Causes

Heartburn occurs when stomach acid backs up into the tube that carries food from your mouth to your stomach (esophagus).

Normally when you swallow, a band of muscle around the bottom of your esophagus (lower esophageal sphincter) relaxes to allow food and liquid to flow down into your stomach. Then the muscle tightens again.

If the lower esophageal sphincter relaxes abnormally or weakens, stomach acid can flow back up into your esophagus (acid reflux) and cause heartburn. The acid backup may be worse when you're bent over or lying down.