Also known as acid indigestion, heartburn is a painful but usually temporary condition caused by gastric acid pushing up into the esophagus from the abdomen. It can be caused by the ingestion of foods and drinks which push the sphincter muscle in the esophagus down, allowing gastric acid into the upper chest and throat. Lying down after eating can also trigger heartburn. Chronic heartburn is a key symptom of gastroesophageal reflux disease (GERD). Both acute cases of heartburn as well as GERD can be controlled with the use of antacids and other over-the-counter and prescription medications, but a doctor should be consulted if acid indigestion occurs regularly.