Which H1 antihistamine causes the least sedation and is preferred for patients who must operate machinery?
- A Diphenhydramine
- B Fexofenadine ✓
- C Promethazine
- D Chlorpheniramine
Explanation
Second-generation antihistamines (cetirizine, loratadine, fexofenadine, bilastine) have minimal CNS penetration due to their greater polarity and P-glycoprotein-mediated efflux at the blood-brain barrier. Among these, fexofenadine has the lowest CNS penetration (it is a P-gp substrate that is effectively kept out of the brain) and is consistently the least sedating in objective studies. Diphenhydramine and promethazine are highly sedating first-generation agents. Chlorpheniramine, though less sedating than diphenhydramine, still crosses the BBB significantly. Fexofenadine is therefore preferred when alertness is required.
Reference: KD Tripathi, Essentials of Medical Pharmacology, 8th ed.
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