H₁ Antagonist - First-generation (non-selective, Classical)

First-generation (non-selective, Classical)

These are the oldest H₁-antihistaminergic drugs and are relatively inexpensive and widely available. They are effective in the relief of allergic symptoms, but are typically moderately to highly potent muscarinic acetylcholine receptor (anticholinergic) antagonists as well. These agents also commonly have action at α-adrenergic receptors and/or 5-HT receptors. This lack of receptor selectivity is the basis of the poor tolerability profile of some of these agents, especially compared with the second-generation H₁-antihistamines. Patient response and occurrence of adverse drug reactions vary greatly between classes and between agents within classes.