A 25-year-old woman with severe allergic rhino-conjunctivitis is prescribed a second-generation H1 antihistamine for daytime symptom control. Compared to first-generation agents, what pharmacological property primarily accounts for the reduced sedation of second-generation antihistamines?
- A They are H1 receptor inverse agonists rather than antagonists
- B They do not cross the blood-brain barrier due to being substrates for P-glycoprotein efflux ✓
- C They selectively block peripheral H1 receptors and have no central receptor binding
- D They are rapidly metabolized in the liver by first-pass effect before reaching the brain
Correct answer: B. They do not cross the blood-brain barrier due to being substrates for P-glycoprotein efflux
Explanation
Second-generation antihistamines (cetirizine, loratadine, fexofenadine) are substrates for P-glycoprotein (MDR1) efflux pumps at the blood-brain barrier, limiting CNS penetration and thus sedation. Fexofenadine is the least sedating. This is the key difference from first-generation agents like chlorpheniramine and diphenhydramine, which readily enter the brain and block central H1 receptors causing drowsiness.
Reference: Katzung Basic and Clinical Pharmacology, 14th ed.
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Written and medically reviewed by the StethoPrep medical team.