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They block the vasodilation mediated by the H2 receptors in blood vessels, possibly leading to less edema formation in urticaria.

The combination of H1 and H2 antagonists may be useful in acute urticaria as well as chronic idiopathic urticaria not responding to H1 antagonists alone.

They act by competitive inhibition of histamine at the H1 receptor, which mediates wheal and flare reactions, bronchial constriction, mucus secretion, smooth muscle contraction, and edema.

Because they have significant anticholinergic activity, especially the older sedating antihistamines, they may cause hypotension, CNS depression, urinary retention, and cardiac arrhythmias, especially in higher therapeutic doses or overdoses. Peak plasma levels are reached within 1 hour and the half-life is about 8 hours.

It selectively binds to Ig E and inhibits binding to Ig E receptors on the surface of mast cells and basophils.

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