Why are first-generation antihistamines considered high risk in elderly patients?

First-generation antihistamines like diphenhydramine are included on the Beers Criteria list of potentially inappropriate medications for older adults because their strong anticholinergic effects increase the risk of confusion, delirium, cognitive impairment, falls, urinary retention, and constipation in elderly patients. Older adults are more susceptible to these effects due to decreased hepatic metabolism, increased blood-brain barrier permeability, and pre-existing cholinergic deficits. Second-generation antihistamines like cetirizine and loratadine are preferred alternatives. ---