Aspirin’s most common side effects involve the stomach: heartburn, nausea, bloating, and general stomach discomfort affect roughly 15 to 20% of people taking even low doses. Beyond these everyday complaints, aspirin carries rarer but more serious risks, including internal bleeding, hearing changes, and dangerous reactions in certain people. The specific side effects you experience depend on your dose, how long you take it, and your individual health profile.
Stomach Problems Are the Most Common
Aspirin works by blocking an enzyme called cyclooxygenase-1 (COX-1). This is the same mechanism that makes it useful for pain and heart protection, but COX-1 also helps maintain the protective lining of your stomach. Without that protection, the stomach becomes more vulnerable to its own acid.
The result, for many people, is acid reflux, heartburn, bloating, belching, nausea, and a vague discomfort in the upper abdomen. A large survey of low-dose aspirin users (the UGLA survey) found that 15.4% reported upper GI symptoms, with acid reflux and regurgitation accounting for 70% of those complaints. These symptoms were bothersome enough that about 12% of users said the side effects made them less likely to keep taking the medication.
These issues can show up even at the 81 mg “baby aspirin” dose that many people take daily for heart protection. Taking aspirin with food or water helps some people, but for others the irritation persists regardless.
Bleeding Risk: What the Numbers Look Like
Aspirin permanently disables platelets, the blood cells responsible for clotting. Your body replaces its platelet supply over roughly 7 to 10 days, which means a single dose affects clotting for about that long. This antiplatelet effect is exactly why aspirin helps prevent heart attacks, but it also means you bleed more easily and bleeding is harder to stop.
The most concerning bleeding events are gastrointestinal bleeds (from ulcers or erosions in the stomach or intestines) and hemorrhagic strokes (bleeding in the brain). According to a systematic review by the U.S. Preventive Services Task Force, aspirin use adds an estimated 1.39 extra major GI bleeding events per 1,000 people per year, and roughly 0.32 extra hemorrhagic strokes per 1,000 people per year. Those numbers sound small, and for any individual they are. But across millions of daily aspirin users, they add up significantly.
You may notice this bleeding tendency in less dramatic ways first: bruising more easily, bleeding longer from small cuts, or having nosebleeds that take a while to stop. These minor signs reflect the same underlying mechanism that drives the more serious events.
Hearing Changes and Tinnitus
At higher doses, aspirin can cause ringing in the ears (tinnitus) and temporary hearing loss. Research has shown that aspirin can produce a reversible hearing loss of up to 40 decibels depending on the dose. In studies using 3.9 grams per day (a dose sometimes used for inflammatory conditions, far above the 81 mg heart-protection dose), participants exposed to loud sounds experienced significantly more temporary hearing loss than they otherwise would have.
The good news is that aspirin-related hearing changes are almost always reversible. They typically resolve once you stop taking it or reduce the dose. At the low doses used for cardiovascular prevention, tinnitus is uncommon.
Aspirin-Exacerbated Respiratory Disease
A small percentage of people have a condition sometimes called Samter’s triad, now more formally known as aspirin-exacerbated respiratory disease (AERD). It involves three overlapping problems: asthma, recurring nasal polyps, and a sensitivity to aspirin and related painkillers like ibuprofen and naproxen. Taking aspirin triggers respiratory symptoms in these individuals, including sudden worsening of asthma, nasal congestion, and sometimes facial flushing.
Only a small fraction of people with asthma develop AERD, but if you have both asthma and nasal polyps, the risk is considerably higher. If you’ve ever noticed breathing problems after taking aspirin or ibuprofen, this is worth discussing with your doctor before taking aspirin regularly.
Reye’s Syndrome in Children
Aspirin should not be given to children or teenagers recovering from viral infections. The combination of aspirin with certain viruses, particularly influenza and chickenpox, is linked to Reye’s syndrome, a rare but potentially fatal condition. In Reye’s syndrome, blood sugar drops while ammonia and acid levels in the blood rise. The liver swells and accumulates fat, and dangerous swelling can develop in the brain.
This risk is the reason aspirin labels carry warnings against use in anyone under 18 with a fever or viral illness. Acetaminophen and ibuprofen are the standard alternatives for children.
How Ibuprofen Can Interfere With Aspirin
If you take aspirin for heart protection, timing matters when you also use ibuprofen. Both drugs compete for the same binding site on the COX enzyme in platelets. Aspirin locks onto this site permanently, which is what gives it lasting antiplatelet power. But ibuprofen can physically block aspirin from reaching the site. Since ibuprofen’s own grip is temporary, the net result is that neither drug effectively inhibits clotting.
The FDA has noted that this interference occurs when ibuprofen is taken within 8 hours before aspirin or within 30 minutes after it. To avoid the problem, take ibuprofen at least 8 hours before your aspirin dose, or wait at least 30 minutes after taking immediate-release aspirin before using ibuprofen. This timing applies to single ibuprofen doses. If you take ibuprofen regularly alongside daily aspirin, the interaction is harder to manage and worth a conversation with a pharmacist or physician.
Who Should Think Twice About Daily Aspirin
Guidelines around daily aspirin have shifted considerably in recent years. The U.S. Preventive Services Task Force now recommends against starting aspirin for heart disease prevention if you’re 60 or older, because the bleeding risks at that age tend to outweigh the cardiovascular benefits. For adults 40 to 59 with elevated heart disease risk (10% or greater chance of a cardiovascular event over the next decade), the task force calls it an individual decision, noting the net benefit is small.
These recommendations apply to people without existing heart disease or a history of stroke. If you’ve already had a heart attack or stroke, daily aspirin is a different calculation, and the benefits are generally much clearer. The standard preventive dose is 81 mg per day. People who already have a history of stomach ulcers, recent bleeding episodes, or who take other blood-thinning medications face higher bleeding risk and are generally not good candidates for daily aspirin therapy.
Allergic reactions to aspirin, while not common, can include hives, facial swelling, and in severe cases, anaphylaxis. If you’ve had an allergic reaction to aspirin or other anti-inflammatory painkillers in the past, that rules out aspirin use entirely.