Histamine H2 antagonists, sometimes referred to as H2RAs and also called H2 blockers, are a class of medications that block the action of histamine at the histamine H2 receptors of the parietal cells in the stomach.
The H2 antagonists are competitive antagonists of histamine at the parietal cell’s H2 receptor, suppressing the normal secretion of acid by parietal cells and the meal-stimulated secretion of acid.
H2 receptor antagonists decrease the production of stomach acid.
H2 antagonists can be used in the treatment of dyspepsia, peptic ulcers and gastroesophageal reflux disease.
Histamine released by ECL cells in the stomach is blocked from binding on parietal cell H2 receptors, which stimulate acid secretion; therefore, other substances that promote acid secretion, gastrin and acetylcholine, have a reduced effect on parietal cells when the H2 receptors are blocked.
H2-antagonists are used for:
Peptic ulcer disease (PUD)
Gastroesophageal reflux disease (GERD)
Dyspepsia
Prevention of stress ulcer
Prevention of aspiration pneumonitis during surgery.
The H2-antagonists advantages over antacids, include: longer duration of action (6-10hours vs 1-2 hours for antacids), greater efficacy, and ability to be used prophylactically before meals to reduce the chance of heartburn occurring.
Their use has been surpassed by proton pump inhibitors (PPIs).
Proton pump inhibitors, are the preferred treatment for erosive esophagitis since they have been shown to promote healing better than H2-antagonists.
H2 antagonists are, in general, are well tolerated.
All drugs in the H2 receptor blocker class of medicines have the potential
to cause vitamin B12 deficiency, secondary to a reduction in food-bound vitamin B12 absorption.
PPI omeprazole was found to be more effective at both healing and alleviating symptoms of ulcers and reflux oesophagitis than the H2 blockers ranitidine and cimetidine.
H2 antagonists are a type of antihistamine.
The common use the term antihistamine is often reserved for H1 antagonists, which relieve allergic reactions.
The prototypical H2 antagonist, called cimetidine, sold under the trade name Tagamet.
Cimetidine interferes with drug metabolism and elimination through the liver cytochrome P450 (CYP) pathway.
Ranitidine is not as potent a CYP inhibitor as cimetidine, although it still shares several of the latter’s interactions: warfarin, theophylline, phenytoin, metoprolol, and midazolam.
Famotidine has negligible effect on the CYP system, and appears to have no significant interactions.
Ranitidine, first sold as Zantac, which has fewer adverse effects and drug interactions and is more potent.
This list of H2 agents:
cimetidine
ranitidine
famotidine
roxatidine
lafutidine
70% report relief of gastroesophageal reflux symptomatology.
H2RAs appear useful in GERD for controlling nocturnal acid breakthrough discomfort.
Tachyphylaxis to H2As develops rapidly, and they may therefore have a role only if used intermittently.
Cimetidine, famotidine, nazatidine and ranitidine have similar clinical efficacy.
With cimetidine adverse drug reactions (ADRs) are common.
Proton pump inhibitors are associated with greater risk of G.I. bleeding, pneumonia, and C. difficile infections than H2RAs in mechanically ventilated patients (MacLaren R et al.).
Histamine H2 receptor antagonists, used to treat peptic ulcer disease, include cimetidine (Tagamet�), famotidine (Pepcid), and ranitidine (Zantac). These medications can interfere with the absorption of vitamin B12 from food by slowing the release of hydrochloric acid into the stomach.
Infrequent adverse drug reactions include: hypotension.
Rare adverse drug reactions include: headache, tiredness, dizziness, confusion, diarrhea, constipation, and rash.
The overall risk of pneumonia is about 1 in 4 higher among H2 antagonist users.