Zantac^® is a medication that is licensed for the treatment of certain conditions related to the esophagus, stomach, and intestines. The medicine works by blocking a specific kind of histamine receptor in the body. Zantac belongs to a class of drugs known as H2 blockers; it is sometimes referred to as an "acid reducer." Over-the-counter Zantac may be used for heartburn, acid indigestion, and sour stomach. Prescription forms of Zantac are used to treat gastroesophageal reflux disease (GERD), erosive esophagitis, duodenal ulcers, gastric ulcers, and other conditions. Side effects of the drug may include headache, constipation, and upset stomach.