Mechanism of Action

At therapeutic doses, the first- and second-generation antihistamines are equilibrium-competitive inhibitors of H1-receptor-mediated responses. Certain second-generation drugs are noncompetitive inhibitors at high concentrations. Both first- and second-generation compounds have negligible abilities to block the H2-, H3-, or H4-receptors. The therapeutic effectiveness of these

TABLE 38.2 Representative H1Receptor Antagonists

Trade Duration of Sedative Anti-Motion Anticholinergic

Drug Name Action (hr) Activity Sickness Activity Activity

First-Generation Antihistamines

Ethanolamines Carbinoxamine Clemastine Diphenhydramine Dimenhydrinate Ethylenediamines Pyrilamine Tripelennamine

Alkylamines

Chlorpheniramine Brompheniramine Piperazines Cyclizine Hydroxyzine Meclizine

Phenothiazines

Promethazine Piperidines

Cyproheptadine

Second-Generation Antihistamines

Promethazine Piperidines

Cyproheptadine

Second-Generation Antihistamines

Periactin 4-6

Piperidines

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