antihistaminicum

Antihistaminicum is a term used for substances that block the action of histamine, typically by antagonizing the H1 receptor. In clinical practice, antihistamines are used to counteract allergic symptoms such as sneezing, runny nose, nasal itching, conjunctival irritation, and hives. Some agents also have antiemetic or sedative effects, depending on their ability to cross the blood–brain barrier.

Mechanism of action is mainly competitive inhibition of histamine at H1 receptors located on vascular endothelium, smooth muscle, and sensory nerves. By preventing histamine-induced vasodilation, increased vascular permeability, and itch, antihistamines reduce many symptoms of allergic reactions. First-generation antihistamines tend to cross the blood–brain barrier and cause sedation and anticholinergic side effects, while second-generation antihistamines are designed to be less sedating and have lower brain penetration.

Classification typically divides antihistamines into generations. First-generation examples include diphenhydramine and chlorpheniramine; they are effective but more likely to cause drowsiness and anticholinergic effects. Second-generation examples include cetirizine, loratadine, and fexofenadine; these are preferred for daytime use due to reduced sedation. Antihistamines are available in oral, intranasal, ophthalmic, and topical forms and are used for allergic rhinitis, conjunctivitis, urticaria, and pruritus; some agents are employed as antiemetics for motion sickness.

Safety considerations include drowsiness and dry mouth, particularly with first-generation drugs, and anticholinergic effects in the elderly. Caution is advised in glaucoma, urinary retention, and during pregnancy or lactation, depending on the agent. Potential interactions exist with alcohol, other central nervous system depressants, and certain drugs that affect metabolism or cardiac conduction. Historically, some antihistamines were linked to cardiac risks and have been withdrawn from use. See also H1 receptor antagonists and histamine.