H1 Antagonists/’Anti-histamines’

H1 Antagonists/’Anti-histamines’:
• selectively block H1 receptors  prevent action of histamine at receptors (do not affect histamine release)
• inhibit dilation effects of histamine; reduce oedema (decrease in capillary permeability); reduce itching (due to affects on sensory nerves); CNS depression; anticholinergic effects (due to blockage of muscarnic cholinergic receptors)
• indicated for mild allergy (eg allergic rhinitis); severe allergy (adjuncts in management of anaphylaxis); motion sickness (due to blockage of H1 and muscarnic receptors in vestibular system); insomnia (from CNS depression)
• adverse effects - sedation

First generation H1 antagonists:
• alkylamines (eg bromopheniramine; dexachloropheniramine (Polaramine®); ethanolamines (eg clemastine); ethylenediamines (eg tripelennamine); phenothiazines (eg promethazine); piperidines (eg azatadine)
• sometimes called sedating H1 antagonists (can be a significant problem)

Second generation H1 antagonists:
• eg azelastine; cetrizine; fexofenadine; loratadine (eg Claritin®)
• nonsedating
• astemizole and terfenadine withdrawn from market due to risk of dysrhythmias

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