antihistaminer

Antihistaminer, or antihistamines, are medicines that block the action of histamine at histamine H1 receptors. They are used to relieve symptoms of allergic conditions such as sneezing, itching, runny nose, conjunctival irritation, and hives. The class is commonly divided into first-generation (sedating) and second-generation (non-sedating) agents, with some medicines available in intranasal, ophthalmic, or topical forms.

Mechanism and effects: Most antihistamines act as inverse agonists at the H1 receptor, reducing histamine-induced vasodilation, capillary permeability, and itch. First-generation agents readily cross the blood–brain barrier and often cause sedation and cognitive slowing, while second-generation compounds are designed to be less sedating and have fewer central effects. Some drugs also possess anticholinergic or mild antiemetic properties, contributing to a broader side‑effect profile.

Examples and forms: Common first-generation drugs include diphenhydramine and chlorpheniramine; second-generation options include cetirizine, loratadine, fexofenadine, desloratadine, and levocetirizine. Ocular and nasal antihistamines such as azelastine and olopatadine are used for conjunctival or nasal symptoms. Topical forms may be employed for dermatitis, while oral forms address systemic allergic conditions.

Indications and safety: Indications include allergic rhinitis, conjunctivitis, and urticaria. They are not substitutes for epinephrine in severe anaphylaxis. Side effects vary by generation: sedation, dizziness, dry mouth, and urinary retention are more common with first-generation agents, especially in the elderly. Caution is advised in glaucoma, urinary retention, and during pregnancy, with choices differing by drug.

Interactions: Alcohol and other central nervous system depressants can enhance sedation. Some agents interact with fruit juice (notably fexofenadine) or other drugs that prolong the QT interval. Always review concurrent medications.