First generation H1 blocking antihistamines are used to treat hypersensitivity reactions as they are known to reduce itching and urticaria that are caused by release of histamine granules as seen in Type I hypersensitivity reactions, of which some examples include allergic rhinitis and anaphylaxis.
First generation antihistamines are occasionally used as sedatives for patients experiencing insomnia, as H1 blockade in the CNS causes sedation. It is important to also remember that while some patients may take antihistamines specifically for this effect, for other patients taking antihistamines for other reasons this may represent an undesired side-effect.
First generation H1 blocking antihistamines can be used to treat anxiety. This is because H1 receptors in the CNS have a stimulatory effect when activated, therefore blockade leads to CNS sedation and relaxation.
First generation H1 blocking antihistamines (especially diphenhydramine) can be given as local anesthetic for patients who have allergies to the more commonly used "-caine" drugs. This mechanism is mediated by blockade of H1 receptors located on nerve endings, which normally cause pain when stimulated.
First generation antihistamines are used as anti-tussives (especially diphenhydramine). This mechanism is mediated by the partially anti-cholinergic actions of first generation H1 blockers in the areas of the CNS that control coughing, as well as by the direct anti-inflammatory effect of H1 blockers within the airway and surrounding tissues.
First generation H1 antagonist antihistamines are used as anti-emetics. Some examples include diphenhydramine (Benadryl), promethazine (Phenergan), and dimenhydrinate (Dramamine).
First generation H1 antagonist antihistamines have some anti alpha-adrenergic activity. Alpha-1 adrenergic receptors are located throughout the body, particularly in smooth muscle tissue where they mediate contraction. Some examples of this include blood vessels, in which activation leads to vasoconstriction, and sphincters such as the urethral and anal sphincters, which similarly undergo constriction in response to alpha-adrenergic activation. They also have an antagonistic effect on muscarinic acetylcholine receptors, leading to effects such as dry mouth and dizziness.
Diphenhydramine (AKA benadryl) is an antihistamine used as a sedative, to treat hay fever/allergic reactions, and as an anti-tussive.
Chlorpheniramine is a first generation H1 antagonist antihistamine and is used to treat allergies.
Promethazine is a first generation H1 antagonist antihistamine and is used as an anti-emetic and local anesthetic.
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