Formulary Monographs [ANTIHISTAMINICS, 0400]

There are a multitude of effective antihistamines, now more correctly classified as H1-histamine antagonists. They differ little from each other except in duration of action and the degree of drowsiness, the commonest side effect.

General Indications

Nasal allergy (hay fever or allergic rhinitis), urticaria (prevention and treatment) and pruritus associated with skin disorders. In allergic emergencies chlorphenira-mine maleate 10mg i.m., i.v. may be life saving. Dimenhydrinate is primarily an antiemetic (see p. 369).

Side Effects

All antihistamines cause sedation to some degree. Patients vary widely in their response and a single dose may produce severe sedation within minutes in some patients. This is potentiated by alcohol and other sedatives and patients MUST BE WARNED about this and not to drive until they are sure that sedation is not occurring. In some individuals, especially children and the elderly, and in overdose, bizarre Central Nervous System side effects can occur. Anti-cholinergic effects (dry mouth, blurred vision, urinary retention, constipation and palpitations) may be dose limiting.