Antihistaminics

Antihistaminics are drugs that counteract the effects of histamine by blocking histamine H1 receptors in tissues involved in allergic responses. Their primary action is competitive antagonism of histamine, which reduces symptoms such as sneezing, itching, rhinorrhea, and conjunctival irritation. Many antihistaminics also exhibit secondary effects, including anticholinergic or antiemetic properties. First-generation agents tend to cross the blood–brain barrier and can cause sedation and cognitive impairment, while newer second-generation agents are more peripherally selective and are generally less sedating.

Classification and examples: Antihistaminics are commonly divided into first-generation (e.g., diphenhydramine, chlorpheniramine, hydroxyzine) and second-generation (e.g., loratadine, cetirizine, fexofenadine, desloratadine). In addition to oral formulations, they are available as topical preparations for skin symptoms and as nasal sprays or eye drops for targeted relief.

Indications and use: They treat allergic rhinitis, conjunctivitis, and urticaria, and are used for pruritus associated with dermatitis or insect bites. Some agents are employed to prevent motion sickness or vestibular symptoms. Dosing varies by age and condition, with second-generation agents often taken once daily.

Safety and cautions: Common adverse effects include drowsiness (more with first-generation drugs), dry mouth, blurred vision, and urinary retention in susceptible individuals. Caution is advised in the elderly, people with glaucoma or urinary retention, and during pregnancy. They can interact with alcohol and other central nervous system depressants; some agents may prolong the QT interval in rare cases, though modern medicines are usually safer.