Antihistamines (Diphenhydramine, Cetirizine, Loratadine)

H1 antihistamines are drugs that treat allergic rhinitis, urticaria, pruritus and motion sickness, and they are given as premedication before some infusions. The key split is by generation. First-generation drugs such as diphenhydramine, hydroxyzine and chlorpheniramine cross the blood-brain barrier and block muscarinic receptors, so they sedate and add anticholinergic effects, and the client is taught no driving or alcohol. Second-generation drugs such as cetirizine, loratadine and fexofenadine cause little sedation and are preferred for daily allergies. In anaphylaxis, epinephrine comes first; an antihistamine is only an adjunct and never replaces it. Older adults should avoid first-generation drugs, which cause confusion, falls and urinary retention and are on the Beers list. Use them with caution in angle-closure glaucoma, BPH or urinary retention. In children they can cause paradoxical excitation, and promethazine is contraindicated under two years because of fatal respiratory depression. Teach the client not to take fexofenadine with fruit juice, and to check combination products for duplicate diphenhydramine.