Acid reflux, also known as HEARTBURN, is a condition in which stomach acid flows back up into the esophagus, causing a burning sensation in the chest. It is caused by a weakened lower esophageal sphincter, which allows stomach acid to escape and irritate the lining of the esophagus. Symptoms of acid reflux include chest pain, difficulty swallowing, and a sour taste in the mouth. Treatment for acid reflux includes lifestyle changes, such as avoiding certain foods and eating smaller meals, as well as medications to reduce stomach acid production.