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H2 Receptor Antagonists

Sep 19, 20243 min read

Cimetidine, Ranitidine, Famotidine and nizatidine

Pharmacological Actions

  1. Competitive block of H2 receptors 
  2. Endocrine action and Enzyme inhibition ( for cimetidine only).

A Effects due to H2-blockade:Z

  • Block of H2 receptors of parietal cells of gastric mucosa,
  • there by reduces gastric HCl secretion induced by different stimuli.
  • Block H2 receptors on mast cells, thus may exaggerate histamine release during hypersensitivity reactions.

B Endocrine effects:Y

(occur following use of very high doses of cimetidine) Block of androgenic receptors:

  • decreases libido, sperm count and produces impotence

Hyperprolactinaemia:

  • produces galactorrhea in female and gynaecomastia in male.   Enzyme inhibition: ( These effects occur following use of cimetidine)

  • Inhibits cytochrome oxidase P-450 hepatic microsomal enzyme system, this will lead to inhibition of the metabolism of some drugs metabolized by this enzyme system e.g. B-blockers, Ca2+ channel blockers, sulphonylureas, warfarine, … etc.

  • Decreases glucouronation of acetaminophen. Thus may increase its effect.

Therapeutic Uses

  1. Duodenal and gastric ulcers.
  2. Zollinger-Ellison syndrome (gastrin-secreting tumour which increases HCl secretion) usually larger doses are required according to the severity of the condition.
  3. Reflux oesophagitis.
  4. Gastritis.
  5. With ulcerogenic drugs e.g. antirheumatics as a prophylaxis against injury of gastric mucosa
  6. Prophylaxis against gastric ulceration and bleeding in stress (e.g. after burns , trauma or major surgery) and bleeding oesophageal varices.
  7. Eradication of H. pylori infections.

Side Effects

  1. Nausea, vomiting and diarrhea, but constipation is less common.
  2. Antiandrogenic side effects observed with very high doses (cimetidine).
  3. Hyperprolactinaemia leading to gynecomastia (male) & galactorrhea (female) (cimetidine).
  4. Metabolic enzyme inhibition with subsequent drug interactions (cimetidine).
  5. Myalgia, arthralgia and fatigue.
  6. CNS:
    • headache, slurred speech, delirium, confusion and occasionally coma (particulary cimetidine).
    • Occur primarily in elderly patients and after I.V. administration.
  7. Granulocytopenia and aplastic anaemia.
  8. Liver: reversible hepatitis, cholestasis.

Precautions

  • It crosses the placenta and passes with milk, so, better avoided during pregnancy and lactation.

  • Rebound ulcers due to sudden withdrawal of the drug are possible.

Drugs (cc) Table

  RANITIDINE

  • It is more potent than cimetidine.

  • The oral dose in peptic ulcer is 300 mg for 4-8 weeks, then, 150 mg daily for 6 months or more as maintenance dose.

  • Ranitidine doesn’t significantly affect the cytochrome oxidase P-450, thus the subsequent drug interactions are not reported. The risk of untoward antiandrogenic effects and hyperprolactinaemia from ranitidine use appears to be minimal.

  • Adverse CNS effects of cimetidine have been less reported.

FAMOTIDINE

  • It is the most potent.

  • It has a longer half-life than cimetidine or ranitidine.

  • Pharmacodynamics and adverse effects are similar to those of ranitidine, without any antiandrogenic or enzyme inhibitory effects.   NIZATIDINE

  • Similar to ranitidine in pharmacological action and potency.

  • In contrast to cimetidine, ranitidine and famotidine, which are metabolized by the liver, it is eliminated principally by the kidney.

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