Pharmacology. Cimetidine, ranitidine, famotidine, and nizatidine are selective competitive inhibitors of histamine on H2 receptors. These receptors modulate smooth muscle, vascular tone, and gastric secretions and may be involved in clinical effects associated with anaphylactic and anaphylactoid reactions as well as ingestion of histamine or histamine-like substances (eg, scombroid fish poisoning). Cimetidine, as an inhibitor of cytochrome P-450 enzymes, has been proposed or studied in animals as an agent to block the production of toxic intermediate metabolites (eg, acetaminophen, carbon tetrachloride, halothane, Amanita mushroom poisoning, dapsone), but this has not been shown to be beneficial for human poisonings or toxicity with the possible exception of patients on chronic dapsone therapy (see “Indications”). Cimetidine is also an inhibitor of alcohol dehydrogenase (see “Drug or Laboratory Interactions”) and has been suggested for use in patients with an atypical aldehyde dehydrogenase enzyme to minimize a disulfiram reaction (“Oriental flushing”) to acute alcohol ingestion.