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First-generation antihistamines can easily cross the blood-brain barrier into the central nervous system to reach the H-1 receptors within, often causing drowsiness. [7] Second-generation antihistamines selectively bind to the peripheral H-1 receptors outside the blood-brain barrier, therefore they are less likely to cause sedation.
It is a first-generation antihistamine in the piperazine family of chemicals. [8] [4] Common side effects include sleepiness, headache, and dry mouth. [8] [9] Serious side effects may include QT prolongation. [9] It is unclear if use during pregnancy or breastfeeding is safe. [8]
The newer, second-generation H 1-antihistamines are far more selective for peripheral histamine H 1-receptors and have a better tolerability profile compared to the first-generation agents. The most common adverse effects noted for second-generation agents include drowsiness, fatigue, headache, nausea and dry mouth.
The introduction of the first-generation antihistamines marked the beginning of medical treatment of nasal allergies. [28] Research into these drugs led to the discovery that they were H 1 receptor antagonists and also to the development of H 2 receptor antagonists , where H 1 -antihistamines affected the nose and the H 2 -antihistamines ...
Doxylamine and other first-generation antihistamines are the most widely used sleep medications in the world. [6] Typical side effects of doxylamine (at recommended doses) include dizziness, drowsiness, grogginess, and dry mouth, among others. [7] [4] As an antihistamine, doxylamine is an inverse agonist of the histamine H 1 receptor.
Clemastine, also known as meclastin, is a first-generation H1 histamine antagonist (antihistamine) with anticholinergic properties (drying) and sedative side effects. [1] Like all first-generation antihistamines, it is sedating. [2] [3] Patented in 1960, it came into medical use in 1967. [4]
Azatadine (Optimine) is a first-generation antihistamine and anticholinergic drug that was synthesized in 1963 by Schering-Plough, a former American pharmaceutical company. [ 1 ] [ 2 ] It was patented in 1967. [ 3 ]
Phenbenzamine, sold under the brand name Antergan and known by the former developmental code name RP-2339, is an antihistamine of the ethylenediamine class which also has anticholinergic properties. [ 1 ] [ 2 ] It was introduced in 1941 or 1942 and was the first antihistamine to be introduced for medical use.