What are the differences between the H2-receptor antagonists?

The H2-receptor antagonists which are used for ulcer therapy fall into four main structural classes. Cimetidine is an imidazole derivative; ranitidine belongs to the basically substituted furans, famotidine is a member of the guanidinothiazole 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. Cimetidine, ranitidine and famotidine are competitive antagonists, while the long-acting H2-receptor antagonists, e.g. loxtidine and lamitidine, are insurmountable H2-receptor blockers. Famotidine has a longer duration of action than either ranitidine or cimetidine. Because 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 number 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 a first post-marketing study, the number of patients with side-effects was only 0.43%. Side-effects such as headache, dizziness, constipation and diarrhoea have been observed only occasionally. Thus, famotidine is a safe and potent H2-receptor blocker of acid secretion.