Cimetidine is in a group of drugs called histamine receptor antagonists. Cimetidine works by decreasing the amount of acid your stomach produces. Cimetidine is used to treat and prevent certain types of ulcer, and to treat conditions that cause the stomach to produce too much acid. Cimetidine is also used to treat gastroesophageal reflux disease (GERD), when stomach acid backs up into the esophagus and causes heartburn. Cimetidine may also be used for other purposes not listed in this medication guide.