Which generation of H1 blocker antihistamines has greater CNS penetration and more CNS side effects?

• A. 1st Generation
• B. 2nd Generation
• C. Both Generations Equally
• D. Neither Generation

Answer: (A)

The main idea is how well these drugs enter the brain, which drives CNS effects. First-generation H1 blockers are lipophilic and not strongly restricted by the blood-brain barrier, so they distribute into the CNS readily. That results in sedative effects, drowsiness, impaired coordination, and anticholinergic side effects. Second-generation H1 blockers are designed to be more polar and often substrates for efflux pumps, so they stay mainly outside the brain and cause far fewer CNS effects. That’s why the older, first-generation group has greater CNS penetration and more CNS side effects. Examples you’ll often learn are diphenhydramine or chlorpheniramine for the sedating group, and cetirizine, loratadine, or fexofenadine for the non-sedating, second-generation group. In practice, this difference informs choices when avoiding drowsiness is important.