Cimetidine is a medication primarily used to treat conditions related to excess stomach acid, such as peptic ulcers and gastroesophageal reflux disease (GERD). It belongs to a class of drugs known as H2 receptor antagonists, which work by blocking histamine receptors in the stomach lining, thereby reducing acid production. In addition to its use for ulcers and GERD, cimetidine can also help prevent complications from acid reflux and is sometimes used to treat Zollinger-Ellison syndrome, a condition that causes excessive stomach acid production. It is available in both prescription and over-the-counter forms.