Cimetidine, sold under the brand name Tagamet among others, is a histamine H₂ receptor antagonist that inhibits stomach acid production. It is mainly used in the treatment of heartburn and peptic ulcers.

With the development of proton pump inhibitors, such as omeprazole, approved for the same indications, cimetidine is available as an over-the-counter formulation to prevent heartburn or acid indigestion, along with the other H2-receptor antagonists.

Cimetidine was developed in 1971 and came into commercial use in 1977. Cimetidine was approved in the United Kingdom in 1976,^{[citation needed]} and was approved in the United States by the Food and Drug Administration in 1979.

Cimetidine is indicated for the treatment of duodenal ulcers, gastric ulcers, gastroesophageal reflux disease, and pathological hypersecretory conditions. Cimetidine is also used to relieve or prevent heartburn.

Reported side effects of cimetidine include diarrhea, rashes, dizziness, fatigue, constipation, and muscle pain, all of which are usually mild and transient. It has been reported that mental confusion may occur in the elderly. Because of its hormonal effects, cimetidine rarely may cause sexual dysfunction including loss of libido and erectile dysfunction and gynecomastia (0.1–0.2%) in males during long-term treatment. Rarely, interstitial nephritis, urticaria, and angioedema have been reported with cimetidine treatment. Cimetidine is also commonly associated with transient raised aminotransferase activity; hepatotoxicity is rare.

Cimetidine appears to be very safe in overdose, producing no symptoms even with massive overdoses (e.g., 20 g).

Due to its non-selective inhibition of cytochrome P450 enzymes, cimetidine has numerous drug interactions. Examples of specific interactions include, but are not limited to, the following:

The mechanism of action of cimetidine as an antacid is as a histamine H₂ receptor antagonist. It has been found to bind to the H₂ receptor with a K_d of 42 nM.