What is the difference between first-generation and second-generation antihistamines?

First-generation antihistamines like diphenhydramine and chlorpheniramine cross the blood-brain barrier and cause significant sedation, dry mouth, urinary retention, and blurred vision due to their anticholinergic properties. Second-generation antihistamines like cetirizine, loratadine, and fexofenadine are less lipophilic, minimally cross the blood-brain barrier, and cause much less sedation. Both generations block H1 histamine receptors to reduce allergy symptoms including sneezing, itching, runny nose, and hives.