H1-antihistamines have been used for more than fifty years in treating various allergic manifestations. H1-antihistamines which penetrate into brain elicit drowsiness by inhibiting stimulant effect of histamine.
Antihistamines inhibit histamine receptors from responding to the stimulant affects of histamine such as drowsiness, vasodilation and capillary permeability. They do not prevent cell degranulation, and they do not inhibit antigen/antibodies reactions, nor histamine release, they inhibit H1 effects.
In addition to this antihistamine effect, they can have parallel pharmacological properties, for example antimuscarinic and adrenolytic effects which must be considered.
It is usual to classify H1-antihistamines into two classes: old ones, called first-generation agents, which are sedating and recent ones, called second-generation agents which have slight or no sedating effects because they do not cross the blood-brain barrier.
In addition, each compound can have or not have parallel properties, antimuscarinic effects for example. This distinction between generations must be taken with caution because a product considered as non sedative or not antimuscarinic can, in certain circumstances, large doses or particular susceptibility of the patient ,have these effects.
List of most common H1 Antagonist Antihistamines