Omeprazole is a drug used in the treatment of acid-related diseases such as dyspepsia, peptic ulcer disease, gastroesophageal reflux disease and Zollinger-Ellison syndrome. The drug belongs to the proton-pump inhibitor class of drugs, meaning it reduces acid production in the stomach. Common side effects include constipation, gas, nausea and vomiting.Omeprazole is used alone or with other drugs to treat gastroesophageal reflux disease (GERD). GERD is a condition in which acid flows backward from the stomach and can result in a heartburn and possible injury to the esophagus. Through its action, Omeprazole allows the esophagus to heal and prevents further damage. Omeprazole may also be used the Zollinger-Ellison syndrome, a condition in which the stomach produces a significant and harmful amount of acid. Omeprazole can be used with other medications to prevent reoccurrence of ulcers caused by the H. pylori bacteria. Nonprescription Omeprazole can treat heartburn that occurs for two or more days. Omeprazole is in a class of medicines known as proton-pump inhibitors.