Rennie is an over-the-counter antacid tablet used to relieve heartburn, indigestion, and acid reflux. It’s one of the most widely recognized antacid brands in the UK and Europe, sold as a chewable tablet that neutralizes excess stomach acid. Rennie works fast: in laboratory testing using an artificial stomach model, it began neutralizing acid within seconds and raised stomach pH to a comfortable level in under a minute.
How Rennie Works
Rennie tablets contain two active ingredients: calcium carbonate and magnesium carbonate. Both are alkaline minerals that react directly with hydrochloric acid in your stomach, converting it into water and harmless salts. This chemical reaction is almost immediate. In a study simulating stomach conditions, a Rennie tablet brought the stomach’s acidity to a significantly less acidic level (pH 3.0) within 40 seconds. It continued neutralizing acid over the next couple of minutes, reaching a peak pH of about 5.24, which it maintained for nearly 10 minutes.
This is a different approach from medications like omeprazole or lansoprazole, which reduce acid production at its source and take hours or days to reach full effect. Rennie doesn’t stop your stomach from making acid. It simply neutralizes what’s already there, which is why it provides quick but relatively short-lived relief.
What Rennie Treats
Rennie is designed for occasional digestive discomfort rather than chronic conditions. Its listed uses include heartburn (that burning feeling behind the breastbone), acid indigestion, upset stomach, and acid reflux (when stomach acid rises into the throat). It’s a good option for symptoms that flare up after eating, during stress, or when lying down.
If you’re experiencing heartburn or indigestion more than a couple of times a week, an antacid like Rennie can help in the moment, but the frequency itself is worth investigating. Persistent symptoms could point to gastroesophageal reflux disease (GERD) or other conditions that benefit from longer-term treatment.
Different Versions of Rennie
Rennie comes in several varieties. The standard tablets are available in peppermint and spearmint flavors, along with a sugar-free version. These all work the same way, using calcium carbonate and magnesium carbonate to neutralize acid.
Rennie Deflatine is a slightly different product. In addition to 680 mg of calcium carbonate and 80 mg of magnesium carbonate, each Deflatine tablet contains 25 mg of simeticone, an anti-foaming agent. Simeticone works by causing small gas bubbles in your stomach to merge into larger ones that are easier to pass. This makes Deflatine useful if your indigestion comes with bloating, trapped wind, or flatulence alongside the acid symptoms. Each Deflatine tablet also contains sorbitol as a sweetener.
Using Rennie During Pregnancy
Heartburn is extremely common during pregnancy, particularly in the second and third trimesters, as the growing uterus pushes stomach contents upward and pregnancy hormones relax the valve between the stomach and esophagus. Antacids like Rennie are considered the most appropriate first-line treatment for heartburn during pregnancy. They are safe at recommended doses for both pregnant and breastfeeding women. Rennie Deflatine specifically lists pregnancy-related indigestion and heartburn among its intended uses.
How to Take Rennie
Rennie tablets are chewed or sucked rather than swallowed whole. You take them when symptoms appear, not on a fixed schedule. Because they work by direct chemical neutralization, they start providing relief within about a minute. The effect is temporary, typically lasting around 10 to 20 minutes at peak effectiveness, though some residual benefit continues after that.
Like all antacids, Rennie isn’t meant for continuous daily use over long periods. The calcium and magnesium it contains are minerals your body absorbs, and taking large amounts regularly can shift your body’s mineral balance. If you find yourself reaching for Rennie every day, that’s a sign to look into what’s driving the symptoms rather than continuing to neutralize the acid after the fact.
How Rennie Compares to Other Antacids
Rennie belongs to the same category as products like Tums (also calcium carbonate-based) and Gaviscon, but each works a bit differently. Tums and Rennie both neutralize acid directly. Gaviscon takes a different approach: it forms a foam-like barrier that floats on top of stomach contents, physically preventing acid from rising into the esophagus. This makes Gaviscon particularly useful for reflux, while Rennie tends to act faster for general heartburn and indigestion.
The calcium carbonate in Rennie does double as a small calcium supplement. Each standard tablet delivers a meaningful dose of elemental calcium (the Deflatine version provides 272 mg per tablet). This isn’t a reason to take it unnecessarily, but it does mean Rennie is unlikely to cause the magnesium-related loose stools that some other antacids can produce, since calcium tends to have the opposite effect.