Indigestion, also known as dyspepsia or upset stomach, is a condition of impaired digestion. Symptoms may include upper abdominal fullness, heartburn, nausea, belching, or upper abdominal pain. People may also experience feeling full earlier than expected when eating. Indigestion is relatively common, affecting 20% of people at some point during their life, and is frequently caused by gastroesophageal reflux disease (GERD) or gastritis.

Indigestion is subcategorized as "organic" or "functional", but making the diagnosis can prove challenging for physicians. Organic indigestion is the result of an underlying disease, such as gastritis, peptic ulcer disease (an ulcer of the stomach or duodenum), or cancer. Functional indigestion (previously called nonulcer dyspepsia) is indigestion without evidence of underlying disease. Functional indigestion is estimated to affect about 15% of the general population in western countries and accounts for a majority of dyspepsia cases.

In elderly patients (60 years of age or older) or with worrisome symptoms such as trouble swallowing, weight loss, or blood loss, an endoscopy (a procedure whereby a camera attached to a flexible tube is inserted down the throat and into the stomach) is recommended to further assess and find a potential cause. In patients younger than 60 years of age, testing for the bacteria H. pylori and if positive, treatment of the infection is recommended. More details about how indigestion is diagnosed and treated can be found below.