Taking aspirin daily thins your blood by permanently disabling the clotting ability of platelets, the tiny cell fragments that form blood clots. This is why a daily low-dose aspirin (typically 81 mg) can prevent heart attacks and strokes in people who already have cardiovascular disease. But it also increases bleeding risk, which is why the calculus has shifted significantly against daily aspirin for otherwise healthy people.
How Aspirin Affects Your Blood
Aspirin works differently from most drugs. It permanently shuts down an enzyme in your platelets that helps them clump together. Most medications wear off as your body processes them, but aspirin’s effect on each platelet lasts for that platelet’s entire lifespan, roughly 7 to 10 days. Your body produces new platelets continuously, so after you stop taking aspirin, it takes about a week for your clotting ability to fully return.
This permanent effect is what makes even a small daily dose powerful. Each day’s pill disables the fresh batch of platelets your body made overnight, keeping your blood consistently less “sticky.” That reduced stickiness is the entire basis for aspirin’s role in heart disease prevention.
The Case for Daily Aspirin: Secondary Prevention
If you’ve already had a heart attack, stroke, or been diagnosed with cardiovascular disease, daily aspirin remains a cornerstone of treatment. By keeping platelets from clumping, it lowers the chance of another clot forming in an already-narrowed artery. This is called secondary prevention, and decades of evidence support it.
One practical detail worth knowing: the standard 81 mg “baby aspirin” works just as well as the full 325 mg dose. The ADAPTABLE trial, a large randomized study, found no difference in effectiveness or safety between the two doses for people with established heart disease. Most physicians now default to 81 mg because there’s no upside to taking more.
Why Healthy People Are Now Told to Skip It
For years, millions of healthy adults took a daily aspirin hoping to prevent a first heart attack. That advice has changed. The U.S. Preventive Services Task Force now recommends against starting daily aspirin for heart disease prevention if you’re 60 or older, concluding with moderate certainty that the bleeding risks cancel out any cardiovascular benefit in this group.
The ASPREE trial, which followed more than 19,000 healthy adults over age 70 for nearly a decade, reinforced this shift. Aspirin did not extend healthy, disability-free life. It did, however, increase the risk of major bleeding events by 24% over the long term. For participants who were 80 or older at the start of the study, the results were worse: aspirin was associated with a 34% higher rate of persistent physical disability compared to placebo.
The core problem is math. In people without existing heart disease, the chance of a first heart attack or stroke is already low. Aspirin nudges that number down slightly, but it nudges bleeding risk up by a similar or greater amount. The trade-off simply doesn’t favor healthy people the way it favors those who’ve already had a cardiovascular event.
Gastrointestinal Bleeding
The most common serious side effect of daily aspirin is bleeding in the stomach or intestines. A large prospective study of men found that regular aspirin users had about 2.3 major GI bleeding events per 1,000 people per year, compared to 1.4 among non-users. That’s roughly one extra bleeding event for every 1,075 regular users annually. The numbers sound small individually, but across millions of daily users, they add up to a significant number of hospitalizations.
Daily users taking standard doses had a 42% higher risk of GI bleeding overall and a 63% higher risk of upper GI bleeding specifically, which includes stomach and esophageal bleeds. Higher doses pushed the upper GI risk even further, more than doubling it. This is one more reason the lower 81 mg dose is preferred when aspirin is genuinely needed.
Brain Bleeding Risk
Less commonly discussed is the increased risk of bleeding inside the skull. In the ASPREE trial of healthy older adults, those randomized to aspirin had a 38% higher rate of intracranial bleeding, including hemorrhagic stroke. The absolute numbers were small (about 2.5 events per 1,000 person-years on aspirin versus 1.8 on placebo), but intracranial bleeds are far more dangerous than GI bleeds, frequently causing permanent disability or death.
This risk is particularly relevant for older adults and anyone with uncontrolled high blood pressure, both of which independently raise the chance of a brain bleed.
Possible Colorectal Cancer Reduction
One of the more intriguing findings about long-term aspirin use is a potential reduction in colorectal cancer risk. A large follow-up study of women taking low-dose aspirin every other day found a 20% lower incidence of colorectal cancer overall, with a 42% reduction emerging in the years after the trial ended. The protective effect didn’t appear until at least 5 to 10 years of use, and it grew stronger with longer duration.
This is promising but comes with important context. The benefit takes a decade to materialize, and it has to be weighed against years of increased bleeding risk. No major guideline currently recommends starting aspirin solely for cancer prevention.
The Ibuprofen Interaction
If you take daily aspirin for your heart and occasionally use ibuprofen for pain, timing matters. Ibuprofen can block aspirin from reaching its target on platelets, essentially canceling out aspirin’s heart-protective effect. The FDA has laid out specific guidance: take your aspirin at least 30 minutes before ibuprofen, or wait at least 8 hours after taking ibuprofen before taking your aspirin. This applies to immediate-release (non-enteric-coated) aspirin at the standard 81 mg dose.
This interaction doesn’t apply to acetaminophen (Tylenol), which works through a completely different mechanism and is generally the safer choice for occasional pain relief if you’re on daily aspirin.
Why You Shouldn’t Stop Abruptly
If you’ve been taking daily aspirin for cardiovascular protection, stopping suddenly carries real risk. A Swedish study of more than 600,000 long-term aspirin users found that those who discontinued had a 37% higher rate of heart attacks, strokes, and other cardiovascular events compared to those who continued. That translated to roughly one additional cardiovascular event per year for every 74 people who stopped.
The risk increased almost immediately after discontinuation, with no safe grace period. Experimental evidence suggests a rebound effect: after aspirin is withdrawn, the body may temporarily overshoot in the other direction, producing more of the clotting compounds that aspirin had been suppressing. If you and your doctor decide to stop daily aspirin, it should be a deliberate, planned decision rather than something that happens because you ran out of pills or forgot for a few weeks.
Who Benefits and Who Doesn’t
The simplest way to think about daily aspirin in 2024: it remains valuable for people who already have heart disease, and it’s generally not worth starting if you don’t. The benefit-to-risk ratio is clearest for someone who has had a heart attack, stroke, stent placement, or bypass surgery. For a healthy 55-year-old with no cardiovascular disease, the math is far less favorable, and for a healthy person over 60, the major guidelines now advise against it entirely.
Age is the biggest factor driving this divide. Older adults bleed more easily, and their blood vessels are more fragile. The same drug that prevents a clot in a diseased artery can cause a dangerous bleed in the gut or brain when the baseline risk of bleeding is already elevated by age alone.