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Antihistamines

Last updated: March 9, 2021

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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). 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.

Antihistamine agents

Effects of antihistamine agents [1]

Overview of effects of antihistamine agents
Agents Antiallergic Muscarinic Sedation Anti-motion sickness
Diphenhydramine
  • +
  • +++
  • +++
  • +++
Promethazine
  • +
  • +++
  • +++
  • ++
Chlorpheniramine
  • ++
  • ++
  • ++
  • ++
Meclizine
  • +
  • ++
  • ++
  • ++++
Cetirizine
  • +++
  • +/none
  • +
  • None
Loratadine
  • +++
  • +/none
  • None
  • None
Fexofenadine
  • +++
  • +/none
  • None
  • None
Key: +: low; ++: moderate; +++: high

References:[2]

Examples

General physiology

Effects

Side effects

Indications

Antihistamines are ineffective in acute hereditary angioedema attacks. [3]

Contraindications

References:[2]

Examples

  • Ranitidine [4]
  • Famotidine
  • Nizatidine

General physiology

Effects

Competitive, reversible antagonism of histamine H2 receptors (Gs protein-coupled receptor) on parietal cells adenylyl cyclase activity → cAMP levels protein kinase A activity → phosphorylation and activation of H+/K+ ATPase gastric acid (H+) secretion

Side effects

Indications

Relative contraindications

Drink a glass of H2O before dining: H2 blockers need to be taken before dinner.

References:[2]

  1. Nzeako UC. Hereditary Angioedema. Arch Intern Med. 2001; 161 (20): p.2417. doi: 10.1001/archinte.161.20.2417 . | Open in Read by QxMD
  2. Katzung B,Trevor A. Basic and Clinical Pharmacology. McGraw-Hill Education ; 2014
  3. FDA Requests Removal of All Ranitidine Products (Zantac) from the Market. https://www.fda.gov/news-events/press-announcements/fda-requests-removal-all-ranitidine-products-zantac-market. Updated: April 1, 2020. Accessed: December 15, 2020.
  4. Ozdemir PG et al. Assessment of the effects of antihistamine drugs on mood, sleep quality, sleepiness, and dream anxiety. International Journal of Psychiatry in Clinical Practice. 2014; 18 (3): p.161-168. doi: 10.3109/13651501.2014.907919 . | Open in Read by QxMD