<P> H antagonists, sometimes referred to as H2RA and also called H blockers, are a class of medications that block the action of histamine at the histamine H receptors of the parietal cells in the stomach . This decreases the production of stomach acid . H antagonists can be used in the treatment of dyspepsia, peptic ulcers and gastroesophageal reflux disease . They have been surpassed by proton pump inhibitors (PPIs); the PPI omeprazole was found to be more effective at both healing and alleviating symptoms of ulcers and reflux oesophagitis than the H blockers ranitidine and cimetidine . </P> <P> H antagonists are a type of antihistamine, although in common use the term "antihistamine" is often reserved for H antagonists, which relieve allergic reactions . Like the H antagonists, some H antagonists function as inverse agonists rather than receptor antagonists, due to the constitutive activity of these receptors . </P>

What does a histamine 2 receptor antagonist do