Gastro-oesophageal reflux disease (GERD) is a disorder of the oesophagus that causes frequent symptoms of heartburn. The oesophagus is the tube connecting the mouth to the stomach. GERD occurs when a muscular ring called the lower oesophageal sphincter (LES) is weakened, which permits irritating stomach contents to pass up into the oesophagus, resulting in heartburn.