An antacid tablet is an over-the-counter medication that neutralizes stomach acid to relieve heartburn, indigestion, and sour stomach. It works within minutes by using a mild base (the chemical opposite of an acid) to counteract the hydrochloric acid your stomach naturally produces. Antacids are one of the oldest and most widely used remedies for digestive discomfort, and they’re available without a prescription at virtually every pharmacy and grocery store.
How Antacid Tablets Work
Your stomach lining produces hydrochloric acid to break down food. Sometimes the stomach makes too much acid, or acid splashes upward into the esophagus, causing that familiar burning sensation. Antacid tablets contain weak bases, typically hydroxides, carbonates, or bicarbonates, that chemically react with the excess acid and convert it into water and harmless salts. Some formulations also produce a small amount of carbon dioxide gas during this reaction, which is why you might burp after taking one.
This neutralization happens quickly. Unlike other heartburn medications that need hours or even weeks to reach full effect, antacids begin working almost as soon as they dissolve in your stomach. The tradeoff is that the relief is relatively short-lived, usually lasting anywhere from 30 minutes to a couple of hours depending on the formulation and whether you’ve recently eaten.
Common Active Ingredients
Most antacid tablets on the market use one or a combination of these four ingredients:
- Calcium carbonate is the most widely sold type, found in brands like Tums. It’s fast-acting and doubles as a calcium supplement, though that’s not its primary purpose.
- Magnesium hydroxide (the active ingredient in Milk of Magnesia) neutralizes acid effectively and tends to have a mild laxative effect.
- Aluminum hydroxide works more slowly than calcium or magnesium and tends to cause constipation.
- Sodium bicarbonate (essentially baking soda) is very fast-acting but introduces extra sodium into your diet, which can be a concern for people managing blood pressure.
Many products combine aluminum and magnesium together. This is intentional: aluminum tends to cause constipation while magnesium tends to cause diarrhea, so pairing them helps cancel out both side effects.
When and How to Take Them
The best time to take an antacid is about one hour after eating or whenever heartburn symptoms appear. If you’re using them to manage nighttime symptoms, take them on an empty stomach rather than with food. Most tablets are designed to be chewed thoroughly before swallowing, though some are meant to be swallowed whole or dissolved in water. Check the label for your specific product.
There are limits to how many you should take. For a typical extra-strength calcium carbonate tablet (750 mg), the recommended ceiling is no more than 9 tablets in 24 hours, or 6 tablets if you’re pregnant. Federal labeling rules require all antacids to carry a warning: do not use the maximum dosage for more than 2 weeks without a doctor’s guidance. If you find yourself reaching for antacids daily for weeks on end, the underlying problem likely needs a different approach.
Side Effects
Side effects from occasional antacid use are uncommon and usually mild. The most frequently reported ones include diarrhea (more common with magnesium-based formulas), constipation (more common with aluminum-based formulas), loss of appetite, unusual tiredness, and muscle weakness. The chalky taste bothers some people, and taking the tablet with water or milk can help with that.
Heavy, long-term use carries more serious risks. Excessive calcium carbonate intake can lead to a condition where calcium builds up in the blood, potentially causing kidney stones or impairing kidney function. Aluminum-based antacids can interfere with how your body absorbs phosphorus if used frequently over a long period. Sodium bicarbonate antacids can disrupt your body’s acid-base balance if overused, particularly in people with kidney problems.
Antacids vs. Other Heartburn Medications
Antacids are just one of three main categories of acid-reducing medications, and they differ significantly in how they work and how long the effects last.
Antacids neutralize acid that’s already been produced. They’re the fastest option, working in minutes, but the relief is temporary. They’re best suited for occasional, predictable heartburn.
H2 blockers (like famotidine, sold as Pepcid) reduce the amount of acid your stomach produces in the first place. They take 30 to 60 minutes to kick in but last significantly longer, often 6 to 12 hours. They can be taken as needed when you anticipate a problem.
Proton pump inhibitors, or PPIs (sold under names like Prilosec and Nexium), are the strongest option. They shut down acid production more completely, but they need to be taken daily for 4 to 8 weeks to reach full effectiveness because not all acid-producing cells in the stomach are active at the same time. An as-needed strategy with PPIs won’t reliably control symptoms. PPIs are designed for more persistent conditions like gastroesophageal reflux disease, not the occasional bout of heartburn after a spicy meal.
Drug Interactions to Know About
Antacids can interfere with the absorption of other medications you take by mouth. Because they change the acid level in your stomach, they can affect how well your body absorbs certain antibiotics, thyroid medications, and heart drugs. If you take prescription medications regularly, spacing them at least two hours apart from an antacid dose is a common strategy, but checking with a pharmacist about your specific medications is the most reliable approach.
Lifestyle Alternatives
For people who experience mild or occasional heartburn, some simple adjustments can reduce the need for antacids altogether. Eating smaller meals, avoiding caffeine and spicy foods, not lying down within two to three hours of eating, and elevating the head of your bed can all help keep stomach acid where it belongs. These strategies are especially worth trying during pregnancy, when heartburn is common but medication choices require more caution.