Acid reflux is the backup of stomach acid into the esophagus, the
muscular tube that connects the throat to the stomach. It may cause burning
pain in the esophagus.
Acid reflux may occur because of weakness in the ring of muscle
(lower esophageal sphincter, or LES) that controls the flow of food from the
esophagus into the stomach. Frequent acid reflux can lead to irritation of the
esophagus (esophagitis), narrowing of the esophagus, or development of sores
(ulcers) in the esophagus. Long-term exposure of the esophagus to stomach acid
causes the cells that line the inside of the esophagus to be replaced by cells
similar to those that line the inside of the stomach (Barrett's esophagus).