Paper I
2015 September (Supplementary) (2010 Scheme) · 40 marks · 120 min

Question

Second generation antihistamines

Q53 marksShort Notes

Answer

Second-generation antihistamines are H1-receptor antagonists developed to overcome the sedative and anticholinergic drawbacks of first-generation agents.

Examples

  • Cetirizine, Loratadine, Fexofenadine, Desloratadine

Properties

  • Do not readily cross the blood-brain barrier (poorly lipid-soluble, substrates for P-glycoprotein efflux) → minimal sedation (“non-sedating antihistamines”)
  • More selective for peripheral H1 receptors — minimal anticholinergic and antiadrenergic side effects
  • Longer duration of action — allow once-daily dosing

Uses

  • Allergic rhinitis, urticaria, allergic conjunctivitis, other allergic conditions

Adverse effects

  • Generally well tolerated; minimal sedation
  • Rarely: headache, dry mouth
  • Some (e.g., older agents like terfenadine, now withdrawn) had QT prolongation risk — modern agents like cetirizine/loratadine/fexofenadine are considered safer in this regard

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