H2RAs, such as ranitidine and cimetidine, block the effects of released histamine at H2 receptors, therefore treating vasodilatation and possibly some cardiac effects, as well as glandular hypersecretion.

Why is ranitidine given for anaphylaxis?

H2 blockers with H1 blockers have additive benefit over H1 blockers alone in treating anaphylaxis. Ranitidine (Zantac) probably preferred over cimetidine (Tagamet) in anaphylaxis in light of the risk for hypotension with rapidly infused cimetidine and the multiple, complex drug interactions with cimetidine.

Why is famotidine used for anaphylaxis?

H2 antihistamines — An H2 antihistamine (eg, famotidine) given with an H1 antihistamine may provide some additional relief of hives [77]. Although H2 antihistamines are sometimes administered in anaphylaxis treatment, H2 antihistamines do not relieve upper or lower airway obstruction or shock.