Non-antisecretory activities of H2 antagonists

Besides the main effect of the H2 antagonists--that is, the inhibition of gastric acid secretion--these drugs possess several other pharmacological activities, which in most cases may be evident only for doses higher than those required to produce the H2 blockade. The non-secretory activities of the H2 antagonists may be classified into true side effects--that is, those independent of the primary action, which may or may not depend on the primary action. They concern the central and autonomic nervous systems, cardiovascular and endocrine systems, digestive system (gut, liver, pancreas), immune system, and so forth. It is obvious that when the actions of the different H2 blockers are completely at variance with regard to the same factor (cimetidine increase the TSH response to TRH, whereas ranitidine decreases it; ranitidine stimulates gastrointestinal motility, whereas oxmetidine inhibits it; ranitidine increases the exocrine pancreatic response to cholecystokinin, whereas oxmetidine decreases it; metiamide and cimetidine increase the activity of histamine methyltransferase, whereas burimamide decreases it), this is a clear demonstration that we are dealing with non-specific effects rather than with H2-receptor blockade. All these effects may be of interest because sometimes they may be useful in potentiating the primary action, and sometimes they may represent adverse reactions. In any case, they characterize pharmacologically the individual molecules of the family.