Antihistamines

H1 antagonists, and the family holds both generations: the sedating first-generation agents — promethazine, doxylamine, hydroxyzine, cyclizine, dimenhydrinate, chlorphenamine — alongside the second-generation ones built not to cross into the brain, cetirizine and loratadine. The difference is the whole point. The first generation is antimuscarinic as well as antihistaminergic, and that is where its sedation, its dry mouth and its delirium at high dose come from.

That is also the boundary. Diphenhydramine is filed under Deliriants, because at the doses taken for that purpose the antimuscarinic delirium is the experience; the first-generation agents here share the pharmacology and differ in how they are met. What hurts at class level is additive — sedation stacked onto opioids, alcohol or benzodiazepines, and antimuscarinic load stacked onto the tricyclics and the deliriants. Astemizole was withdrawn over QT.