Heartburn is a symptom of acid reflux or gastro-esophageal reflux disease (GERD) and can cause a painful, burning sensation in your chest or throat after eating, during physical activities, or when you lie down in bed. Gaviscon Extra Strength Heartburn Relief Tablets work as an alginate, which means not in your bloodstream, but directly in your stomach where it forms a protective barrier that helps block stomach acid from back-flowing into your esophagus. So your stomach acid is still able to do its jobs without interference, while preventing it from splashing up and causing your heartburn and acid reflex symptoms.