The H2-receptor antagonists which are used for ulcer therapy fall into four main structural classes: cimetidine is an imidazole derivative; ranitidine and nizatidine belong to the basically substituted furans and thiazoles, respectively; famotidine is a member of the guanidino-thiazole group; and roxatidine belongs to the aminoalkylphenoxy series. Famotidine is the most potent, selective H2-receptor antagonist yet available for ulcer therapy. On a weight basis, famotidine is approximately eight times more potent than ranitidine and 40 times more potent than cimetidine. Furthermore, it increases the gastric mucosal blood flow, resulting in an increased haemostatic effect. Famotidine has a longer duration of action than either ranitidine or cimetidine. Since famotidine does not interact with cytochrome P-450 of the hepatic enzyme system, it does not appear to affect the metabolism of drugs metabolized by this system. The overall incidence of side-effects of the H2-receptor antagonists is in the range of 2-3% and no irreversible adverse effects are known. Famotidine has been found to be generally well tolerated. In post-marketing studies, the percentage of patients with side-effects is in the range 1.2-2%. Thus, famotidine is a safe and potent H2-receptor blocker of acid secretion.