abruptly carries real risk. A large Swedish study found that discontinuing low-dose aspirin without a medical reason (like upcoming surgery or active bleeding) was associated with more than a 30% increased risk of cardiovascular events. The risk appeared almost immediately after stopping, with no safe grace period. Patients who missed their next scheduled refill experienced their first cardiovascular event roughly three times sooner than those who continued on schedule.
The likely explanation is a rebound effect. When aspirin is withdrawn, the body’s clotting system can temporarily overshoot, producing higher-than-normal levels of the clotting chemicals that aspirin had been suppressing. If you and your doctor decide aspirin is no longer appropriate, the transition should be managed rather than just skipped one day.
Aspirin and Colorectal Cancer
Aspirin has been shown to reduce the incidence of colorectal cancer, which generated excitement about an additional reason to take it daily. However, the picture is more complicated than early findings suggested. A recent phase 3 trial tested whether aspirin taken for three years after standard colorectal cancer treatment could prevent recurrence. Five-year disease-free survival was 77% in the aspirin group and 74.8% in the placebo group, a difference that was not statistically significant. While aspirin may still play a role in cancer prevention over longer time horizons, it is not currently a recommended reason to start daily aspirin on its own.
Ibuprofen Can Block Aspirin’s Benefits
If you take daily aspirin for heart protection and also use ibuprofen for pain, the timing matters. Ibuprofen competes for the same enzyme that aspirin targets on platelets. If ibuprofen gets there first, it can temporarily occupy the site and prevent aspirin from binding permanently, essentially canceling out aspirin’s heart-protective effect.
The FDA recommends a simple rule for immediate-release (non-enteric-coated) aspirin: take your aspirin at least 30 minutes before ibuprofen, or take ibuprofen at least 8 hours before your aspirin. Either approach preserves aspirin’s antiplatelet effect. Occasional ibuprofen use is unlikely to cause problems if you follow this timing, but regular daily use of both warrants a conversation about alternatives.