Antihistamines

Background

• Also known as histamine antagonists
• In common use, the term antihistamine refers only to compounds that inhibit action at the H₁ receptor (and not H₂, etc.).

Types

H₁ antagonists

• First-generation (sedating; High-risk geriatric medication!)
  • Diphenhydramine (Benadryl)
  • Chlorpheniramine
  • Dimenhydrinate
  • Doxylamine
  • Hydroxyzine (Vistaril)
  • Meclizine
• Second-generation (non-sedating, preferred)
  • Cetirizine
  • Levocetirizine
  • Loratadine (Claritin)
  • Desloratadine
  • Fexofenadine (Allegra)
  • Azelastine

H₂ antagonists

• Cimetidine
• Famotidine
• Nizatidine
• Ranitidine
• Roxatidine

Indications

• H₁ antagonists
  • Allergic reactions
  • Insomnia
  • Vertigo
  • Motion sickness
  • Nausea and vomiting in pregnancy
• H₂ antagonists
  • GERD/peptic ulcers
  • Allergic reactions

Side Effects

• Sedation
  • Second-generation antihistamines cross the blood–brain barrier to a much lower degree (therefore are less sedating), however high doses can still induce the drowsiness

See Also

• Anticholinergic toxicity