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H2 Receptor Antagonist – Effects, Uses & Side Effects

H2 receptor antagonists are medications that reduce stomach acid production. They are used to treat heartburn, peptic ulcers, and acid reflux disease.

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Things worth knowing about "H2 Receptor Antagonist"

H2 receptor antagonists are medications that reduce stomach acid production. They are used to treat heartburn, peptic ulcers, and acid reflux disease.

What is an H2 Receptor Antagonist?

An H2 receptor antagonist (also called an H2 blocker) is a type of medication that reduces the production of acid in the stomach. The name reflects its mechanism of action: these drugs block histamine H2 receptors located on the acid-producing cells (parietal cells) of the stomach lining. Normally, histamine binds to these receptors and stimulates the secretion of hydrochloric acid. By blocking this process, H2 antagonists significantly lower stomach acid levels.

H2 receptor antagonists are among the most widely used drugs in gastroenterology. Common active ingredients in this class include famotidine, cimetidine, and nizatidine. Ranitidine, once a widely prescribed H2 blocker, was withdrawn from the market due to contamination concerns.

Indications

H2 receptor antagonists are prescribed for a range of conditions associated with excessive stomach acid production:

Heartburn and acid indigestion
Gastroesophageal reflux disease (GERD): backflow of stomach acid into the esophagus
Peptic ulcers: ulcers in the stomach (gastric ulcer) or the first part of the small intestine (duodenal ulcer)
Gastritis: inflammation of the stomach lining
Zollinger-Ellison syndrome: a rare condition causing excessive acid production
Prevention of stress-related or NSAID-induced stomach ulcers

Mechanism of Action

Stomach acid secretion is regulated by three key stimulants: histamine, gastrin, and acetylcholine. Histamine acts on H2 receptors on the parietal cells of the gastric mucosa, triggering the activation of the proton pump (H+/K+-ATPase), which pumps hydrogen ions into the stomach to form hydrochloric acid.

H2 receptor antagonists competitively bind to these H2 receptors, preventing histamine from attaching and thereby reducing the signal that activates acid secretion. Compared to proton pump inhibitors (PPIs) such as omeprazole, H2 blockers act more quickly but produce a less pronounced and shorter-lasting reduction in acid.

Dosage and Administration

H2 receptor antagonists are available in various forms including tablets, capsules, and injectable solutions. Depending on the country and specific drug, some formulations are available over the counter.

Famotidine: typically 20–40 mg per day, often taken as a single evening dose
Cimetidine: 400–800 mg per day, divided into multiple doses
Treatment duration depends on the underlying condition and may range from a few weeks to several months

These medications are generally taken before meals or at bedtime, as stomach acid production is often highest at night.

Side Effects

H2 receptor antagonists are generally well tolerated. Possible side effects include:

Headache, dizziness, and fatigue
Diarrhea or constipation
Rarely: confusion or mental status changes, particularly in elderly patients or those with impaired kidney function (especially with cimetidine)
Cimetidine is known to inhibit liver enzymes (cytochrome P450 system), leading to interactions with a wide range of other medications
Very rarely: changes in blood cell counts

H2 Receptor Antagonists vs. Proton Pump Inhibitors

Both H2 blockers and proton pump inhibitors (PPIs) lower stomach acid but differ in their mechanisms and potency:

H2 blockers take effect more rapidly (within 1–2 hours) but are less potent than PPIs
PPIs (e.g., omeprazole, pantoprazole) directly inhibit the proton pump and provide stronger, longer-lasting acid suppression
For mild to moderate symptoms, H2 blockers may be sufficient; for severe GERD or peptic ulcers, PPIs are generally preferred
H2 blockers can lose effectiveness over time due to tolerance development (tachyphylaxis), whereas PPIs do not

References

Loscalzo J. et al. (eds.) - Harrison's Principles of Internal Medicine, 21st edition, McGraw-Hill Education, 2022
Fock KM et al. - Asia-Pacific consensus on the management of gastro-oesophageal reflux disease. Gut, 2016; 65(7): 1164-1180. PubMed PMID: 27261337
U.S. National Library of Medicine - MedlinePlus: H2 Blockers, medlineplus.gov

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