• Clinical science

Antihistamines

Summary

Histamine is a biologically active substance that potentiates the inflammatory and immune responses of the body, regulates physiological function in the gut, and acts as a neurotransmitter. Drugs that antagonize these effects by blocking or inhibiting histamine receptors (H receptors) are called antihistamines. Antihistamines are divided into two classes (H1 antihistamines and H2 antihistamines), based on the type of H receptor targeted. H1 antihistamines are mostly used to treat allergic reactions and mast cell-mediated disorders. This subtype is further divided into two generations. While the first-generation H1 antihistamines have a central effect and, thus, are also used as sedatives, second-generation H1 antihistamines have less central effects and are used primarily as antiallergenic drugs. H2 antihistamines are indicated primarily for gastric reflux disease because they reduce the production of stomach acid by reversibly blocking the H2 histamine receptors in the parietal cells of the gastric mucosa. Use of most H1 and H2 antihistamines is contraindicated during pregnancy and childhood. First-generation H1 antihistamines are specifically contraindicated in angle-closure glaucoma and pyloric stenosis.

Overview

Antihistamine agents

Group Generation Drug Uses Characteristics
H1 antihistamines 1st-generation antihistamines
2nd-generation antihistamine
  • Nonsedative/mildly sedative
H2 antihistamines

Antihistamine effects [1]

Antiallergic Muscarinic Sedation Antimotion sickness
Diphenhydramine
  • +
  • +++
  • +++
  • +++
Promethazine
  • +
  • +++
  • +++
  • ++
Chlorpheniramine
  • ++
  • ++
  • ++
  • ++
Meclizine
  • +
  • ++
  • ++
  • ++++
Cetirizine
  • +++
  • +/none
  • +
  • None
Loratadine
  • +++
  • +/none
  • None
  • None
Fexofenadine
  • +++
  • +/none
  • None
  • None
Key: +: low; ++: moderate; +++: high

References:[2][2][2][3][3][4][5]

H1 antihistamines

General physiology

Effects

  • Competitive, reversible antagonism of histamine H1 receptors
    • Inhibition of increased vascular permeability
    • Inhibition of allergic bronchial constriction
    • Central action: sedation

Side effects

Indications

Contraindications

[3]

H2 antihistamines

Target receptors

Effects

Side effects

Indications

Relative contraindications

References:[3][6]

last updated 12/02/2019
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