The pharmacodynamic, therapeutic, and toxicologic properties of famotidine are evaluated and compared with those of cimetidine and ranitidine. Famotidine, an H2-receptor antagonist with a thiazole nucleus, is approximately 7.5 times more potent than ranitidine and 20 times more potent than cimetidine on an equimolar basis. Therapeutic trials indicate that famotidine 20 mg b.i.d. or 40 mg at bedtime is as effective as standard doses of cimetidine and ranitidine for healing duodenal ulcers. A dose of 40 mg at bedtime appears to heal benign gastric ulcers. A single nocturnal dose of 20 mg is effective in preventing duodenal ulcer relapse. Further studies are required that compare the efficacy of famotidine with cimetidine and ranitidine in the treatment of gastric ulcers and in the prevention of recurrent duodenal ulcers. The overall incidence of adverse effects observed with famotidine appears to be similar to that reported for cimetidine and ranitidine. Like ranitidine, famotidine does not have antiandrogenic effects or substantially inhibit the hepatic metabolism of drugs. Because of its increased antisecretory potency and lack of antiandrogenic effects at higher doses, famotidine may be the H2-receptor antagonist of choice in treating Zollinger-Ellison syndrome. Additional clinical experience, as well as cost and safety factors, will determine the place of famotidine in treating and preventing acid-peptic disorders.