• 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. Antihistamines are drugs that antagonize these effects by blocking or inhibiting histamine receptors (H receptors), and they are categorized as either H1 or H2 according to 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 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 primarily used as antiallergic 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. Most H1 and H2 antihistamines are 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
  • Reduce production of stomach acid

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

Examples

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

Antihistamines are ineffective in acute hereditary angioedema attacks! [6]

Contraindications

[3]

H2 antihistamines

Examples

Target receptors

Effects

  • Competitive, reversible antagonism of histamine H2 receptors leads to reduced production of stomach acid.

Side effects

Indications

Relative contraindications

References:[3][7]

last updated 06/18/2020
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