Nexium (esomeprazole) is a proton pump inhibitor, which treats different stomach and esophagus illnesses. This drug prevents production of acid in the stomach that can cause heartburn, gastroesophageal reflux disease (GERD), and other stomach conditions. It also helps to repair possible damage to the esophagus made by stomach acid. There are no generic forms of the drug available. Nexium is a medicine used in the treatment of conditions caused by excessive acid production. These include gastroesophageal reflux disease (GERD), Zollinger-Ellison syndrome, and erosive esophagitis. Nexium can also be used in the treatment of dyspepsia and peptic ulcer disease. The active ingredient of Nexium, esomeprazole, is a proton pump inhibitor. This means that it inhibits the formation of gastric hydrochloric acid by inhibiting the pumps that produce this acid - specifically in the parietal cells of the stomach. Nexium is associated with its own range of potential side effects. Among these include headache, drowsiness, diarrhea, stomach pain, gas, constipation, and dry mouth.