Daily aspirin carries a well-documented set of side effects, most of them tied to the same property that makes it useful: it prevents blood from clotting normally. For the millions of people taking a daily low-dose aspirin (typically 81 mg), this means an increased risk of bleeding throughout the body, from minor bruising to serious gastrointestinal or brain bleeds. The balance between benefit and risk depends heavily on your age, your cardiovascular history, and what other medications you take.
How Daily Aspirin Affects Your Body
Aspirin works by permanently disabling an enzyme called cyclooxygenase (COX) in your platelets, the tiny blood cells responsible for forming clots. Once aspirin has acted on a platelet, that platelet can never clot normally again for the rest of its roughly 10-day lifespan. This is what makes daily aspirin effective at preventing heart attacks and strokes in people who already have cardiovascular disease. But COX also plays protective roles elsewhere, particularly in the stomach lining, where it helps produce the mucus barrier that shields tissue from digestive acid.
By suppressing that protective system, daily aspirin makes your stomach lining more vulnerable to injury. It reduces mucus and bicarbonate secretion, changes the physical properties of the mucus layer, and lowers the surface’s ability to repel acid. The drug itself also has a direct toxic effect on stomach tissue. These two mechanisms work together, which is why gastrointestinal problems are the most common side effect of long-term use.
Stomach and Intestinal Bleeding
Gastrointestinal bleeding is the side effect that gets the most attention, and for good reason. Daily aspirin can cause anything from mild stomach irritation and heartburn to ulcers and serious internal bleeding. Bleeding events overall are about 38% higher among people taking daily aspirin compared to those who aren’t, regardless of gender, age, or baseline cardiovascular risk.
You might first notice dark or tarry stools, which signal bleeding somewhere in the digestive tract, or you might experience persistent stomach pain, nausea, or vomiting that looks like coffee grounds. Some people bleed slowly enough that they develop iron-deficiency anemia over months without any obvious symptoms. The risk climbs with age, with a history of ulcers, and if you drink alcohol regularly.
One question many people have is whether the lower 81 mg dose is meaningfully safer for the stomach than the full 325 mg tablet. A large trial comparing the two doses in patients with established heart disease found no significant difference in hospitalization rates for major bleeding: 0.63% for the 81 mg group versus 0.60% for the 325 mg group. That said, people on the higher dose were more likely to switch down because of bruising and bleeding side effects, which may have blurred the comparison. In practice, most doctors prescribe the lower dose because it offers the same cardiovascular benefit with at least some reduction in day-to-day discomfort.
Increased Risk of Brain Bleeding
The same anti-clotting effect that protects against clot-caused strokes slightly raises the risk of the other type: hemorrhagic stroke, where a blood vessel in the brain ruptures. In pooled clinical data, hemorrhagic stroke occurred at a rate of about 1.2 per 1,000 person-years in aspirin users compared to 0.9 per 1,000 person-years with placebo. That’s a small absolute increase, but it becomes more relevant as you get older or if you have uncontrolled high blood pressure, both of which independently raise the risk of brain bleeds.
Falls add another layer of concern. For older adults who fall frequently, daily aspirin can turn a minor head bump into a more serious bleeding event inside the skull.
Bruising and Prolonged Bleeding
Easy bruising is one of the most noticeable everyday side effects. Because your platelets can’t clump together as effectively, even minor bumps leave visible bruises, and small cuts take longer to stop bleeding. This is generally harmless but can be alarming if you’re not expecting it. Some people notice bruises appearing with no memory of an injury at all. If bruising becomes severe, spreads rapidly, or is accompanied by bleeding gums, nosebleeds that won’t stop, or blood in your urine, those are signs that the bleeding effect has become more than cosmetic.
Effects on Kidney Function
Long-term daily aspirin use has been linked to a faster decline in kidney function. In a large study of chronic kidney disease patients, aspirin users had a 30% higher likelihood of experiencing a steeper drop in their kidney filtration rate compared to non-users. The leading explanation is that aspirin alters blood flow within the kidneys by blocking the same prostaglandins that help regulate it. This doesn’t appear to increase the risk of complete kidney failure, but for people whose kidneys are already compromised, the accelerated decline matters.
Hearing Changes at Higher Doses
Ringing in the ears (tinnitus) and temporary hearing loss are well-established effects of aspirin, though they typically require doses far above the standard daily 81 mg. These symptoms generally appear at doses of 4 grams per day or more, which is roughly 50 times a low-dose tablet. At those levels, hearing loss can range from 15 to 40 decibels, roughly the difference between hearing a whisper easily and not hearing it at all.
The good news is that with short-term use, these effects are completely reversible. In documented cases, even severe hearing loss from a large single dose resolved within a day or two as aspirin cleared the body, with hearing tests returning to normal. Long-term high-dose use, however, may cause a more lasting form of nerve damage. At the 81 mg dose used for heart protection, clinically significant hearing effects are unlikely.
Interactions With Common Pain Relievers
If you take daily aspirin and reach for ibuprofen when you have a headache, you could be undermining the reason you’re taking aspirin in the first place. Ibuprofen binds to the same enzyme site on platelets that aspirin targets. When ibuprofen gets there first, it physically blocks aspirin from doing its job. The result is that your platelets retain more of their clotting ability, reducing aspirin’s protective cardiovascular effect.
This isn’t a theoretical concern. Studies show that when ibuprofen is taken before aspirin, the anti-clotting effect is clearly suppressed. If you need an occasional pain reliever while on daily aspirin, the safest approach is to take your aspirin first and wait at least 30 minutes, or choose a pain reliever from a different class, like acetaminophen, which doesn’t interfere with platelet function.
What Happens if You Stop Suddenly
Abruptly stopping daily aspirin after long-term use carries its own risk. A large Swedish study of over 600,000 long-term aspirin users found that the 15% who stopped taking it experienced a 37% increase in cardiovascular events. For people taking aspirin because they’d already had a heart attack or stroke, the jump was even steeper: a 46% increase. Even among those using it purely for prevention, stopping was associated with a 28% rise in events.
The spike in risk appears to reflect a rebound effect. After months or years of suppressed clotting, the body may overcompensate when aspirin is withdrawn, making blood temporarily more prone to clotting than it was before you ever started taking it. This is why doctors taper or carefully time any discontinuation, especially before surgery.
Who Should and Shouldn’t Take It
Current guidelines from the U.S. Preventive Services Task Force draw a clear line by age and cardiovascular history. Adults 40 to 59 who have a higher risk of cardiovascular disease should discuss with their doctor whether the benefits outweigh the bleeding risks. People 60 or older should not start taking daily aspirin to prevent a first heart attack or stroke, because the bleeding risks at that age generally cancel out the cardiovascular benefit.
For people who have already had a heart attack, stroke, or stent placement, the calculus is different. In that group, daily aspirin’s ability to prevent a second event usually outweighs the side effects. The decision is most nuanced for the middle group: people with risk factors like high blood pressure, diabetes, or high cholesterol who haven’t yet had an event. For them, the side effect profile described above is exactly what needs to be weighed against the potential benefit.