**R12 — Heartburn** describes a burning sensation in the chest, typically retrosternal, often after meals or when lying down. It is the cardinal symptom of gastro-oesophageal reflux disease (GERD) but can occur without confirmed GERD.
When pathology is identified, codes shift to K21.0 (GERD with esophagitis) or K21.9 (GERD without esophagitis).
Belongs to **Chapter R** — Symptoms, signs.
