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Antihistamines (H1 receptor antagonists) are commonly found in over-the-counter and prescription medications used for motion sickness, control of allergy-related itching, and cough and cold palliation and used as sleep aids (Table II–9). Acute intoxication with antihistamines results in symptoms very similar to those of anticholinergic poisoning. H2 receptor blockers (cimetidine, ranitidine, and famotidine) inhibit gastric acid secretion but otherwise share no effects with H1 agents, do not produce significant intoxication, and are not discussed here. Common combination products containing antihistamines include Actifed, Allerest, Contac, Coricidin, Dimetapp, Dristan, Drixoral, Excedrin PM, Nyquil, Nytol, Pamprin, PediaCare, Tavist, Triaminic, Triaminicol, Unisom Dual Relief Formula, and Vicks Pediatric Formula 44.

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Table II-9 Antihistamines

  1. Mechanism of toxicity

    1. H1 blocker antihistamines are structurally related to histamine and antagonize the effects of histamine on H1 receptor sites. They have anticholinergic effects (except the “nonsedating” agents: cetirizine, desloratadine, fexofenadine, levocetirizine, and loratadine). They may also stimulate or depress the CNS, and some agents (eg, diphenhydramine) have local anesthetic and membrane-depressant effects in large doses.

    2. Pharmacokinetics. Drug absorption may be delayed because of the pharmacologic effects of these agents on the GI tract. Volumes of distribution are generally large (3–20 L/kg). Elimination half-lives are highly variable, ranging from 1–4 hours for diphenhydramine to 7–24 hours for many of the others (see also Table II–61).

  2. Toxic dose. The estimated fatal oral dose of diphenhydramine is 20–40 mg/kg. In general, toxicity occurs after ingestion of 3–5 times the usual daily dose. Children are more sensitive to the toxic effects of antihistamines than are adults. The nonsedating agents are associated with less toxicity. Up to 300 mg of loratadine is expected to cause only minor effects in pediatric ...

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