Indigestion, also known as dyspepsia or upset stomach, is a condition of impaired digestion. Symptoms may include upper abdominal fullness, heartburn, nausea, belching, flatulence 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 either "organic" or "functional dyspepsia", 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 non-ulcer 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 patients who are 60 or older, or who have 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.

Patients experiencing indigestion likely report one, a combination of, or all of the following symptoms:

There may be abdominal tenderness, but this finding is nonspecific and is not required to make a diagnosis. However, there are physical exam signs that may point to a different diagnosis and underlying cause for a patient's reported discomfort. A positive Carnett sign (focal tenderness that increases with abdominal wall contraction and palpation) suggests an etiology involving the abdominal wall musculature. Cutaneous dermatomal distribution of pain may suggest a thoracic polyradiculopathy. Tenderness to palpation over the right upper quadrant, or Murphy's sign, may suggest cholecystitis or gallbladder inflammation.

Also known as Alarm features, alert features, red flags, or warning signs in gastrointestinal (GI) literature.

Alarm features are thought to be associated with serious gastroenterologic disease and include:

Indigestion is a diagnosis related to a combination of symptoms that can be attributed to "organic" or "functional" causes. Organic dyspepsia should have pathological findings upon endoscopy, like an ulcer in the stomach lining in peptic ulcer disease. Functional dyspepsia is unlikely to be detected on endoscopy but can be broken down into two subtypes, epigastric pain syndrome (EPS) and post-prandial distress syndrome (PDS). In addition, indigestion could be caused by medications, food, or other disease processes.