Rennie is an over-the-counter antacid tablet used to treat heartburn, indigestion, and acid reflux. Sold mainly in the UK and Europe, each chewable tablet contains 680 mg of calcium carbonate and 80 mg of magnesium carbonate, two mineral compounds that neutralize excess stomach acid on contact. It’s one of the fastest-acting antacids available, raising stomach pH in under a minute.
How Rennie Works
When you chew a Rennie tablet, the calcium carbonate and magnesium carbonate react directly with hydrochloric acid in your stomach. This chemical reaction converts the acid into water, carbon dioxide, and harmless mineral salts (calcium chloride and magnesium chloride). The result is a rapid drop in stomach acidity.
In laboratory testing using an artificial stomach model, Rennie raised stomach pH from 1.0 (highly acidic) to above 3.0 within 40 seconds of contact. Within about two minutes, the pH climbed to 4.5, which is the threshold where most heartburn symptoms ease. The peak neutralizing effect lasted roughly 10 minutes, with overall acid suppression maintained for about 56 minutes. In real-world use, people in observational studies reported symptom improvement within five minutes of taking a tablet.
This makes Rennie a good option for quick, short-term relief rather than long-lasting acid control. If you need something that works for hours, a different class of medication (like an H2 blocker or proton pump inhibitor) may be more appropriate.
What Rennie Treats
Rennie is designed for occasional digestive discomfort, not chronic conditions. The standard tablets (available in peppermint, spearmint, and orange flavors, plus a sugar-free version) target heartburn and indigestion. A separate product called Rennie Deflatine adds an anti-foaming agent to help with trapped wind and bloating. There’s also a liquid suspension for heartburn relief and a medicated chewing gum containing calcium carbonate.
Typical situations where people reach for Rennie include that burning feeling behind the breastbone after a heavy meal, sour taste in the back of the throat from acid reflux, or general stomach discomfort after eating too quickly or drinking too much coffee or alcohol.
Safety During Pregnancy and Breastfeeding
Heartburn is extremely common during pregnancy, and antacids like Rennie are widely used. Both calcium carbonate and magnesium carbonate can be used during breastfeeding at standard doses. Both minerals occur naturally in breast milk, and neither absorbs well enough through the gut to reach significant levels in milk. The NHS Specialist Pharmacy Service considers antacids a first-line option for managing heartburn during breastfeeding, and no special monitoring of the infant is usually needed.
Side Effects and Interactions
Most people tolerate Rennie well when used occasionally at recommended doses. The most common minor issue is the carbon dioxide produced during the neutralization reaction, which can cause mild belching. Prolonged or excessive use of calcium-based antacids can lead to constipation, while magnesium-based ones can have a mild laxative effect. Rennie contains both, which partially balances these effects.
People with severe kidney problems should avoid calcium and magnesium antacids, since impaired kidneys may struggle to clear the extra minerals. Very high long-term intake of calcium carbonate can, in rare cases, lead to elevated calcium levels in the blood.
Timing matters if you take other medications. Antacids containing calcium and magnesium can interfere with the absorption of certain drugs, including some antibiotics and iron supplements. A good rule of thumb is to leave at least two hours between taking Rennie and any other medication, so the antacid doesn’t block what your body needs to absorb.
How to Take Rennie
You chew or suck one or two tablets when symptoms appear. If symptoms return, you can repeat the dose. The tablets are designed for adults and children over 12, and you should stay within the maximum daily amount listed on your specific product’s packaging. Because Rennie works by direct contact with stomach acid, chewing thoroughly rather than swallowing whole gives faster results.
If you find yourself reaching for antacids most days for more than two weeks, that’s a signal your symptoms may need a different approach. Frequent heartburn can point to gastroesophageal reflux disease (GERD) or other conditions that benefit from longer-acting treatments or lifestyle changes rather than repeated antacid use.