Antihistamine

An antihistamine is a drug which serves to reduce or eliminate effects mediated by histamine, an endogenous chemical mediator released during allergic reactions, through action at the histamine receptor. Only agents where the main therapeutic effect is mediated by negative modulation of histamine receptors are termed antihistamines - other agents may have antihistaminergic action but are not true antihistamines.

Second-generation H₁-receptor antagonists

These are newer drugs that are much more selective for peripheral H1 receptors in preference to the central nervous system histaminergic and cholinergic receptors. This selectivity significantly reduces the occurrence of adverse drug reactions compared with first-generation agents, while still providing effective relief of allergic conditions.

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Systemic

• acrivastine
• astemizole
• cetirizine
• loratadine
• mizolastine
• terfenadine (withdrawn from most markets due to risk of cardiac arrhythmias and replaced with fexofenadine)

Topical

• azelastine
• levocabastine
• olopatadine