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Activation of H2-receptors in parietal cells increases gastric acid secretion. Stimulation of H1-receptors leads to contraction of intestinal smooth muscle.

1. H1-Receptor Antagonists

Mechanism of Action

Diphenhydramine is one of a diverse group of drugs that competitively blocks H1-receptors. Many drugs with H1-receptor antagonist properties have considerable antimuscarinic, or atropine-like, activity (eg, dry mouth) or anti-serotonergic activity (antiemetic).

Clinical Uses

Like other H1-receptor antagonists, diphenhydramine has a multitude of therapeutic uses: suppression of allergic reactions and symptoms of upper respiratory tract infections (eg, urticaria, rhinitis, conjunctivitis); vertigo, nausea, and vomiting (eg, motion sickness, Ménière disease); sedation; suppression of cough; and dyskinesia (eg, parkinsonism, drug-induced extrapyramidal side effects). Although H1-blockers prevent bronchoconstriction from histamine, they are ineffective in treating bronchial asthma, which is primarily due to other mediators. Likewise, H1-blockers will not completely prevent the hypotensive effect of histamine unless an H2-blocker is administered concomitantly.

2. H2-Receptor Antagonists

Mechanism of Action

H2-receptor antagonists include cimetidine, famotidine, nizatidine, and ranitidine. These agents competitively inhibit histamine binding to H2-receptors, thereby reducing gastric acid output and raising gastric pH.

Clinical Uses

All H2-receptor antagonists are equally effective in the treatment of peptic duodenal and gastric ulcers, hypersecretory states (Zollinger–Ellison syndrome), and gastroesophageal reflux disease (GERD). Intravenous preparations have been used to prevent stress ulceration in critically ill patients. By decreasing gastric fluid volume and hydrogen ion content, H2-blockers reduce the perioperative risk of aspiration pneumonia. These drugs affect the pH of only those gastric secretions that occur after their administration. Recently, ranitidine drugs have been removed from the market due to the concern for the presence of a contaminant known as N-Nitrosodimethylamine (NDMA).

Side Effects

Cimetidine is now much less commonly used because of its many side effects, including hepatotoxicity, interstitial nephritis, granulocytopenia, thrombocytopenia, and occasional gynecomastia and erectile dysfunction in men. Finally, it has been associated with mental status changes, including lethargy, hallucinations, and seizures, particularly in older adult patients.


As a premedication to reduce the risk of aspiration pneumonia, H2-receptor antagonists should be administered at bedtime and again at least 2 h before surgery.


Mechanism of Action

Antacids neutralize the acidity of gastric fluid by providing a base (usually hydroxide, carbonate, bicarbonate, citrate, or trisilicate) that reacts with hydrogen ions to form water.

Clinical Uses

Common uses of antacids include ...

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