Most minor Alka-Seltzer side effects, like stomach discomfort, bloating, or nausea, resolve within a few hours as your body processes the medication. If pain or upset stomach persists beyond 10 days in adults or 5 days in children, that’s a sign something else is going on. The timeline depends on which side effect you’re experiencing and whether it’s caused by the aspirin, the sodium, or the fizzing action of the tablet.
What’s in Alka-Seltzer and Why It Matters
Each original Alka-Seltzer tablet contains three active ingredients: 325 mg of aspirin, 1,916 mg of sodium bicarbonate, and 1,000 mg of citric acid. The recommended dose is two tablets, which means you’re taking 650 mg of aspirin (equivalent to two regular aspirin) and nearly 4 grams of sodium bicarbonate. That’s a significant amount of both a pain reliever and a sodium-heavy antacid, and each ingredient carries its own set of possible side effects with different timelines.
Minor Side Effects: A Few Hours
The most common side effects are digestive: mild nausea, gas, bloating, or a slightly upset stomach. These typically fade on their own within one to three hours as the sodium bicarbonate neutralizes and the aspirin is absorbed. Your body adjusts to the medicine during treatment, so these effects often don’t need any special attention.
Heartburn or a burning sensation in the stomach can also occur, particularly if you take Alka-Seltzer on an empty stomach. The aspirin component irritates the stomach lining, but the antacid is designed to buffer that effect. If the irritation lingers past a few hours, it usually means the aspirin is causing more inflammation than the antacid can offset.
Sodium-Related Swelling: Hours to Days
Because each dose delivers a large amount of sodium bicarbonate, fluid retention is a real concern. Your body holds onto extra water to balance the sodium, which can cause swelling in your face, fingers, ankles, feet, or lower legs. For most people, this resolves within 24 hours as the kidneys flush out the excess sodium. Drinking extra water helps speed this along.
If you take Alka-Seltzer repeatedly over several days, the swelling and elevated blood pressure can build up. This is why the general guidance is to avoid using it for more than five consecutive days, especially for older adults. People with high blood pressure, heart disease, or kidney problems are more vulnerable to these sodium-driven effects, and the timeline for resolution stretches longer when the sodium has been accumulating.
Aspirin-Related Stomach Bleeding
The most serious side effect is gastrointestinal bleeding, which the FDA has specifically warned about for aspirin-containing antacid products. This isn’t just a theoretical risk. Warning signs include feeling faint, vomiting blood, passing black or bloody stools, and abdominal pain that doesn’t go away. These symptoms can appear within hours of taking the medication, particularly if you’re also drinking alcohol or taking other blood thinners.
Because aspirin thins the blood by reducing your platelets’ ability to clot, a single dose can affect clotting for several days. The aspirin’s blood-thinning effect lasts for the lifespan of the affected platelets, roughly 7 to 10 days. This doesn’t mean you’ll bleed for that long, but it does mean your stomach lining is more vulnerable to irritation during that window if it was already damaged.
Ringing in the Ears and Other Toxicity Signs
Ringing in the ears (tinnitus), nausea, vomiting, and heavy sweating are the earliest signs that you’ve taken too much aspirin. This can happen if you exceed the recommended dose or take Alka-Seltzer alongside other aspirin-containing products without realizing the overlap. The first phase of aspirin toxicity involves rapid breathing and can last up to 12 hours. A second phase, involving more significant metabolic disruption, can begin within hours and persist for 12 to 24 hours. In adults, more severe symptoms may not appear until 24 hours or more after ingestion.
Tinnitus from a mildly excessive dose usually fades within a day once you stop taking the medication. If the ringing persists or is accompanied by confusion, rapid breathing, or persistent vomiting, that points to a more serious level of toxicity that needs immediate medical attention.
Allergic Reactions
Some people are sensitive to aspirin and can develop hives, facial swelling, or breathing difficulty after taking Alka-Seltzer. These reactions typically begin within minutes to a couple of hours. Mild skin reactions like hives often clear within a few hours to a day. Difficulty breathing or swelling of the throat is a medical emergency regardless of how quickly it started.
If you’ve ever had a reaction to aspirin, ibuprofen, or naproxen, you’re at higher risk. Aspirin sensitivity is especially common in people with asthma or chronic nasal polyps.
What Affects How Long Side Effects Last
Several factors influence your personal timeline. Taking Alka-Seltzer with food generally reduces stomach irritation and shortens the duration of digestive side effects. Your kidney function matters too, since the kidneys are responsible for clearing both the aspirin metabolites and the excess sodium. Older adults and people with reduced kidney function will process the medication more slowly, meaning side effects linger longer.
How many doses you’ve taken also plays a role. A single dose for occasional heartburn or a headache produces side effects that clear relatively quickly. Multiple doses over several days allow sodium to accumulate and give aspirin more opportunity to irritate the stomach lining, extending the recovery window. If you’re taking other medications, the sodium bicarbonate in Alka-Seltzer can change how your body absorbs or eliminates those drugs, potentially creating effects that outlast the Alka-Seltzer itself.