Antacids work fast to neutralise stomach acid already present in your stomach, relieving indigestion, heartburn and upset stomach. Mylanta Original and Double Strength formulae also contain an "anti-flatulent" which quickly relieves wind pain by dissipating excess gas bubbles in the stomach. New Mylanta 12 Hour Action however, actually works to reduce the amount of stomach acid your body produces. It does this by blocking the receptor connections which release acid into the stomach. It takes a little longer than regular Mylanta Antacids, but provides much longer lasting relief - all day or all night - with a single tablet. Mylanta Antacids provide relief within minutes which lasts for up to 3 hours. Mylanta 12 Hour Action provides relief within 1 hour which lasts for up to 12 hours.