Nexium 40mg is a proton pump inhibitor (PPI) used to treat heartburn or irritation of the esophagus caused by gastroesophageal reflux disease (GERD). It may be used with certain antibiotics to treat ulcers of the small intestines and to prevent them from coming back. It may be used to treat conditions that cause your body to make too much stomach acid (eg, Zollinger-Ellison syndrome). It may also be used for other conditions as determined by your doctor.