antihistaminica

Antihistaminics, or antihistamines, are medicines that block histamine H1 receptors to counteract allergic symptoms. They are widely used to treat conditions such as allergic rhinitis, conjunctivitis, urticaria, and pruritus. The term antihistaminics is used in several languages as a general designation for this drug class.

Most antihistamines act as competitive antagonists at H1 receptors, reducing histamine-mediated vasodilation, increased vascular permeability, mucus production, and itching. First-generation agents cross the blood–brain barrier, often causing drowsiness, sedation, and anticholinergic effects. Second-generation antihistamines are designed to be less sedating by limited central nervous system penetration and often have longer durations of action.

Common first-generation examples include diphenhydramine, chlorpheniramine, and promethazine; common second-generation examples include cetirizine, loratadine, fexofenadine, desloratadine, and levocetirizine. Some antihistamines possess additional antiemetic or anticholinergic properties and may be used for other indications, such as nausea or motion-related symptoms.

Uses include relief of seasonal and perennial allergic rhinitis, conjunctival symptoms, chronic urticaria, and itch due to dermatitis or bites. Routes of administration include oral tablets or liquids, nasal sprays, topical creams, and ophthalmic drops. In general, second-generation agents are preferred for daytime use due to a lower risk of sedation; first-generation agents may be used when sedation is acceptable or beneficial.

Safety and cautions: adverse effects can include dry mouth, blurred vision, urinary retention, and constipation. Anticholinergic effects can worsen glaucoma or urinary problems in some patients. Cautions apply in pregnancy and lactation, in elderly patients, and when consuming alcohol or other CNS depressants.