H1 Antagonists/’Anti-histamines’

The H1 Antagonists are a class of drugs that selectively block the H1 receptors to prevent the action of histamine at the receptors; They do not affect histamine release. The H1 antagonists inhibit the dilation effects of histamine; reduces angioedema (decrease in capillary permeability); reduces itching (due to affects on sensory nerves); CNS depression; anticholinergic effects (due to blockage of muscarnic cholinergic receptors).

They are 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); sedating effects (first generation only).

The main adverse effects are sedation.

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

Second generation H1 antagonists:
Non-sedating H1 antagonists as they are more selective for peripheral histamine H1 receptors rather than the CNS H1 receptors and cholinergic receptors; they are more polar so less likely to cross the blood-brain barrier (becasue of this they will have a different side affect profiles).
eg azelastine; cetrizine; fexofenadine; loratadine (eg Claritin®)

Astemizole and terfenadine were withdrawn from market due to risk of dysrhythmias in the 90’s.

Examples of the H1 Antagonists (Anti-histamines):

In Australia, Promethazine, Desloratadine, Fexofenadine and Loratadine are on the National Podiatry Scheduled Medicines List for use by authorised Podiatrists.

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