Alka-Seltzer typically begins relieving an upset stomach within 3 to 5 minutes of drinking the dissolved solution. Because the tablets fully dissolve in water before you drink them, the active ingredients are already in liquid form and start neutralizing stomach acid almost immediately on contact. This is noticeably faster than swallowing a solid antacid tablet, which needs time to break down in your stomach first.
Why It Works So Quickly
When an Alka-Seltzer tablet hits water, the citric acid and sodium bicarbonate (baking soda) react to produce carbon dioxide gas, which creates the familiar fizz. That reaction also transforms the ingredients into sodium citrate, a liquid antacid that neutralizes excess hydrochloric acid in your stomach. By the time the fizzing stops and you drink it, the neutralizing compound is already dissolved and ready to work the moment it reaches your stomach lining.
The aspirin component takes slightly longer to have its full effect, generally 15 to 30 minutes. But for pure stomach discomfort, the antacid portion is what matters most, and that relief is nearly immediate.
How Long the Relief Lasts
A single dose of Alka-Seltzer provides relief for roughly 4 to 6 hours, depending on how severe your symptoms are and what caused them. The dosing schedule reflects this: adults can take one or two regular-strength tablets every four to six hours as needed. If your upset stomach is mild, one dose may be all you need. For ongoing discomfort from something like overindulgence in food or drink, you may need a second dose a few hours later.
What’s Actually in Each Tablet
Each Original Alka-Seltzer tablet contains three active ingredients: 1,916 mg of sodium bicarbonate (the antacid), 1,000 mg of citric acid (which drives the fizzing reaction), and 325 mg of aspirin (a pain reliever). Once dissolved, the solution you drink contains sodium citrate as the primary antacid along with a soluble form of aspirin.
This combination is designed for a specific situation: upset stomach accompanied by headache or body aches. If your only symptom is heartburn or acid indigestion without any pain, the aspirin is unnecessary, and Alka-Seltzer makes aspirin-free versions for that purpose. Choosing the right product matters because aspirin can actually irritate the stomach lining in some people, potentially making things worse rather than better.
The Sodium Factor
Each tablet contains 567 mg of sodium. A standard two-tablet dose delivers over 1,100 mg, which is nearly half the daily sodium limit recommended by most health guidelines. If you’re watching your salt intake due to high blood pressure, heart disease, or kidney problems, this is a significant amount. Most people don’t think of an antacid as a major sodium source, but Alka-Seltzer is one of the highest-sodium over-the-counter stomach remedies available.
When Alka-Seltzer Isn’t the Right Choice
The aspirin in Original Alka-Seltzer creates several situations where it’s the wrong pick for an upset stomach. If you’re over 60, have a history of stomach ulcers or bleeding problems, take blood thinners, or drink three or more alcoholic beverages daily, the risk of stomach bleeding increases. Signs of a problem include black or tarry stools, or vomiting material that looks like coffee grounds.
Children and teenagers should not take it, particularly if they have or are recovering from chickenpox or flu, due to the risk of a rare but serious condition linked to aspirin use in young people. People with high blood pressure, kidney disease, liver cirrhosis, or those on a sodium-restricted diet should also check with a doctor first, largely because of the high sodium content.
Tips for Faster, Better Relief
Drop the tablets into a full glass of water (about 4 ounces) and wait for the fizzing to completely stop before drinking. Drinking it while it’s still actively fizzing won’t hurt you, but you’ll swallow more carbon dioxide gas, which can cause bloating and burping that may add to your discomfort. Room-temperature water dissolves the tablets faster than cold water.
Taking it on a mostly empty stomach speeds up relief because there’s less food buffering the antacid from reaching the stomach lining. If your upset stomach is from overeating, though, you’ll still feel results within a few minutes. Just don’t lie down immediately after drinking it, since the carbonation and liquid volume can worsen any acid reflux symptoms you might have alongside the stomach upset.