Sedating antihistamines effect is related to
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.
They are valuable drugs to reduce symptoms due to allergic diseases, especially those mediated by histamine.Antihistamines are inhibitors of histamine receptors. H1-antihistamines, inhibit competitively H1 receptors and the corresponding effects i. They do not inhibit antigen/antibodies reactions, nor histamine release, they inhibit H1 effects. H1-antihistamines have been used for more than fifty years in treating various allergic manifestations. H1-antihistamines which penetrate into brain elicit, by inhibiting stimulant effect of histamine, drowsiness.Certain H1 antagonists, such as promethazine, have a local anesthetic effect.
Other H1-antihistamines having an important antimuscarinic activity like diphenhydramine and dimenhydrinate, are used in preventive and curative treatment of motion sickness, but scopolamine which has no antihistamine effect seems more effective than them in preventive treatment.(There are other types of antihistamines that affect H-2, H-3, and H-4 receptors.) People build up tolerance to them over time and when they stop taking them experience rebound insomnia.