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.The reason for their peripheral selectivity is that most of these compounds are zwitterionic at physiological p H (around p H 7.4).As such, they are very polar, meaning that they do not cross the blood–brain barrier and act mainly outside the central nervous system.
Significant anticholinergic adverse effects, as well as sedation, are observed in this group but the incidence of gastrointestinal adverse effects is relatively low.
This term is used to describe drugs that antagonise histamine H1 receptors.
They are used primarily to treat disorders where abnormal or excessive histamine release by inflammatory cells is thought to underly illness.
Most of the second-generation antihistamines have been shown to benefit chronic idiopathic urticaria.
Once-daily fexofenadine appears to offer effective and well-tolerated relief from the symptoms of this illness.