H1 Blockers

Overview

• Mechanism
• reversible inhibition of H₁ histamine receptors

1st generation

• Drugs
  • diphenhydramine (Benadryl)
  • dimenhydrinate (Dramamine)
  • chlorpheniramine
  • hydroxyzine
  • promethazine
    • abused as a recreational drug (“purple drank” or “sizzurp”)
• Clinical uses
  • allergy
  • motion sickness 
  • sleep aid
• Toxicity
  • antimuscarinic 
  • anti-α-adrenergic
• sedation (lipophilic and thus crosses brain barrier)

2nd generation

• Drugs
  • loratadine (Claritin)
  • fexofenadine (Allegra)
  • cetirizine (Zyrtec)
  • desloratadine
• Clinical uses
  • allergy
• Toxicity
  • decreased entry into CNS (lipophobic) makes this generation far less sedating