Cetirizine is a second-generation H1 antihistamine. Compared to diphenhydramine, its reduced sedation is primarily because:

• A Cetirizine is a prodrug and must be activated peripherally before entering the brain
• B Cetirizine is a zwitterion at physiological pH, limiting blood-brain barrier penetration ✓
• C Cetirizine undergoes rapid hepatic first-pass extraction, reducing systemic levels
• D Cetirizine selectively blocks peripheral H1 receptors over central H1 receptors

Explanation

Cetirizine (a carboxylated metabolite of hydroxyzine) exists as a zwitterion at physiological pH — it has both a positive and negative charge that reduce its lipophilicity and limit passive diffusion across the blood-brain barrier. This physicochemical property, not receptor selectivity per se, reduces CNS penetration and sedation. Diphenhydramine is highly lipophilic with excellent CNS penetration. Levocetirizine (active enantiomer) shares this property.

Reference: KD Tripathi, Essentials of Medical Pharmacology, 8th ed.

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Written and medically reviewed by the StethoPrep medical team.