Pharmacology · MU Sofia

25. Histamine antagonists.

Covers histamine antagonists in three classes: H1 blockers (first and second generation), H2 blockers, and the mast cell stabilizer ketotifen. It goes through pharmacodynamics, pharmacokinetics, side effects, drug interactions and therapeutic uses, with a classification picture and recap questions on the differences between generations, H2 duration and ketotifen's mechanism.

Inside: 9 sections · 4 labelled pictures · 10 flashcards · 4 min

Topic 24 of 93 in the MU Sofia syllabus for Pharmacology (Dental Medicine, Year 3). The free AceMed note covers it in 9 sections, about 532 words (4 min of reading), with 10 flashcards.

In this note

  1. Histamine Antagonists
  2. Classification
  3. Pharmacodynamics
  4. Pharmacokinetics
  5. Side Effects
  6. Drug Interactions
  7. Therapeutic Uses
  8. SAFE IN PREGNANCY
  9. Recap Questions

Histamine Antagonists

Histamine is a chemical messenger mostly generated in mast cells. Histamine, via multiple receptor systems, mediates a wide range of cellular responses, including allergic and inflammatory reactions, gastric acid secretion, and neurotransmission in parts of the brain.

Classification

  1. 1. H1 receptor blockers
  • First-generation: Diphenhydramine, chlorpheniramine, promethazine
  • Second-generation: Cetirizine, loratadine, fexofenadine (less or no CNS penetration, thus non-sedating)
  1. 2. H2 receptor blockers: Cimetidine, ranitidine, famotidine, nizatidine
  2. 3. Mast cell stabilizers: Ketotifen

Pharmacodynamics

Histamine antagonists function by blocking the action of histamine at its receptors:

  • H1 receptor blockers (located in endothelial cells and smooth muscles):
  • First-generation H1 blockers competitively inhibit histamine at H1 receptors. These drugs can cross the blood-brain barrier, leading to sedative effects.
  • Second-generation H1 blockers have minimal or no CNS penetration, reducing sedation.
  • H2 receptor blockers (located on parietal cells in the stomach lining):
  • By blocking histamine at H2 receptors, they reduce gastric acid secretion, making them effective for peptic ulcer disease, GERD, and Zollinger-Ellison syndrome.
  • By decreasing stomach acid production, they help to promote the healing of gastric and duodenal mucosa.
  • Ketotifen, a mast cell stabilizer:
  • Prevents the release of histamine and other inflammatory mediators by inhibiting Ca²⁺ influx into mast cells.
  • Used in allergic and inflammatory conditions such as asthma, allergic conjunctivitis, and atopic dermatitis.

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